DYNAMO/AMIE S-PolKa Scientist Summaries - December 2011
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OTHER MONTHS:
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| The active phase of the MJO
continues to propagate towards the east with increased
speed (Figure 1). As of 30
November the MJO was exiting phase 3
with convection associated with the active phase
concentrated in the eastern Indian Ocean and Maritime
Continent. The large negative anomalies of 200 hPa
velocity potential continue to be concentrated over the
entire Indian Ocean basin in a wavenumber one structure;
however, these anomalies are starting to move over the
Maritime continent and Western Pacific as of 30 November (Figure 2). At the 200 hPa level, easterlies continue to persist across most of the Indian Ocean today, with the large subtropical ridge over India increasing in amplitude; however, the southern anticyclone has once again mostly broken down (Figure 3). At 500 hPa, the rotation between the easterlies north of 5 N and the westerlies south of the equator remains stationary. The winter monsoon circulation over India is seen to have shifted westward over Gujurat as the subtropical ridge developed a tilt in the ridge axis. The westerlies north of the equator have disappeared entirely as the easterlies penetrate further west towards Ethiopia. The westerlies south of the equator remain intense between 55-80 E, but meet the easterlies just east of the DYNAMO array. At 850 hPa, the enhanced westerlies are backing off towards the west, with the exit region of the jet now directly over the DYNAMO array. the flow splits at this juncture between a cyclonic region SE of Sri Lanka and a broad cyclonic region SW of Diego Garcia. Easterlies between 5-10 N now stretch into the Arabian Sea. Most of the DYNAMO array remained very quiet today. In the morning the main convection areas remained to our east, west, and south (Figure 4). The two convective lines near the southern portion of the array were very slowly moving south, with scattered lightning throughout each. The convective region in the eastern Indian Ocean remains mostly stationary with most of its lightning confined to its northern and eastern edges (Figure 5). By 1200 UTC, the convective lines to the north and south of Diego Garcia significantly weakened and at 1800 UTC convection began to reform in a line just to the west of Diego Garcia. By the end of the day, lightning was prevalent within the line of convection to the west of Diego Garcia, in a small region of enhanced convection north of S-PolKa, south of the former location of the Mirai, and east of the Revelle. At Diego Garcia, the mid-levels started very dry, with a deep layer of westerlies extending up to 350 hPa and easterlies above that level at 0000 UTC 1 December (Figure 6). Throughout the day the entire troposphere moistened over Diego Garcia and saturated conditions were present through 450 hPa by 1800 UTC. However, the vertical wind profile remained relatively constant with deep layer westerlies changing to easterlies between 400-300 hPa throughout the day at Diego Garcia. Although the low levels were dry at the Revelle yesterday, the mid-levels began to dry out as well over the course of the day and mid-level winds shifted to have a strong westerly component (Figure 7). By 1800 UTC, moistening had occurred below 500 hPa and above approximately 400 hPa, while the layer between 500-400 hPa dried. Additionally, deep westerlies were present below 400 hPa and easterlies above this level. The Revelle radar saw a few weak lines of convective cells throughout the day that were oriented NW to SE that tended to move towards the northeast (Figure 8). At S-PolKa, the sky mostly had high cirrus streaks and shallow cumulus in the morning, building to slightly deeper cumulus cloud lines that are leaning towards the east (Figure 9). These thin, high clouds are visible on the DOE KAZR (Figure 10). Time lapse imagery shows that these shallow lines of cumulus are oriented NW to SE and propagated towards the SE (Figure 11), which is consistent with the WNW winds close to the surface as well as the speed shear seen between the surface and 800 hPa (Figure 12). Several humidity gradients are also evident in the Bragg scattering rings of the higher elevation scans (right panel of Figure 11), which are consistent with the numerous humidity layers seen in the DOE Gan sounding (Figure 12). The dry layer between 450 - 350 hPa remained throughout the day; however, conditions below 600 hPa began to moisten and conditions began to dry above 350 hPa. During the afternoon and overnight convective activity increased in the northern portion of the S-PolKa domain (Figure 4). Beginning around 1230 UTC convective cells began to develop within NW-SE lines that moved towards the southeast. These cells were taller than the cells witnessed the previous two days, reaching up to 16 km. These cells has had strong divergence signals at upper levels and graupel was present just above the 0 deg C level in convective cores. The convective cells grew in areal coverage and organization to form a SW-NE oriented convective line that propagated towards the southeast between 1800 - 2100 UTC (Figure 13). This line not only produced a trailing stratiform region at upper levels, but also a strong low-level outflow boundary, which was visible as a fine line of reflectivity and high differential reflectivity (Figure 14). While time lapse imagery of radial velocity shows the convection moving away from the radar (warm colors), the outflow can be seen emanating from the storm and moving towards the radar (cool colors). Both the convective and stratiform regions moved towards the southeast. The stratiform region was notably weak, which low reflectivity at the melting layer and just a few melting aggregate signatures in the particle type field. Weak stratiform regions were a characteristic of the back edge of the October active MJO phase (e.g., see the 31 October summary). In general, the heights of the echoes seen have dropped dramatically today compared to the past week (Figure 15). Compared with the onset of the MJO, where the field of convective clouds were more commonly deep, the depth of all except the most intense echoes decreased dramatically as the active phase passed to the east. The frequency of the echoes also dropped, signifying the change from widespread, disorganized precipitation on 22-23 November to the organized convective lines like those seen 26-27 November. |
| Figure 1. Austrailian Bureau of
Meteorology RMM MJO Index for 30 November |
| Figure 2. 200 hPa velocity potenial
anomolies (green contours negative, brown contours
positve) overlaid on daily infrared imagery for 30
November. |
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| Figure 3. IMD model analyses for 01
December at 200 hPa, 500 hPa, and 850 hPa. |
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| Figure 4. Infrared satellite imagery at
0600, 1200, 1800, and 2300 UTC 01 December. |
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| Figure 5. Infrared imagery overlaid with
WWLN lightning data at 0600 and 2300 UTC 01 December. |
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| Figure 6. Diego Garcia soundings for 01
December. |
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| Figure 7. Revelle soundings for 01 December. |
| Figure 8. Revelle reflectivity PPI for 1629 UTC 01
December. |
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| Figure 9. Photos looking east at 0540 UTC,
north at 0841 UTC and north at 1006 UTC 01 December. |
| Figure 10. ARM KAZR reflectivity for 1
December. |
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| Figure 11. S-PolKa reflectivity PPI at the
0.5 and 7 elevation angle (top), and reflectivity RHIs at
76 (yellow line in left PPI) and 14 (yellow line in right
PPI) degrees azimuth at 0900 UTC 01 December. |
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| Figure 12. Gan soundings for 0600 and 2100
UTC 01 December. |
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| Figure 13. S-PolKa S-Band reflectivity PPI
at 1516, 1616, 1716, 1816, 1916, and 2016 1 December (top
two rows), and RHIs of reflectivity, radial velocity, and
hydrometer classification at 44 degrees (yellow line in
PPI) at 1516 UTC 1 December (bottom row). |
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| Figure 14. S-PolKa S-Band reflectivity PPI
reflectivity, radial velocity, and differential
reflectivity (top row) and RHIs of reflectivity, radial
velocity, differential reflectivity, and hydrometer
classification at 20 degrees (yellow line in PPI) at 1746
UTC 1 December (bottom row). |
| Figure 15. Nine-day echo-height statistics
for 22 November - 30 November. |
| At S-PolKa, the day was marked by
lines of isolated convective cells oriented
northwest-southeast transitioning into squall lines
oriented north to south. n the large scale, the 200 hPa
velocity potential continues to indicate divergence across
the eastern Indian Ocean, although the Arabian Sea is
becoming less negative (Figure 1).
Positive velocity potential, indicating increased
upper-level convergence, is also beginning to cover Africa
and is moving eastward. Most of the convection in the
basin has moved to our south and east, with very active
convection over the Maritime Continent and an electrified
convective line SW of Diego Garcia
(Figure 2). Squall lines have appeared in the ending stages of the two active phases of the MJO sampled by DYNAMO: on 31 October after the first, and over the past few days for the most recent. However, the large-scale setup is somewhat different on these days. On 31 October, the 200 hPa winds were diverging over the array in association with a large subtropical ridge to the SE, but on 2 December the subtropical ridge to the north was more zonal (see comparison of the two days in Figure 3). The intense westerlies seen at the end of the current active phase then were not apparent south of the equator from 500 hPa to 925 hPa on 31 October. In general the low-level flow on 31 October was much less coherent as cyclonic rotations in the Arabian Sea, off the southern tip of India, and NE of Madagascar channeled 700 hPa flow from the Arabian desert towards the array. Today, the 200 hPa winds are easterly across the entire Indian Ocean from 10 N-10 S with no hint of diffluence around the equator. The subtropical ridge to the north has a fairly large amplitude, while the subtropical jet to the south is mostly zonal (Figure 3). The 200 hPa winds abruptly slow down at 70 E between the equator and 5 S, creating a small region of upper-level convergence that would suggest a localized subsidence just west of the northern array. 500 hPa vorticity is getting compressed into a thinner region along a shear line between the easterlies and the westerlies north of the array, but over the array the winds remain westerly from 500 hPa to the surface. Flow at 700 hPa instead seems to be coming from Ethiopia, funneled between four cyclonic gyres that are north and south of the equator at approximately 55 E, southwest of the array at 70 E, and north of the array at 80 E. The main similarity of the two cases is that the winds over Gan in the present case were westerly from 700 hPa downward and stronger; whereas, the westerlies were in a shallower layer and weaker on 31 October compared to 02 December. Also, in both cases lower-tropospheric flow seems to be coming from Africa and/or Arabia. The localized upper-level convergence seen today may explain why the upper-level soundings are so dry on this day (Figure 4), compared to the relatively moist soundings above 600 hPa seen on 31 October. These differences may also explain the relative lack of trailing stratiform associated with the passing squall lines, except in the early morning hours. A region of mixed stratiform and convection appeared in the NNW sector of the radar at ~2200 UTC 1 December (Figure 5). A convective line began propagating out of this region towards the southeast at roughly 9 m/s (estimated from timelapse imagery) at ~2330 UTC 01 November, trailing streamers of stratiform behind it. The convection in this line was relatively deep compared to that seen over the past few days, reaching up to ~12 km with 45 dBZ echo reaching ~5.5 km (Figure 6). The PID suggests that this storm was forming large amounts of aggregates, yet the stratiform of this line was relatively limited in height with some fallstreaks, only a few patches of brightband, and not much signal of melting aggregates, as is seen in more robust stratiform echoes (compare Figure 8 of 16 October). The morning was bright, mostly clear, and hot, but over the course of the morning and afternoon the small cumulus (Figure 7) began building in height. The 0600 UTC sounding indicates that as the surface heated up, the low levels became incredibly unstable, and the shear below 700 hPa was only ~ 5 m/s (Figure 4). Above 700 hPa, there was some directional shear between 600-700 hPa, and again between 400-500 hPa as the winds switch to easterlies above that level. Distinct dry layers were especially strong around 800 hPa and 450 hPa, although the column was relatively dry between those levels as well. Most of the morning, convection consisted of lines of isolated cells oriented northwest-southeast and moving towards the southeast. The pace with which the clouds built sped up around 0700 UTC (Figure 7), corresponding to a squall line that moved eastward over Gan (Figure 8). The squall line had thick trailing anvil, but only a small amount of trailing stratiform (Figure 9). The collapse of the convective cells that appeared on the radar ~0600 UTC seemed to spawn new convection ahead of it, which began to merge ~0830 UTC before forming a complete line by 0930 UTC (Figure 10). Bookend vortices appeared on the northern and southern extremities of the line as it began to bow in the middle ~1030 UTC. At its most intense, the convection was shallow in depth, especially compared to what was seen 31 October, but similar in depth to what has been seen in the squall lines over the past few days at the end of this active phase (Figure 11). This convection was intense, especially closest to the radar, with over 50 dBZ echo and high differential reflectivity (not shown), which the hydrometeor classification indicating heavy rain up to 4 km. The northern end of the line had some trailing stratiform, whereas the southern end did not, which is another feature many of the squall lines of the past few days have had. In contrast, the 31 October squall line had the entire back end of the bow echo filled with stratiform. After the passage of the squall, the layer between 500-700 hPa completely saturated by 1200 UTC, and then began to slowly dry out (Figure 4). The low levels were briefly capped by an inversion starting at 900 hPa, but this inversion disappeared by 1500 UTC. The sky was covered in anvil from the passing squall for some time (Figure 7). The convective cells returned to being isolated and moving towards the southeast for several hours, but new convective lines began to form about 1800 UTC that continued into the next day, which will be a subject of tomorrow's summary. |
| Figure 1. 200 hPa velocity potential with
infrared satellite for 01 December. |
| Figure 2. Infrared satellite overlaid with
WWLN lightning data at 0600 UTC 02 December. |
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| Figure 3. Comparisomn of IMD ARW model
analyses for 0000 UTC 31 October and 02 December 2011 at
200 hPa, 500 hPa, 700 hPa, and 850 hPa. |
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| Figure 4. Gan soundings for 0000-1500 UTC
02 December. |
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| Figure 5. S-PolKa reflectivity and
convective/stratiform separation PPI from 2300 UTC 01
December - 0200 UTC 02 December. |
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| Figure 6. S-Polka reflectivity PPI (top)
and reflectivity, velocity, and PID RHI at 8 degrees
azimuth (bottom) for 0100 UTC 02 December. |
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| Figure 7. Photos looking east at 0438 UTC,
0701 UTC, 0821 UTC, 0852 UTC, and 1128 UTC 02 December. |
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| Figure 8. Photos facing SSW at 0821 UTC,
0913 UTC, and 1129 UTC 02 December. |
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| Figure 9. ARM KAZR reflectivity and
vertical velocity for 02 December. |
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| Figure 10. S-PolKa reflectivity PPI for
0630-1130 UTC 02 December. |
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| Figure 11. S-PolKa reflectivity PPI (top)
and reflectivity, velocity, and PID RHI at 8 degrees
azimuth for 0915 UTC UTC 02 December. |
| The MJO has entered phase 4 (Figure 1) as enhanced convection is
now over the Maritime Continent (Figure 2).
The upper-level support for convection is decreasing as
the negative velocity potential moves farther east and
positive velocity potential, proportional to upper-level
convergence, moves off of Africa into the western Indian
Ocean (Figure 3). Localized
upper-level diffluence has returned to the southern
portion of the DYNAMO array, but the northern array
remains in intense easterlies (Figure
4). The circulation associated with a developing
tropical disturbance is apparent at 500 hPa east of the Revelle, and the
northerly flow over India is intensifying as the
subtropical ridge to the north flattens into a a more
zonal flow. A second potential tropical invest circulation
is most apparent at 850 hPa and below, and is currently
southwest of Diego Garcia. Weaker 850 hPa cyclonic
circulations off the southern tip of India as well as off
the coast of Myanmar (Burma) are also spinning up as the
circulation around the tropical invest west of Indonesia
and the northerly flow off of India intensify. The Revelle left station the evening of 02 December and is presently doing transects across the equator. Small amounts of lightning persists in the storm SW of Diego Garcia, as well as in some of the isolated convection east of Diego Garcia and east of the former station of the Revelle (Figure 5). The amount of lightning increased in the middle of the array by the end of the day. At Diego Garcia, the soundings began very dry between 500-800 hPa and moist below 800 hPa and between 375-500 hPa (Figure 6). At 066 the dry layer was between 400-500 hPa and it was moister below this level. However, the layer between 400-700 hPa became drier again by 1800 UTC, with an inversion just above 700 hPa. Gan continued to experience squall lines from the evening of 2 December until about mid-day 3 December. Initially the convection, which was mostly disorganized, moved in a band across the radar domain from WNW to ESE, trailing thick anvils of irregular ice behind them (Figure 7). The upper-level clouds were generally streaming behind the line to the west until about 1800 UTC 3 December, when the anvils began separating from the convective cells and moving towards the SW (Figure 8). These high clouds persisted throughout most of the day, but finally dissipated around 1200 UTC 3 December (Figure 9). Although the sounding above and below 600 hPa was dry at 1800 UTC , by 0100 UTC the sounding was a mostly saturated with an "onion" sounding below 600 hPa (Figure 10). The sounding also shows high deep-level directional shear resulting from a switch from westerlies to easterlies at ~450 hPa and little speed shear. About 0000 UTC a particularly large squall line congealed over the radar as it moved towards the southeast at roughly 13 m/s (Figure 11; estimated from the radar). This squall line was notable for the thick trailing anvil it left as well as a comparatively (to previous days) robust stratiform region (Figure 12). As it was reaching maximum intensity, the convective cells in the line were producing graupel and aggregates, and the region behind had a sloping rear inflow (Figure 13). By 0113 UTC, the echo had reached its peak of ~15 km, which is deeper than most of the squall lines seen over the past few days, and the storm had trailing stratiform with a weak melting signature and fallstreaks (Figure 14). After this squall, several other smaller convective lines passed through the domain, although the convective lines seemed to get less and less organized as the day wore on (Figure 15). By the end of the day, the regime had returned to being a band of isolated cells north of the radar. The soundings regained various dry layers with inversions by 1800 UTC (Figure 10) |
| Figure 1. Australian Bureau of Meteorology
MJO phase diagram for 1 December. |
| Figure 2. Infrared satellite imagery for
0600 UTC 03 December. |
| Figure 3. CPC 200 hPa velocity potential
overlaid infrared satellite imagery for 02 December. |
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| Figure 4. IMD model analyses at 200 hPa,
500 hPa, and 850 hPa for 0000 UTC 03 December. |
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| Figure 5. WWLN lightning data overlaid
infrared satellite imagery for 0000 UTC and 1500 UTC 03
December. |
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| Figure 6. Diego Garcia soundings for 03
December. |
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| Figure 7. S-PolKa reflectivity PPI (top)
and reflectivity, velocity, and PID RHI along the 126
degree azimuth (bottom) for 2230 UTC 02 December. |
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| Figure 8. S-PolKa cartesian reflectivity
PPI at 2.5 and 8 km for 0220 UTC 03 December. |
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| Figure 9. Photos looking NE at 0511 UTC,
east at 0818 UTC, and SE at 1150 UTC 03 December. |
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| Figure 10. Gan soundings for 02-03
December. |
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| Figure 11. S-PolKa reflectivity PPI for
2316 UTC 02 December - 0116 UTC 03 December. |
| Figure 12. ARM KAZR data for 03 December. |
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| Figure 13. S-PolKa reflectivity PPI and
reflectivity, velocity, and PID RHI along the 120 degree
azimuth for 0030 UTC 03 December. |
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| Figure 14. S-PolKa reflectivity, velocity,
and PID RHI along the 120 degree azimuth for 0113 UTC 03
December. |
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| Figure 15. S-PolKa reflectivity PPI from
0516-2216 UTC 03 December. |
| Positive 200 hPa velocity
potential continues to creep into the Indian Ocean,
indicating that upper-level convergence and associated
large-scale subsidence is slowly moving into the area (Figure 1). Convection over the Indian
Ocean has quieted dramatically, with the main active
regions being the newly upgraded Cyclone 01S at 12 S 90 E,
the weak tropical disturbance SW of Diego Garcia, and a
wispy stream of convection north of the DYNAMO array that
stretches from the SW tip of India across the Arabian Sea
(Figure 2). The easterlies at 500 hPa in
the Arabian Sea (Figure 3) seem to
be pushing moisture west towards Ethiopia (Figure 4) as part of the flattened
monsoonal circulation over India. However, dry enhanced
westerlies continue to flow towards the DYNAMO array at
500 hPa. The low-level westerlies are weaker than they
have been in previous days. At 200 hPa, easterlies
continue to stretch across most of the Indian Ocean
between 10 N and 10 S, although as seen yesterday the
speed temporarily drops over the northern array before
accelerating again just west of the array. The oppositely
directed easterly and westerly components at upper and
lower levels, respectively, affected the motions of
convective cells and anvils seen near the S-PolKa radar. Convection continues to initiate in the middle of the DYNAMO array as well as to the northwest of Gan (Figure 5). Interestingly, several clouds near Diego Garcia are electrified while not having particularly cold cloud tops. The P3 concentrated their mission in this region (Figure 6). Soundings at Diego Garcia indicate the appearance of a dry layer between 500-700 hPa on 2 December that has moved upward to 400-650 hPa by today, along with a deepening of the westerlies up to the top of that layer (Figure 7). Dry air also descended over the same period from 100 hPa to just below 200 hPa. The sky today was mostly clear, and it was sunny and hot at S-PolKa (Figure 8). The streamer of eastward-moving convective cells to our north was visible in the distance. By midday the boundary layer was highly unstable but capped by an stable layer at 850 hPa, with subsequent inversions ~500 hPa and again at 350 hPa (Figure 9). Like Diego Garcia, dry air at ~100 hPa has descended to ~250 hPa, but dry air is also appearing at many other levels in the column (Figure 10). The westerlies extend up to ~500 hPa. Because of the shear between the westerlies and easterlies at higher levels, the convective cells rooted in lower levels were moving eastward while their anvils were traveling westward (Figure 11). When these cells moved underneath the anvil of the cells that preceded them, they did not seem to directly interact with the anvil above (Figure 12; see the summaries for 25-26 October and 18 November for discussion of other examples). At this cell's maximum intensity, it was fairly deep and reached ~14 km, but produced minimal amounts of aggregates, no graupel, and had a weak divergence signature aloft (Figure 13). Later in the day, a convective cell formed on a cold pool boundary (evident by the Bragg scattering in the reflectivity) and intensified when this boundary intersected another (Figure 14). The cell's anvil was visible off to the far NE of S-PolKa (Figure 15). This cell did produce graupel; at the moment that graupel was first identified by the PID, the top of the cell had a very healthy divergence signature above the closer reflectivity core (Figure 16). This core had high reflectivities up to 5 km, but nearly zero differential reflectivity (ZDR) and specific differential phase (KDP) above 3 km, which is consistent with an identification of graupel by the PID. In contrast, the more distant reflectivity core had more positive ZDR and KDP up to 5 km, which is consistent with heavy rain. Short-lived isolated convective cells of this nature continued to propagate across the radar domain for the rest of the day, becoming somewhat more numerous as the day wore on (Figure 17). |
| Figure 1. CPC 200 hPa velocity potential
overlaid IR satellite imagery for 3 December. |
| Figure 2. Infrared satellite imagery at
0600 UTC 4 December. |
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| Figure 3. IMD model analyses for 0000 UTC
4 December at 200 hPa, 500 hPa, and 850 hPa. |
| Figure 4. CIMSS MIMIC total precipitable
water for 0700 UTC 4 December. |
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| Figure 5. Infrared satellite imagery with
WWLN lightning data (top) and infrared satellite imagery
(bottom) for 0000 UTC 4 December. |
| Figure 6. NOAA P3 flight tracks overlaid
visible satellite imagery at 0600 UTC 4 December. |
| Figure 7. Diego Garcia weekly sounding
series for 28 November to 4 December. |
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| Figure 8. Photos looking south at 0403
UTC, east at 0655 UTC, SE at 0851 UTC, and north at 1025
UTC 4 December. |
| Figure 9. Gan sounding for 0600 UTC 4
December. |
| Figure 10. Gan weekly sounding series for
27 November to 5 December. |
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| Figure 11. S-PolKa Cartesian reflectivity
(top) and velocity (bottom) PPI for 0.5 km (left) and 10
km (right) at 0050 UTC 4 December. |
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| Figure 12. S-PolKa reflectivity PPI (top)
and RHI (bottom) for 0016-0046 UTC 4 December. |
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| Figure 13. PID and velocity RHI for 0054
UTC 4 December (same as rightmost panels in Figure). |
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| Figure 14. S-PolKa reflectivity PPI from
0816-1016 UTC 4 December. |
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| Figure 15. Zoomed photos looking NE at
0655 UTC and 0922 UTC 4 December. |
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| Figure 16. S-PolKa reflectivity, velocity,
differential reflectivity (ZDR), specific differential
phase (KDP), and particle ID (PID) RHI at 30 degrees
azimuth for 0900 UTC 4 December (yellow line in Figure 14). |
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| Figure 17. S-PolKa reflectivity PPI (top)
and reflectivity, velocity, PID RHI at 8 degrees azimuth
for 2200 UTC 4 December. |
| The Wheeler-Hendon RMM index
indicates that the MJO has continued to propagate eastward
(Figure 1). While the amplitude of the
MJO has remained relatively constant over the past few
days, the speed of its eastward propagation has slightly
increased. Consistent with the eastward shift of the
active phase of the MJO, positive 200 hPa velocity
potentials and large-scale subsidence have begun to enter
the western Indian Ocean (Figure 2).
However, a band of negative 200 hPa velocity potential
along the equator extends further to the west and covers
the DYNAMO array. At 200 hPa, the anticyclone that was
located located just to north of Sri Lanka yesterday, has
shifted eastward and is presently over the Arabian Sea (Figure 3). The 200 hPa easterly flow and
geopotential heights have increased near the DYNAMO array
and strong easterlies are located between 10 N and 5 S.
Upper and mid-level flow over India continues to be zonal,
while a 500 hPa gyre has developed over the Equator near
65 E. Midlevel southwesterly flow is being funneled over
the DYNAMO array between this gyre and the circulation
from cyclone 01S to the southeast. At 850 hPa, the
cyclonic circulations of the two tropical disturbances are
relatively unchanged from yesterday. Low-level westerlies
along the Equator and throughout the DYNAMO array continue
to advect dry air into the region; however, the speed of
the westerlies has slightly reduced (Figure 4).
A narrow band of low level convergence is also present
just to the southeast of the former location of the Mirai, along the
northwestern edge of cyclone 01S. While convective activity began to slightly increase in the DYNAMO array around 1200 UTC, suppressed conditions persist (Figure 5). A band of convection in the Arabian Sea, extending from Somalia to the southern tip of India, began to shift slightly northward. Convection within this band became increasingly deep and electrified throughout the day. Tropical Cyclone 01S is located at approximately 12 S 87 E and continued to move towards the west-southwest at approximately 3 knots. A small amount of lightning has been consistently observed within this tropical circulation. A band of convection began to form along the northern edge of the tropical cyclone's outflow around 1200 UTC, which merged with a region of enhanced convection in the middle of the DYNAMO array. The tropical disturbance located at approximately 17 S 70 E looks increasingly organized in satellite imagery, and the Joint Typhoon Warning Center is indicating that this circulation has a high probability of developing into a tropical cyclone within the next 24 hours. The Revelle has been moving eastward along the equator and continues to take soundings every six hours (Figure 6). While winds were consistently westerly below 450 hPa and above approximately 125 hPa, easterly flow occurred within the intervening layer and was particularly strong between 200-150 hPa. While drying occurred between 850 and 500 hPa, significant moistening was observed below 850 hPa. Conditions above 300 hPa and from 500-400 hPa also experience notable moistening. Radar imagery from the Revelle shows that scattered showers slowly moving towards the southeast throughout the day (Figure 7). While winds at Diego Garcia were characterized by a transition from low-level westerlies to upper-level easterlies between 350-300 hPa throughout the day, the moisture profile varied widely throughout the day (Figure 8). At 0000 UTC, relatively moist conditions existed through 750 hPa, a moderately dry layer occurred between 750 and 400 hPa, and moisture increased between 400-300 hPa. Early in the day, drying occurred within the layer between 900 - 400 hPa. However, by 1800 UTC extremely moist conditions were present below 650 hPa and extremely dry conditions existed between 650 and 350 hPa. Satellite imagery indicates that the convection near Diego Garcia at 0000 UTC dissipated and convection was scattered by 1200 UTC (Figure 5). Dry conditions prevailed above the 900 hPa level at Gan, and for the majority of the day convection near S-PolKa was limited to shallow cumulus that propagated rapidly towards the southeast (Figure 9). S-PolKa continued to see echoes characteristic of suppressed conditions throughout the day, with scattered convection predominately in the northern portion of the radar domain. The day started with a prominent cold pool boundary released from convection to the north of S-PolKa (Figure 10). Similar to cold pools previously mentioned, this outflow was characterized by a fine line of high reflectivity, high differential reflectivity, and Bragg scatter in the region between the fine line and the convection. Additionally, radial velocities indicate that the cold pool was propagating towards the radar as the storm was moving away from the radar. The hydrometeor classification identifies this fine line as clutter, which is consistent with the highly variable phi DP along the line. While this outflow was not distinctly observed in the SMART-R reflectivity field, it may be seen as a thin line of inbound velocities in the radial velocity field (Figure 11). From approximately 0700-1100 UTC a persistent line of Bragg scatter cut across the northern portion of the S-PolKa domain. This line is particularly interesting in light of the cold pool observed around 0000 UTC since this line is not associated with high differential reflectivity or phi DP (Figure 12). Convective cells tended to form along this boundary and propagate towards the southwest. Most precipitation echoes during the day were narrow convective cells, had a maximum height below 8 km, had a only small amount of frozen aggregates above the 0 deg C level, lacked stratiform echo, but nevertheless produced a cold pool that emanated from the storm (Figure 13). During the late afternoon, the cold pools that came within approximately 25 km of S-PolKa had fine lines of reflectivity and were classified by the hydrometeor classification scheme as clutter. However, the differential reflectivity and phi DP were quite noisy, making it difficult to see the cold pool. Cold pools outside of the is range were often only discernible through Bragg scatter. Convection slowly began shift from the northern portion to the central portion of the S-PolKa domain. At approximately 2200 UTC a cell passed over Gan as it propagated towards the southeast (Figure 14). This convective cell later became the tallest cell observed today as it reached about 9 km in height (Figure 15). This cell was also unique since graupel was present just above the freezing layer within convective core and a divergence signature aloft was observed above 8 km. |
| Figure 1. Australian Bureau of Meteorology
RMM MJO index valid for 3 December. |
| Figure 2. CPC 200 hPa velocity potential
overlaid IR satellite imagery valid for 4 December. |
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| Figure 3. IMD model analyses for 0000
UTC December at 200 hPa, 500 hPa, and 850 hPa. |
| Figure 4. CIMSS MIMIC total precipitable
water for 1200 UTC 05 December. |
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| Figure 5. WLLN Lightning overlaid on
METEOSAT infrared imagery at 0000, 0600, 1200, and 1800 5
December and 0000 UTC 6 December. All lightning in the
prior 30 minutes is shown. |
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| Figure 6. Soundings from the Revelle taken at
0000, 0600, 1200, and 1800 UTC 5 December along the
equator, moving towards the east. |
| Figure 7. PPI of reflectivity from the Revelle at 1159 UTC 5
December. |
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| Figure 8. Soundings from Diego Garcia at
0000, 0600, 1200, and 1800 UTC 5 December. |
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| Figure 9. DOE Gan sounding at 1200 UTC 5
December and photos taken at S-Polka looking north at 0715
and 1008 UTC (middle two panels) and looking northeast at
1256 UTC 5 December. |
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| Figure 10. S-PolKa S-Band PPIs (left
column) and RHIs (right column) along the yellow line of
reflectivity, radial velocity, differential reflectivity,
hydrometeor classification, and phi DP at 2316 UTC 4
December. |
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| Figure 11. SMARTR reflectivity (left) and
radial velocity (right) at 2318 UTC 4 December. |
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| Figure 12. S-PolKa S-band reflectivity,
radial velocity, differential reflectivity, and phi DP at
0931 UTC 5 December. |
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| Figure 13. S-PolKa S-Band PPIs (left
column) and RHIs (right column) of reflectivity, radial
velocity, differential reflectivity, phi DP, and
hydrometeor classification at 1501 UTC 5 December. |
| Figure 14. KAZR reflectivity for 5
December. |
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| Figure 15. S-PolKa S-Band PPI of
reflectivity and RHIs of reflectivity, radial velocity,
and hydrometeor classification along the yellow line at
2246 UTC 5 December. |
| Suppressed conditions continue as
positive 200 hPa velocity potential spreads eastward and
now covers half of the Indian Ocean (Figure
1). The MJO signal remains strongly wavenumber-1,
with the peak negative velocity potential over the
Maritime Continent, as the MJO active phase continues to
steadily move through phase 4 and into phase 5 (Figure 2). Strong easterlies remain
aloft along the equator until 60 E, where the 200 hPa flow
splits away from the equator (Figure 3).
While 200-500 hPa flow over northern India is now
completely zonal, the 500 hPa flow in the southern Indian
Ocean over the DYNAMO array is southwesterly as it goes
around the circulation of Tropical Cyclone Alenga (01S).
The low-level westerlies continue to advect dry air into
the northern portion of the array, and Tropical Cyclone
2S's circulation continues to wrap dry air around its
northern side close to Diego Garcia (Figure
4). The low-level easterlies in the Arabian Sea are
also accelerating towards into a point just east of
Somalia, creating a zone of low-level convergence just off
of the coast with enhanced convection and lightning (Figure 5). Portions of the two
cyclones and the large convective line east of Diego
Garcia were also highly electrified. Waves were visible in
the satellite imagery in the afternoon and evening hours
emanating towards the NW from this convective line. At
Diego Garcia itself, conditions were moist up to 750 hPa,
dry between 325-750 hPa, but relatively moist again above
325 hPa (Figure 6). Westerlies
extended up to ~400 hPa, and easterlies extended down to
250 hPa, with a transitional zone in between. Skies were partly cloudy for most of the day (Figure 7) as a band of isolated convective cells stretched across the northeastern half of the S-PolKa domain (Figure 8). The air was very dry above 750 hPa, and stable layers existed between 650-750 hPa as well as 450-500 hPa (Figure 9 ). By late morning the low-levels became highly unstable because of daytime heating. A particularly intense cell occurred in the late afternoon (Figure 10). At 0839 UTC the cell was only 9 km high yet the low-level convective core had > 55 dBZ echo to 3 km and 40 dBZ echo up to 5 km as well as a very distinctive updraft and downdraft signature (Figure 11). The heavy rain classification by the PID is consistent with the high differential reflectivity and decorrelation in the region of the most intense reflectivities. The upper levels of the cell have a strong divergence signal, a relatively large amount of aggregates, but a small amount of other ice hydrometeors. In contrast, a convective cell that developed 2 hours later was larger in diameter and deeper (up to 10 km) with 40 dBZ echo up to 6 km and only a very small amount of > 55 dBZ echo (Figure 12). The updraft structure is still apparent but less distinct. Although the differential reflectivity is high in several areas of the cell that also have >45 dBZ echo, only the portion with the highest reflectivities had a distinct decorrelation signature; the other regions of > 45 dBZ echo did not have a distinct decorrelation. This cell had a larger amount of irregular ice and a stronger melting signature that was likely mixed in with graupel. A different cell developed not long after this one passed over Gan (Figure 13), releasing a short burst of heavy rain. In general, the rain statistics over the past two months suggest a different frequency in heavy rain events during the first active phase sampled compared to the second (Figure 14). Heavy and light rain events seemed to oscillate at a higher frequency during the first active phase, whereas the this oscillation seems to have a longer period during the second active phase. However, similar to the previous active phase, this ending period seems to be marked by a switch to a highly convective regime, with relatively minimal contribution from stratiform, rather than a more equal contribution between convective and stratiform precipitation during the active phase. |
| Figure 1. 200 hPa velocity potential
overlaid IR satellite imagery for 5 December. |
| Figure 2. Australian Bureau of Meteorology
RMM MJO index valid for 4 December. |
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| Figure 3. IMD model analyses for 0000 UTC
6 December at 200 hPa, 500 hPa, and 850 hPa. |
| Figure 4. CIMSS MIMIC total precipitable
water for 0700 UTC 6 December. |
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| Figure 5. Infrared satellite imagery
overlaid with WWLN lightning data for 0830 UTC and 1630
UTC 6 December. |
| Figure 6. Diego Garcia sounding for 1200
UTC 6 December. |
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| Figure 7. Photos looking east at 0451 UTC,
east at 0645 UTC, SE at 0847 UTC, and east at 1227 UTC 6
December. |
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| Figure 8. S-PolKa reflectivity PPI
overlaid with visible satellite imagery at 0300 UTC, 0700
UTC, and 1100 UTC, and infrared satellite imagery at 1500
UTC 6 December. |
| Figure 9. Gan sounding for 0600 UTC 6
December. |
| Figure 10. Photo looking NE at 0847 UTC 6
December. |
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| Figure 11. S-PolKa reflectivity PPI,
reflectivity RHI, velocity RHI (top), differential
reflectivity (ZDR), correlation coefficient (rhoHV) and
particle identification (bottom) for 0830 UTC 6 December. |
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| Figure 12. S-PolKa reflectivity PPI,
reflectivity RHI, velocity RHI (top), differential
reflectivity (ZDR), correlation coefficient (rhoHV) and
particle identification (bottom) for 1000 UTC 6 December. |
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| Figure 13. ARM KAZR reflectivity and
velocity for 6 December. |
| Figure 14. S-PolKa area-averaged daily
rain accumulation from 5 October - 6 December. |
| Positive 200 hPa velocity
potential has reached the DYNAMO array as it now
completely covers the western Indian Ocean and is slowly
propagating eastward (Figure 1). The
wavenumber-1 structure is beginning to break down as a
region of negative velocity potential begins to build over
western Africa, suggesting that the MJO's continued
eastward propagation may not continue. The speed of the
MJO's propagation has definitely slowed and nearly stalled
over the Maritime Continent (Figure 2).
The 200-500 hPa flow over northern India remains mostly
zonal, but the 200 hPa flow over the array is splitting
between southeasterlies heading towards the southern tip
of India, and easterly flow that is part of an
anticyclonic gyre to the south (Figure
3). The flow from 500 hPa downward is westerly, with
the array being between the cyclonic circulations of
Cyclone Alenga (01S) and another weaker tropical
disturbance SW of Diego Garcia. Westerly winds from 850
hPa downward continue to bring in somewhat drier air into
the northern array, but this low-level flow into the
northern array has combined with flow from the Bay of
Bengal to the NE (Figure 4).
Convection continues to initiate in a region of low-level
confluence just off the coast of Somalia, which stretches
in a wave-like stream to just north of the array. This
line is highly electrified, as is the region of convection
to the east of Diego Garcia (Figure
5). The convection near Diego Garcia was initially an intense mesoscale system with relatively cold echo tops (Figure 6). During this time, the sounding was very moist at all levels (Figure 7). As the day wore on, the mid-levels began to steadily dry, with several stable layers beginning to appear. This drying of the mid-levels corresponded to the rapid dissipation of the convection (Figure 6). Meanwhile, the S-PolKa radar domain remained mostly devoid of precipitating convection for much of the day (Figure 8). Mostly clear skies and some isolated shallow cumulus were present locally (Figure 9), but lines of nonprecipitating clouds were evident on the S-band radar (Figure 8). The soundings were very dry above 900 hPa for most of the day, with westerly wind components up to 600 hPa switching to easterlies by 300 hPa (Figure 10). The deep-level shear was fairly intense, but the shear below ~400 hPa was not. By 1800 UTC, the low levels began to moisten below 650 hPa. An interesting radar artifact was observed in a small but intense convective cell seen at ~1600 UTC, which had a core of > 55 dBZ echo up to 3 km (Figure 11). A very large differential reflectivity signature appeared in the low reflectivity zone between the two cells, likely because of three-body scattering between the large raindrops in the high reflectivity core and the ocean surface. As a result, the particle identification identified this region as a mixture of ground clutter and "insects." As the humidity profile of the low levels moistened, the Bragg scattering in the humidity layers also changed (Figure 12). Early in the day, the humidity layers were much more distinct, as were the gradients in humidity in the soundings (compare with the 1200 UTC sounding in Figure 10). The most distinct ring likely corresponds to the sharp humidity gradient seen ~700 hPa. Later, Bragg scattering became much less distinct, with the detectable humidity gradients being mostly close to the surface and further aloft, potentially corresponding to the gradients centered ~900 hPa and ~700 hPa. At about 2100 UTC, convection began moving in from the north, beginning a long series of convective events through 8 December that will be discussed in tomorrow's summary. |
| Figure 1. CPC 200 hPa velocity potential
overlaid infrared satellite imagery for 6 December. |
| Figure 2. Australian Bureau of Meteorology
RMM MJO index for 5 December. |
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| Figure 3. IMD model analyses at 0000 UTC 7
December for 200 hPa, 500 hPa, 700 hPa, 850 hPa, and 925
hPa. |
| Figure 4. CIMSS MIMIC total precipitable
water for 0700 UTC 7 December. |
| Figure 5. WWLN lightning data overlaid
infrared satellite imagery for 0900 UTC 7 December. |
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| Figure 6. Infrared satellite imagery
overlaid with S-PolKa reflectivity PPI for 0000-2100 UTC 7
December. |
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| Figure 7. Diego soundings from 0000-1800
UTC 7 December. |
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| Figure 8. S-PolKa reflectivity PPI for
0000-1800 UTC 7 December. |
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| Figure 9. Photos looking ESE at 0520 UTC,
north at 0820 UTC, and SW at 1020 UTC 7 December. |
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| Figure 10. Gan soundings from 0000-1800
UTC 7 December. |
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| Figure 11. S-PolKa reflectivity PPI (top),
reflectivity and signal-to-noise ratio (SNRC) RHI
(middle), differential reflectivity and PID RHI (bottom)
for 1600 UTC 7 December. |
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| Figure 12. S-PolKa reflectivity PPI at 7
degrees elevation (top) and RHI (bottom) for ~1120 UTC and
~1750 UTC 7 December. |
| Especially when compared to the
very quiet conditions over Gan yesterday, today was a day
full of activity as the broad stream of convection that
has been building across the Arabian Sea moved southward
over Gan (Figure 1). Although
positive 200 hPa velocity potential continued to spread
over the western Indian ocean
(Figure 2),
a wave-like feature across the Arabian Sea has been
steadily building eastward over the past several days,
with enhanced convection extending past Sri Lanka by this
afternoon. The 200 hPa flow shows the undulation in the
winds, with the winds south of India being southeasterly,
turning easterly and then northeasterly as the flow
approaches Somalia (Figure 3). At
the terminus of this feature, the low-level winds
accelerate to a point on the coast of Somalia. The
monsoonal circulation at 500 hPa is substantially weaker
than before. The array itself is under fairly strong
southeasterly flow at 200 hPa, but the intense area of
low-level westerly flow has moved eastward, which would
suggest low-level divergence. The 500 hPa winds are
rotating around a point center just NW of Gan. The
northern array is now relatively moist as the wave-like
feature moves further south, and as moisture from the
Maritime Continent travels with the easterlies into the
eastern Indian Ocean (Figure 4).
Although this feature was electrified, the portion over
the northern array was not
(Figure 1). The 0000 UTC soundings from Male, Gan, and Diego Garcia illustrate the different characteristics of the airmass north, within, and south of the line (Figure 5). In Male, the winds predominately had an easterly component except for a layer from 600-800 hPa. The low levels were very moist but with a series of stable layers, and the air was exceedingly dry above 700 hPa. At Gan the sounding was relatively moist at all levels for the entire day, although the drier air above 700 hPa lifted to ~500 hPa by the middle of the day (not shown). Diego Garcia was dry at most levels, but especially from 300-925 hPa. The precipitation observed by S-PolKa during this event seemed to occur in 4 distinct stages, the characteristics of which can somewhat be seen in the KAZR data (Figure 6): from 2000 UTC 7 December to roughly 0300 UTC 8 December; from ~0400-1100 UTC 8 December; from ~1130-1630 UTC, and from ~1730-2230 UTC. The sky cover was broken to overcast most of the day, with the occasionally open sky seen between passing convective lines (Figure 7). The first period started when S-PolKa began to register convective cells from this feature at about 2000 UTC 7 December (Figure 8). During this stage, which lasted until about 0400 UTC, the convection in the southern radar domain was mostly isolated convective cells, but stratiform from collapsing convection began to collect in the northern sectors. The cells to the south were not particularly deep and had limited ice production, but they did have healthy updrafts and mid-level divergence signatures, as well as some heavy rain in the convective cores (Figure 9). During this period, low-level winds and shear were weak, but the deep-layer shear was relatively intense, and there was a relatively thick transition layer between 450-700 hPa (Figure 5). Beginning around 0000 UTC 8 December, convective lines began to form on the edges of some of these stratiform regions, and propagated away from or through them towards the east as the stratiform moved towards the west (Figure 8). Around 0100 UTC one of these lines formed a coherent line, which had deeper cells that produced a bit more ice cloud, as well as a rear inflow that descended towards the surface within the convective cell (Figure 9). The NOAA P3 and the Falcon flew a coordinated mission into the second phase of this event, which was when the convective cells began to continuously merge into long convective lines (Figure 10). By this point, the transition zone from westerlies to easterlies had narrowed considerably, although low-level shear remained weak (Figure 11). Three distinctive lines formed north of the S-PolKa domain, oriented NW-SE (Figure 12). Around 0730 UTC the largest convective line began to coalesce (Figure 10). At 0900 UTC, Ka-band's humidity retrieval detected a considerable humidity gradient between the region west of the radar and that east of the radar (Figure 13). By the end of this stage, a long convective line coalesced, stretching north to south across the entire radar domain with stratiform trailing off to the west (Figure 10). As it formed, the convection was again not particularly deep, with the cells barely reaching over the melting level (Figure 14). The P3 and Falcon first focused on a convective region to the SE, then flew north to sample the stratiform (Figure 10). After this, the P3 sampled the leading edge of the developing large convective line, and then released dropsondes during two circumnavigations of Gan before the leading and trailing convective lines coalesced. This convective line held its position for several hours, staying almost completely west of the RHI sectors (Figure 15). Stage 3 was marked by the collapse and breakup of the convection in this line. During this time, the upper levels began to dry out, although the low-level westerlies deepened to 550 hPa and remained moist (Figure 11). An embedded line of convection propagated through the stratiform region, although this collapsed before it reached the radar. During the final stage, a region of robust stratiform moved into the western radar domain (Figure 16), from a convective cluster just outside of S-PolKa's range (Figure 17). The three convective lines to the north mentioned before had by this time become two, and had reoriented themselves by this time to be more WNW-ESE. At ~1800 UTC, the upper levels were still dry, although the moist lower layer had moved up to 500 hPa, and the winds through the column were weak and variable (Figure 11). As this cell dissipated, the stratiform eventually began to break up by 2200 UTC (Figure 16). After this time, the convective lines to the north merged to form one large convective region to the NW of S-PolKa, which will be the subject of tomorrow's summary. Figure 18 shows the overall history of the rainfall on this day. In the first panel the line precipitation tagged as convective corresponds to the convective feature seen NE of S-PolKa in the second row of Figure 9. Overall during the day the patterns showed a rather typical mix of convective and stratiform echo coverage. Most of the stratiform precipitation occurred outside of the RHI sectors, so it was hard to determine its vertical structure (1st, 3rd, and 4th rows of Figure 18). From the hourly rainfall summary for S-PolKa (Figure 19) we see that about 59% of the precipitation was classified as convective, which is near normal for this part of the world. |
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| Figure 1. Infrared satellite imagery
overlaid with the WWLN lightning data for 1900 UTC 7
December - 0700 UTC 8 December. |
| Figure 2. CPC 200 hPa velocity potential
for 7 December. |
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| Figure 3. IMD WRF-ARW model analyses at
0000 UTC 8 December for 200 hPa, 500 hPa, 700 hPa, 850
hPa, and 925 hPa. |
| Figure 4. CIMSS MIMIC total precipitable
water for 0700 UTC 8 December. |
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| Figure 5. Male, Gan, and Diego Garcia 0000
UTC soundings for 8 December. |
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| Figure 6. ARM KAZR reflectivity and
velocity for 8 December. |
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| Figure 7. Photos looking SE at 0122 UTC,
SW at 0428 UTC, SW at 0820 UTC, and SE at 1120 UTC 8
December. |
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| Figure 8. S-PolKa reflectivity PPI for
2000 UTC 7 December - 0300 UTC 8 December. |
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| Figure 9. S-PolKa reflectivity PPI and
reflectivity, velocity, and PID RHI for 0000 UTC (top) and
0100 UTC (bottom) 8 December. |
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| Figure 10. S-PolKa reflectivity PPI from
0530-1030 UTC 8 December. |
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| Figure 11. Gan soundings for 0600 UTC,
1200 UTC, and 1800 UTC 8 December. |
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| Figure 12. Infrared satellite imagery
overlaid with the P3 and Falcon flight tracks for
0530-0930 UTC 8 December. |
| Figure 13. S-PolKa Ka-band humidity
retrieval for 0900 UTC 8 December. |
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| Figure 14. S-PolKa reflectivity PPI and
reflectivity and PID RHI for 0900 UTC 8 December. |
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| Figure 15. S-PolKa reflectivity PPI for
1130-1630 UTC 8 December. |
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| Figure 16. S-PolKa reflectivity PPI for
1730-2230 UTC 8 December. |
| Figure 17. Infrared satellite imagery for
1630 UTC 8 December. |
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| Figure 18. S-PolKa radar reflectivity
(left), rain rate (middle), and convective (yellow)
and stratiform (red) areas for 8 December 2011. |
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| Figure 19. S-PolKa hourly rainfall record
for 8 December 2011. |
| Despite the unfavorable
large-scale conditions that cover most of the western
Indian Ocean (Figure 1), the
northern DYNAMO array remained convectively active as a
large convective region persisted to the NW of S-PolKa (Figure 2). In general, the large-scale
support for convection (negative velocity potential,
proportional to upper-level divergence) remains mostly
stationary over the Maritime Continent (Figure
3). The coherence of the MJO signal seems to be
breaking down somewhat, consistent with the weakening of
the MJO signature. In the Indian Ocean, a line of
convection stretching from SE of the former Revelle station
towards the Maritime Continent remained convectively
active most of the day, as well as the remnants of a
tropical disturbance SSW of Diego Garcia (Figure
2). Over the past several days, two distinct
boundaries have formed: between dry air in the Arabian Sea
and the moister air to the south; and along the line of
convection east of the former
Revelle and Mirai stations (Figure 4). As this southeastern
boundary has become more distinct over time, the one north
of the DYNAMO array has become less so as it moves further
south. Lightning was present along both of these
boundaries; in the northern boundary, lightning was mostly
present the northern edge of the enhanced convection,
where the convective line borders the dry air (Figure 5). The lightning frequency
in these boundary regions decreased as the convection
weakened over the course of the day. At most levels, moisture continues to stream with the easterlies from the Maritime Continent into the Bay of Bengal (Figure 5, Figure 6). The 200-500 hPa zonal jet across India remains and has moved south over the past couple of days. The winter monsoon circulation at 500 hPa is very weak and mostly limited to the Arabian Sea, with a limited amount of northerly flow over Gujurat and some southerly flow over the Arabian Peninsula. A subtle rotation continues over Gan at that level as well that extends down to low levels. Low-level westerlies south of the Equator continue to weaken, with the more intense winds even farther to the east then yesterday. The question remains as to what set off convection across the basin during the last few days. At Gan, conditions were moist up to 550 hPa with relatively weak winds and shear, but above 550 hPa conditions were dry with weak winds with a westerly component up to 300 hPa, and a more northerly component above that (Figure 7). Varying degrees of cloud debris were visible throughout the day, especially to the west where most of the active convection came from (Figure 8). However, similar to what has occurred over the past few days, most of the convection did not reach significant depths (Figure 9), although upper-level blowoff from the convective region to our NW was often visible overhead. The convective lines on S-PolKa today seemed to be the result of outflow pulses from the large nearby convective region seen to our NW in satellite imagery (Figure 2); these outflows (evident in the visible satellite imagery in Figure 10) were mesoscale in along-line extent, long-lived and traveled significant distances. These pulses occurred throughout the day, creating relatively long lines of precipitating convective cells that traversed the radar domain (Figure 11). The convective lines, in turn, produced their own outflow boundaries, which were visible in the differential reflectivity field (Figure 12). However, as can be especially seen in the linear differential reflectivity, second-trip echo contamination was a significant problem through much of the day, which sometimes obscured the signatures of these relatively thin convective lines. Convection along the lines was mostly relatively shallow (Figure 13), but the longevity and depth of the convective cells seemed to be helped by mergers of the leading cells with ones behind (Figure 14). When the cells interacted, there was a brief burst in ice cloud production, since it was during this time the cell gained enough depth to reach a greater distance above the melting level. However, the overall intensity in terms of rain hydrometeor production and high reflectivity decreased over this time. Convection also seemed to intensify during the mergers of differing outflow boundaries (Figure 15). Before the merger, the height of the most intense outflow to the SE of S-PolKa was relatively shallow. The intensity and the ice production of the cells in this line, especially at the merger point between it and the other two lines to the north and south, increased as the three lines folded into one. The cells in the middle of this merger produced streamers with a melting signature and gradually settling aggregates. The collapsed convection to the north, in contrast, had shallow echo that barely reached above the melting level (not shown). In general, the common heights of both the convective- and stratiform-classified echoes have been shallow over the past few days, but particularly today (Figure 16). Both types of precipitation reached their most frequent height around 6 km over the past two days, whereas on 7 December the distribution of heights was more flat up to ~8 km. Today the least frequent convective and stratiform echoes were also shorter in height than those seen over the other two days. The relative frequency of graupel decreased over the three days, as well as for heavy rain (Figure 17). |
| Figure 1. CPC 200 hPa velocity potential
overlaid infrared satellite imagery for 8 December. |
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| Figure 2. Infrared satellite imagery for
0000-1800 UTC 9 December. |
| Figure 3. Australian Bureau of Meteorology
RMM MJO phase diagram for 7 December. |
| Figure 4. CIMSS MIMIC total precipitable
water from 0300 UTC 7 December - 0200 UTC 10 December. |
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| Figure 5. Infrared satellite imagery
overlaid with WWLN lightning data for 0700 UTC and 1800
UTC 9 December. |
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| Figure 6. IMD WRF-ARW analyses for 0000
UTC 9 December. |
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| Figure 7. IMD WRF-ARW analyses for 0000
UTC 9 December. |
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| Figure 8. Photos looking north at 0420 UTC
and 0620 UTC, and looking SW at 0920 UTC and 1120 UTC. |
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| Figure 9. ARM KAZR reflectivity and
velocity for 9 December. |
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| Figure 10. Visible satellite imagery for
0700-0900 UTC 9 December. |
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| Figure 11. S-PolKa reflectivity PPI for
0000-2100 UTC 9 December. |
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| Figure 12. S-PolKa reflectivity, velocity,
differential reflectivity (ZDR), and linear differential
reflectivity (LDR) for 0546 UTC (top) and 0631 UTC
(bottom) 9 December. |
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| Figure 13. S-PolKa reflectivity PPI (top)
and RHI (bottom) for 0700 UTC (left) and 0800 UTC (right)
9 December. |
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| Figure 14. S-PolKa reflectivity PPI and
reflectivity, velocity, and PID RHI for 0745-0816 UTC 9
December. |
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| Figure 15. S-PolKa reflectivity PPI and
reflectivity and PID RHI for 1200-1500 UTC 9 December. |
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| Figure 16. S-PolKa convective and
stratiform echo height statistics for 7-9 December. |
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| Figure 17. S-PolKa particle identification
statistics by altitude for 7-9 December. |
| Conditions remain disturbed in the
Indian Ocean as the positive velocity potential anomaly
weakens, especially over the NW portion of the basin (Figure 1). Although the negative
velocity potential anomaly remains focused over the
Maritime Continent and the western Pacific, associated
with the MJO being in phase 5 (Figure
2), the large-scale suppression of convection
following the active phase seems to be losing intensity,
and widespread convection is occurring in a line
stretching from the Bay of Bengal to Tanzania, a line west
of Sumatra, and in the remnants of small tropical
disturbance east of Madagascar (Figure
3). Moisture continues to increase between 10 N and
10 S, and the gradient of moisture in the southern Arabian
Sea and Bay of Bengal continues to sharpen as the air in
their northern parts continues to get drier (Figure 4). The remnants of the
tropical disturbance to the SW of Diego Garcia also closed
off the southern edge of a dry pocket to the west of the
array. A 700 hPa trough to the SE of the array has been
increasing in amplitude (Figure 5),
which is bringing dry air north into the central Indian
Ocean but is also taking moisture south to the west coast
of Australia. All of the boundaries between the dry and
moist air in the Arabian Sea, the Bay of Bengal, and the
line west of Sumatra remained electrified throughout the
day, although this latter line dissipated over the course
of the day (Figure 3). A broad
low-level circulation persists NW of the DYNAMO array, and
the winds on its southern side are increasing in intensity
again, resulting in an area of low-level convergence over
Gan. Gan is also directly underneath an area of 200 hPa
diffluence, since the winds directly along the equator are
easterly but begin to turn northeasterly over the array. A
broad, ill-defined 200 hPa anticyclonic circulation is
forming over southern Arabian Sea as a ridge begins to to
its north, creating further diffluence to the NW of Gan. Conditions at Diego Garcia remained dry, with weak low-level winds with a westerly component that changed over to weak easterlies at 550 hPa (Figure 6). The sky there was mostly clear with scattered shallow cumulus (Figure 7). At S-PolKa, larger cumulus and cumulonimbus clouds were visible throughout the day, as well as some higher ice clouds that were debris from collapsing convection (Figure 8). In the photos taken at 0506 UTC, the clouds to the north corresponded to the streamers of precipitation left behind by two convective cells traveling west to east across the northern radar domain (Figure 9). The leading cell built in intensity while the other collapsed. By 0630 UTC, the rear cell had collapsed completely, and the front cell was relatively deep, reaching up to 14 km (Figure 10). The rear of the streamer consisted of settling and melting aggregates. The head had a deep irregular ice signature and some aggregates closer to the melting level. The front of the cell had a particularly distinct divergence signature, while just behind that the ice crystals aloft were blowing behind in strong winds visible in the velocity RHI. While these cells propagated along, they released an outflow that initiated further convection (Figure 11). A highly positive differential reflectivity signature appeared south of these cells, which as they moved southward their outflow merged with that of another set of cells moving towards the east. The cell at the junction of the two outflows was relatively intense but not particularly deep (not shown). The velocity observation is consistent with the Gan sounding data, which show a reversal in winds from westerlies to easterlies at 4 km (600 hPa) (Figure 12). These easterlies intensified to 15 m/s by 200 hPa. The depth of the moist layer increased from 700 hPa to 550 hPa by the end of the day, and very dry air persisted above that level. At 0600 UTC, the 900-1000 hPa shear was about 5 m/s, but the shear increased at 1200 UTC to about 10 m/s. At about 1200 UTC, a tiny bow echo quickly moved directly over S-PolKa heading towards the east (Figure 13). This small line of convection contained a distinctive straightline wind signature close to the surface. By 1800 UTC, the low-level shear had decreased back to 5 m/s, but through the latter half of the day the upper-level winds got stronger between 200-300 hPa, increasing the deep-layer shear (Figure 12). Late in the evening, S-PolKa observed many more convective cells streaming precipitation behind them, but the decaying convective signatures were much more robust (Figure 14), possibly because of the greater amounts of ice lofted to higher levels and radiatively favorable nighttime conditions. Although the convective portion of this cell was not particularly intense and did not produce particularly heavy rain, ample amounts of settling aggregates followed behind that created a melting signature and fallstreaks. More horizontally-oriented particles appeared above interesting convergence signatures in the velocity field between 6-8 km. These horizontally-oriented ice crystals persisted until the trailing stratiform dissipated at about 2300 UTC. After the passage of this line of cells, the convection returned to being sparse and isolated. |
| Figure 1. CPC 200 hPa velocity potential
overlaid with infrared satellite imagery for 9 December. |
| Figure 2. Australian Bureau of Meteorology
RMM MJO index for 8 December. |
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| Figure 3. Infrared satellite imagery
overlaid with WWLN lightning data for 0700 UTC and 2100
UTC 10 December. |
| Figure 4. CIMSS MIMIC total precipitable
water for 0700 UTC 10 December. |
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| Figure 5. IMD WRF-ARW model analyses for
0000 UTC 10 December. |
| Figure 6. Diego Garcia sounding for 1200
UTC 10 December. |
| Figure 7. Visible satellite imagery for
0800 UTC 10 December. |
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| Figure 8. Photos looking north and
ESE at 0506 UTC 10 December and looking west, ESE, and
SE at 1200 UTC 10 December. |
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| Figure 9. S-PolKa reflectivity PPI (top)
and cross-sections along the red line (bottom) for
0516-0646 UTC 10 December. |
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| Figure 10. S-PolKa reflectivity PPI
(top) and reflectivity, velocity, and PID RHI for 0630 UTC
10 December. |
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| Figure 11. S-PolKa reflectivity (top) and
differential reflectivity (ZDR; bottom) for 0516-0616
UTC 10 December. |
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| Figure 12. Gan soundings for 10 December. |
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| Figure 13. S-PolKa reflectivity and
velocity PPI for 1246 UTC 10 December. |
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| Figure 14. S-PolKa reflectivity PPI and
reflectivity, velocity, and PID RHI for 1931-2216 UTC 10
December. |
| Convection still remains active
across much of the northern Indian ocean
(Figure 1). The line that has
stretched from the Bay of
Bengal to Tanzania over the last several days has broke up
into pockets of convection overnight, with the convective
area far to the NW of Gan still very electrically active.
Convection with electrical activity now spreads in a large
disorganized mass from the Bay of Bengal southeastward to
the island of Sumatra. A small area of electrically-active
convection that flared up just east of the Revelle station, and
the remnants of the tropical disturbance mentioned in
previous days, dissipated by the end of the day, and the
convective area NW of Gan moved further north into the
Arabian Sea. The amount of upper-level confluence increased on 10 December, resulting in less-favorable large-scale conditions for convection over the Indian Ocean, but the overall signal of the positive velocity potential has split into two parts (Figure 2). Generally the 200 hPa flow has become a lot less coherent (Figure 3). A short jet of easterlies still exists over the equatorial central Indian Ocean, but the main diffluence region of the easterlies is now further east. Flow over the southern Arabian Sea, the western Indian Ocean, and the area south of the DYNAMO array has light and variable winds at 200 hPa. The monsoonal circulation over the Arabian Sea has been reorganizing at 500 hPa over the past couple of days, resulting in stronger northwesterly flow over India, but the winds at this level are also light and variable over the array. Light to moderate low-level westerlies dominate just south of the equator. The low-level cyclonic circulation to our NW, associated with the region of convection mentioned over the past few summaries, continues to persist, although it is losing definition above 925 hPa. Gan and Diego Garcia remain in a regime of low-level divergence as the winds to the east accelerate away. However, moisture continues to be plentiful in the northern array (Figure 4), at least to 550 hPa (Figure 5). Above 550 hPa, the air dried out considerably over the course of the day, suggesting upper-level subsidence (Figure 5). Winds were relatively light at all levels, and they switched from westerlies to easterlies at about 600 hPa. The low altitude of the directional shear resulted in the tops of the convective clouds seen being tilted away from the convective core, although not to the drastic degree seen in previous days (Figure 6). The sky was partly cloudy for most of the day, with isolated convection generally moving towards the northwest; most of these cells did not produce significant trails of stratiform. The PPI displays in Figure 7 show that well-defined lines of non-precipitating clouds were present aligned with the low-level WSW winds and shear. In the late morning, a convective cell developed that although not very deep, had a healthy updraft signature, a divergent cloud top, and a small amount of ice production (Figure 7). This cell mostly decayed in place and its top began to be displaced towards the west because of the low-level directional shear; however, this cloud was mostly liquid. At the end of this decay, the cell with its cloud streamer was visible to the north of the radar (Figure 8); the mostly well-defined edges of the cloud are consistent with this cloud being mostly liquid water. Late in the evening, a cell passed over the radar that left a patch of melting ice in its wake (Figure 9). This low reflectivity patch of ice slowly drifted towards the southwest back towards the radars and was sampled by both the S-band and K-band of S-PolKa, as well as by KAZR when it reached Gan (Figure 10). Both KAZR and S-PolKa showed fallstreaks below the melting level. The particle identification indicated that by the time it was reaching Gan, this patch of ice consisted of a mix of horizontally-oriented particles, likely dendrites, as well as irregular ice crystals, and some melting and dry aggregates that were melting into drizzle. |
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| Figure 1. Infrared satellite data overlaid
with WWLN lightning data for 0600 UTC and 1800 UTC 11
December. |
| Figure 2. CPC 200 hPa velocity potential
anomaly with infrared satellite imagery for 10 December. |
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| Figure 3. IMD WRF-ARW model analyses for
0000 UTC 11 December at 200 hPa, 500 hPa, 850 hPa, and 925
hPa. |
| Figure 4. CIMSS MIMIC total precipitable
water at 0700 UTC 11 December. |
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| Figure 5. Gan soundings for 11 December. |
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| Figure 6. Photos taken towards the NE,
east, and SSE at 0421 UTC (top), and looking north at 0715
UTC, north at 1020 UTC, and ESE at 1220 UTC 11 December. |
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| Figure 7. S-PolKa reflectivity PPI and
reflectivity, velocity, and PID RHI at 0546-0616 11
December. |
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| Figure 8. Photos facing NW, north, and NE
at 0620 UTC 11 December. |
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| Figure 9. S-PolKa S-band (left) and
Ka-band (right) reflectivity PPI and RHI at 141.9 degrees
azimuth for 1715 UTC 11 December. |
| Figure 10. ARM KAZR reflectivity for 11
December. |
| Figure 11. S-PolKa PID RHI at 141.9
degrees azimuth for 1715 UTC 11 December. |
| The northern, eastern, and western
regions of the Indian ocean remain convectively active,
but the center of the basin has become clearer today,
especially at S-PolKa (Figure 1). The
Arabian Sea remains under an area of 200 hPa diffluence,
and a new one is now just east of the DYNAMO array,
perhaps creating better upper-level conditions for
convection (Figure 2). A 500 hPa
cyclonic gyre has set up to our SE, and an anticyclonic
gyre to our SW, resulting in weak southerly flow over the
array that diverges in the middle of the array. At low
levels, the winds remain mostly westerly over much of the
array. A cyclonic circulation remains just south of the
Bay of Bengal, but the one to our NW has become much more
poorly defined. Lightning activity remains very active in
the northern part of the basin, but much of the lightning
seen in past days in the eastern and western sides of the
basin (not including that over land) has decreased
somewhat (Figure 3). The
convective region to our east continues to persist close
to the Revelle
station. Moisture remains plentiful over the northern part of the array, and the southern portion is showing signs of moistening from previous days ( Figure 4). The moistening seen at Diego Garcia also appears in the weekly sounding series, show that after having a significant amount of dry air down to nearly 700 hPa over the past few days, higher relative humidities are now beginning to reach up to 500 hPa (Figure 5). Gan, by contrast, has remained moist below 500 hPa but has experienced increasing drying of the upper levels over the past few days. This trend continued today, with the upper levels of the Diego Garcia soundings moistening between 0000-0600 UTC today (Figure 6). Some drying was evident at 1200 UTC, however. At Gan, at 0000 UTC, there were relatively sharp humidity gradients just above 600 hPa and 700 hPa. The 600 hPa gradient, likely the boundary between the moister lower levels and the dry layer aloft, had a distinct Bragg scattering signature (Figure 7). At this level, the low level winds switch from weakly westerly below that level to weakly easterly above (Figure 6). Several shallow isothermal layers above 600 hPa suggest subsidence aloft. The day was bright, sunny, and partly cloudy, with several isolated cumulus clouds heading towards the ENE across the S-PolKa domain ( Figure 8). Even the most intense cells, such as the one in Figure 9, barely reached above 8 km. Because of their limited vertical extent, ice production was somewhat limited, with some aggregates and irregular crystals, but the cells were short-lived and there was little to no stratiform production. There were lines of non-precipitating clouds and some cold pool activity today, but the cold pools were only discernable by the Bragg scattering in the reflectivity field (Figure 10). These cold pools did occasionally result in convective cell initiation along their boundary (Figure 11), but these cells were not obviously more intense, deeper, or longer-lived than the other isolated convective cells. In another case later in the day, the collapse of a cell seemed to briefly intensify the cell downwind of it, resulting in it being one of the deepest of the day (Figure 12). This cell produced a large amount of non-melting aggregates up to nearly 12 km, but when the cell collapsed what it left behind dissipated very quickly (not shown). The tops of the deeper cells were high enough to be pushed towards the SW by the easterlies aloft (Figure 8). Statistically, the echoes of 10 and 11 December do not appear very different, with only the 10-20 dBZ echoes showing a little less frequency at higher altitudes on the 11th than the 10th ( Figure 13). Interestingly, oriented ice crystals were more common on 11 December, as was heavy and moderate rain. However, today all of the echo tops except the 30 dBZ convective echoes dropped in frequency and in height. All of the ice crystals, but especially the oriented crystals, dropped in their frequency at higher heights, and the number of oriented ice crystals dropped in frequency. On the tail of this particular peak in precipitation, the amount of contribution to the rain accumulation by stratiform decreased to below the convective contribution, which is consistent with the rain events seen during the last suppressed phase (Figure 14). |
| Figure 1. Visible satellite imagery for
0700 UTC 12 December. |
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| Figure 2. IMD WRF-ARW analyses for 0000
UTC 12 December at 200 hPa, 500 hPa, and 850 hPa. |
| Figure 3. Infrared satellite imagery
overlaid with WWLN lightning data for 0700 UTC 12
December. |
| Figure 4. CIMSS MIMIC total precipitable
water for 0600 UTC 12 December. |
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| Figure 5. Weekly sounding series for Gan
(left) and Diego Garcia (right) for 6-12 December. |
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| Figure 6. Gan and Diego Garcia soundings
for 12 December. |
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| Figure 7. S-PolKa reflectivity PPI at 11
degrees elevation and RHI at 42 degrees azimuth for 0030
UTC 12 December. |
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| Figure 8. Pictures looking ESE at 0434
UTC, 0710 UTC, 0917 UTC, and 1054 UTC 12 December. |
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| Figure 9. S-PolKa reflectivity PPI and
reflectivity, velocity, and PID RHI for 1000 UTC 12
December. |
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| Figure 10. S-PolKa reflectivity PPI for
0231-0216 UTC (top) and 2246-2331 UTC (bottom) 12
December. |
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| Figure 11. S-PolKa reflectivity PPI and
reflectivity, velocity, and PID RHI for 0446 UTC 12
December. |
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| Figure 12. S-PolKa reflectivity and PID
RHI for 2250-2342 UTC 12 December, for the same azimuth
angle as the yellow line in the lower panels of Figure 10. |
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| Figure 13. S-PolKa echo top and PID
statistics for 10-12 December. |
| Figure 14. S-PolKa daily rainfall from 5
October - 11 December. |
| The area of the positive velocity
potential continues to decrease over the Indian Ocean as
the negative velocity potential anomaly also fades away (Figure 1). Today convection remained
active across much of the northern and eastern part of the
basin. Near Gan the areas of convection to its north and
east moved closer to the S-PolKa domain
(Figure 2). The array remained in
an area of 200 hPa diffluence associated with the
strengthening of two partial anticyclonic gyres to the
NW and SE of Gan, while at 300 hPa the winds were
predominately easterly (Figure
3). Upper-level flow remains highly
zonal north of India but with more wave structure in the
southern hemisphere. At low-levels, the entire equator
region across the basin is fairly moist, with the winds
being predominately westward over the northern array. A
low-level circulation to the NE of the Revelle station is
associated with the region of enhanced convection in that
area. 925 hPa winds briefly slow down upon reaching Gan,
but speed up again near the Revelle station. Throughout the day, the layer below 700 hPa remained moist with westerly winds while the layer above 500 hPA had easterly winds and was initially dry (Figure 4). The region between these two layers had southeasterly winds that were dry for the first half of the day. The source of the dry mid-level winds appears to be a patch of dry air in the southern part of the array that was advected towards Gan (Figure 5). During the first half of the day, the sky was bright blue with little to no high clouds (Figure 6). Non-precipitating clouds were easily visible on the radar during the first half of the day oriented SW-NE (Figure 7). Most of the convection that occurred during this part of the day appeared to be initiated along outflow boundaries, although the source of the outflow boundaries was not obvious (Figure 2). These groups of cells also mostly propagated towards the NE, although the spreading of the outflow boundary altered the direction of some cells somewhat. In particular, one of these outflow boundaries traveled for four hours (evident by Bragg scattering; not shown) from the SW portion of the radar domain before finally initiating convection very close to S-PolKa (Figure 8). The lack of any leaning in the convective towers is consistent with the low-level shear being very low (Figure 9). Once fully developed (Figure 10), these cells were intense although not particularly deep, reaching only about 8 km, but with a decent amount of aggregate and graupel production (Figure 11). This line also produced an outflow boundary that was visible with Bragg scattering and had a positive differential reflectivity signature (Figure 12). The line left behind it a trail of thick anvil between 6-10 km (Figure 6, Figure 13), which seemed to temporarily humidify the 400-500 hPa layer, although this layer quickly dried out again by 1500 UTC (Figure 4). At about 1500 UTC, the surface winds began to shift from westerly the northwesterly (Figure 14), along with the direction of cell propagation. Large amounts of moisture began to move into the S-PolKa domain at 1200 UTC (Figure 15). After about 1200 UTC, collapsing convection began to leave more stratiform behind (Figure 16). Stratiform precipitation also appeared on the fringes of the radar several times during this period as well. However, the mid-levels remained very dry throughout the day (Figure 4), and these patches of stratiform were short lived. The moisture moving in from the east was associated with a moistening of the upper-levels in the soundings from 1500 UTC (Figure 4). |
| Figure 1. CPC 200 hPa velocity potential
anomaly overlaid with infrared satellite imagery for 12
December. |
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| Figure 2. Visible satellite imagery from
0330-0930 UTC 13 December. |
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| Figure 3. IMD WRF-ARW analyses for 0000
UTC 13 December at 200 hPa, 300 hPa, 700 hPa, and 925 hPa. |
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| Figure 4. Gan soundings for 13 December. |
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| Figure 5. MeteoFrance ALADIN 500 hPA
analyses and water vapor satellite imagery for 0000 UTC 13
December. |
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| Figure 6. Photos looking SE at 0400 UTC,
west at 0600 UTC, SSE at 0900 UTC, ESE at 1000 UTC, south
at 1100 UTC, and SSW 1155 UTC 13 December. |
| Figure 7. S-PolKa reflectivity PPI at 0600
UTC 13 December. |
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| Figure 8. S-PolKa reflectivity PPI from
0900-1000 UTC 13 December. |
| Figure 9. Photo looking SSW at 0936 UTC 13
December. |
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| Figure 10. Pictures looking SE, south, and
SW at 0950 UTC 13 December. |
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| Figure 11. S-PolKa reflectivity PPI and
reflectivity, velocity, and PID RHI at 1004 UTC 13
December. |
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| Figure 12. S-PolKa reflectivity and
differential reflectivity (ZDR) at 1001 UTC 13 December. |
| Figure 13. ARM KAZR reflectivity for 13
December. |
| Figure 14. ARM meteogram for 13 December. |
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| Figure 15. Water vapor imagery for
0000-2100 UTC 13 December. |
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| Figure 16. Infrared satellite imagery
overlaid with reflectivity PPI (top) and
convective/stratiform seperation (bottom) for 1500-2200
UTC 13 December. |
| Gan remained perched near the edge
of several moisture gradients and large atmospheric
circulations (Figure 1; Figure 2). At 850 hPa, there
were three distinct cyclonic circulations to the
northwest, northeast, and southeast of Gan (Figure 2, left panel).
The wind at 500 hPa was less well organized, although the
cyclonic feature off the southern tip of Sri Lanka was
still evident (Figure 2, center
panel). This feature has been slowly moving westward
over the past several days, bringing with it large amounts
of moisture and precipitation. Northeasterly winds
over Gan at 500 hPa implied a weak moisture transport into
the array, a property that seen in the increases in mid
and upper-level RH measured by the soundings on Gan (Figure 3). Winds centered on
the equator at 200 hPa were fairly zonal in the eastern
Indian ocean, but became strongly divergent over Gan and
the western side of the DYNAMO array (Figure 2, right
panel). The 200 hPa velocity
potential plots, however, indicate increasing upper-level
subsidence towards the center of the array
(Figure 4). The winds at
500 hPa also indicate weak divergent motion. The soundings from Gan continued to show a moistening trend, particularly above 600 hPa, which began on the afternoon of the 13th (Figure 3). This change is concurrent with a relaxation of the moderate southeasterly winds between 650 and 500 hPa. These winds, which had been advecting dry air from the south central portion of the DYNAMO array toward Gan, were replaced by weak westerlies drawing on more moist air. Soundings from Male varied in sync with Gan while the profile of moisture over Diego Garcia remained quite dry in comparison (not shown). Visible satellite imagery and cloud photography showed decreased amounts of low-level cumulus in mid-morning over the S-PolKa region (Figure 5, top row; Figure 6, top row). In the afternoon clouds began streaming in from the northwest, riding over the ridge feature evident at 850 hPa (Figure 5, bottom row; Figure 6, bottom row). Deeper, precipitating convection was associated with the northern convective line discussed in these summaries over the past several days. This line remained active today. Convection stretched across the entire basin in the region between 5 and 10 degrees N, gradually sloping equatorward over the eastern basin into the Maritime Continent (Figure 7). There were three regions of notable convection near the DYNAMO array: in the center of the surface cyclone to the northwest, in the circulation due south of Sri Lanka, and in an area of velocity convergence south of the southeastern corner of the array. Within range of S-Polka (Figure 8), there were lines of nonprecipitating convection organized along lines, probably parallel to the low-level winds. These lines are best seen east of the radar at 0801 UTC. Lines of precipitating cells with similar orientation were located in the far northern part of the radar area (especially at 0701 UTC in Figure 8). The northern portion of the radar domain was more active than the southern for most of the day. A little later, outflow boundaries were evidently the lifting mechanism responsible for initiating deeper precipitating convection. S-PolKa had an up-close view of this process as an outflow boundary, propagating south from the large region of convection in the north, initiated a new line of cells about 30 km north of the radar at 9 UTC (Figure 8, bottom right panel). This line went on to produce the deepest convection of the day, just over 15 km (Figure 9). This storm was of particular interest since it was sampled in its mature stage by the French Falcon aircraft (see flight tracks in Figure 9). Unlike the cell sampled by the aircraft, convective echo tops were generally below 10 km (Figure 10). Areas of stratiform precipitation occurred relatively frequently throughout the day as active convection dissolved, but only accounted for about 23% of the total rain (Figure 11). |
| Figure 1. CIMMS MIMIC TPW product at 0600
UTC with the approximate location of the DYNAMO array
overlaid. |
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| Figure 2. 0600 UTC analysis from the
Meteo France ARPEGE Model. Shown are RH and winds at
850 hPa and 500 hPa (left and center) and geopotential
height and winds at 200 hPa (right). |
| Figure 3. Time series of the vertical
profile of RH and winds over Gan. |
| Figure 4. CPC 200 hPa velocity potential
anomaly overlaid with infrared satellite imagery for 13
December. |
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| Figure 5. Hourly visible satellite images
between 4 and 9 UTC, overlaid on the range rings of
S-PolKa. |
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| Figure 6. Hourly time series photographs
of clouds moving in from the WNW. |
| Figure 7. Lightning frequency overlaid on
IR imagery at 0730 UTC on December 14th. |
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| Figure 8. Hourly S-Polka radar
reflectivity between 4 and 9 UTC, overlaid on the range
rings of S-PolKa. |
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| Figure 9. S-PolKa observations of the
deep cell sampled by the Falcon. PPI Reflectivity and
flight tracks(top left), RHI Reflectivity (top right), RHI
Velocity (bottom left), and RHI Particle ID (bottom
right). |
| Figure 10. Observed frequency of echotop
heights for varying levels of intensity in convective
precipitation (top) and stratiform precipitation
(bottom). |
| Figure 11. Polarimetrically tuned hourly
integrated rainfall observed over the S-PolKa radar domain
on 14 December 2011. |
| Today marked a significant change
in conditions as the skies clouded over at S-PolKa. At 850
hPa, the cyclonic circulation to the northwest of the
DYNAMO array weakened and moved westward since yesterday,
and the other cyclonic circulation to the northeast of the
array strengthened (Figure
1, left panel). This shift resulted in a
change of low-level wind direction over Gan, from
westerlies to north-westerlies, and the intensification of
the northeasterlies over the Bay of Bengal. At 500 hPa
over the DYNAMO array, the wind was weak
(Figure 1,
Figure 2). In the 500 hPa and
200 hPa geopotential fields,
the highest geopotential heights were centered in the
middle of Indian Ocean along the equator
(Figure 2). Weak positive
velocity potential anomaly at 200
hPa is consistent with weaker upper-level convergence
(Figure
3). The DOE Gan soundings show low-level
northwesterlies and the upper-level easterlies during most
of the day and the humidity increasing by later in the day
(Figure 4). Satellite imagery showed shallow clouds dominating in the vicinity of Gan during much of the day (Figures 5 and 6). On radar lines of convection oriented NW-SE produced cells as hight as 15 km (Figure 7). Wind shear displaced the tops of the clouds towards the southwest (Figure 7). A robust stratiform signal was observed at about 0000 UTC, with brightband reflectivity values up to 60 dBZ as well as intense fallstreaks (Figure 8, bottom right). This stratiform region originated from the collapse of one of the NW-SE oriented convective lines (upper two panels of Figure 8), but was left behind as the convective region moved faster toward the east with the lower-tropospheric southwesterly wind (bottom left panel of Figure 8). From 0500-1000 UTC, several cells passed over S-PolKa. Stratiform precipitation occurred as these cells collapsed, with brightband reflectivities reaching 40 dBZ and containing layers of dry and melting aggregates (Figure 9, Figure 10). The photos in Figure 11 show the stratiform precipitation area away from the radar to the east. The collapsing of convection into stratiform precipitation as illustrated by Figure 8 repeated through the day. This behavior was captured by the convective/stratiform separation. The upper row of Figure 12 shows the algorithm's capture of the event shown in Figure 8. The lower row of Figure 12 shows another example at a later time where a line of convection 60-100 km east and southeast of the radar collapsed into stratiform structure. Figure 13 shows that even though this behavior occurred throughout the day, and the stratiform regions were sizable, they were not long lasting, and were relatively weak, accounting ultimately for only about 23% of the rain over the area observed by the the radar. The convection in this case was not producing enough ice particles to make a widespread stratiform region. This fact is illustrated by comparing the daily polarimetric particle identification summaries for 16 October and today (Figure 14). Look at the upper panels on the two days and compare the dry snow (DS) and wet snow (WS) particle categories to see the difference between the ice producing capacities of the two days (note the two plots have different scales). |
| Convection remains active across
the central portion of the Indian Ocean, with the largest
convectively-active region over the eastern part of the
DYNAMO array (Figure 1). Lightning was
dispersed across most of the convection regions, but was
especially present in the line NE of Gan as well as around
Diego Garcia. Low-level westerlies picked up again along
the equator, flowing eastward across a pressure gradient
at 850 hPa (Figure 2). From 850 hPa down,
strong confluence was occurring directly over the northern
array between northeasterlies south of the tip of India
and the westerlies along the equator, while the flow at
200 hPa exhibited strong diffluence. The 500 hPa flow was
weak and indistinct, with no evident monsoonal
circulation, but with a relatively strong ridge and trough
south of the array. Moisture was plentiful (Figure 3), with clouds occurring
throughout the array; some of these clouds had high cloud
tops (Figure 4). At Gan, the soundings
were very moist throughout the column, with westerly flow
up to 350-400 hPa and easterlies above 250-300 hPa (Figure 5). The low-level confluence near
Gan may was co-located with the active convection seen
near Gan in Figure 1.
Between 0600-1800 UTC, the flow between 300-400
hPa switched from having a sharp transition from
westerlies to easterlies to being mostly southeasterly in
that layer. This change in the circulation was associated
with a saddle-like flow at 300 hPa between the several
anticyclonic circulations north and south of the array (Figure 6). At Gan, skies were overcast or nearly so through most of the day, with multiple layers of cloud visible (Figure 7). Around 0800 UTC the sky temporarily cleared enough to see large amounts of high cloud streaming from the east. In the KAZR data, three distinct layers of cloud were visible at ~5 km, ~7 km, and ~11 km (Figure 8). The low-level reflectivity signatures were likely associated with shallow convection, the middle reflectivity signatures with stratiform precipitation from collapsing convection, and the upper-level reflectivity signatures with deeper convection and anvil. The convective and stratiform precipitation moved primarily together towards the SE (Figure 9). Many of these stratiform regions had large amounts of settling dry aggregates as well as strong melting signatures in the polarimetric variables (Figure 10). A group of intense cells moved across the RHI sectors of S-PolKa ~0400-0600 UTC ( Figure 11). These cells were deeper than what has been seen in previous days, with echo tops up to ~12 km, strong updrafts and upper-level divergence signatures, and significant amounts of heavy rain, aggregate and graupel production. These cells collapsed into a moderately intense stratiform region that had a broad melting signature in the PID (as well as high differential reflectivity and a low correlation coefficient; not shown) and large amounts of settling dry aggregates, but had a broad and only slightly enhanced reflectivity signature along the melting level. Of the past three days, 15-16 December had deeper and more frequent convective and stratiform precipitation than the 14th ( Figure 12). The most frequent occurrences of both types of precipitation were highly similar between the two days, but the outlier echoes were somewhat deeper on 15th versus the 16th. The particle makeup of the precipitation was distributed very similarly between 15th and 16th, although the percentage of irregular ice crystals and drizzle was somewhat higher on the 15th than the 16th (Figure 13). Both of these days had many more ice and liquid hydrometeors than on the 14th. Interestingly, on the 16th the contribution of stratiform-classified rainfall to the total rainfall accumulation surpassed the convective contribution around 1000 UTC, which did not happen on the previous two days (Figure 14). Overall the stratiform rain fraction for the day was 44%, compared to 23% on each of the previous two days. |
| Figure 1. Visible satellite imagery
overlaid with WWLN lightning data for 0700 UTC 16
December. |
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| Figure 2. IMD WRF-ARW model analyses
fields for 0000 UTC 16 December at 200 hPa, 500 hPa, and
850 hPa. |
| Figure 3. CIMSS MIMIC total precipitable
water for 1200 UTC 16 December. |
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| Figure 4. Infrared satellite imagery for
0300 UTC, 1200 UTC, and 2100 UTC 16 December. |
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| Figure 5. Gan soundings for 0000-1800 UTC
16 December. |
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| Figure 6. Meteo-France ARPEGE 300 hPA
analyses for 0600 UTC, 1200 UTC, and 1800 UTC 16 December. |
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| Figure 7. Photos looking ESE at 0400 UTC,
ESE at 0800 UTC, SW at 1000 UTC, and SSW at 1118 UTC 16
December. |
| Figure 8. ARM KAZR reflectivity data for 16
December. |
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| Figure 9. S-PolKa reflectivity PPI, rain
rate, and convective/stratiform seperation for 0159 UTC,
0958 UTC, and 1757 UTC 16 December. |
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| Figure 10. S-PolKa particle identification
PPI at 3.5 degrees elevation for 0732 UTC and 1002 UTC 16
December. |
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| Figure 11. S-PolKa reflectivity PPI and
reflectivity, velocity, and PID RHI at 0446 UTC and 0631
UTC 16 December. |
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| Figure 12. S-PolKa echo top statistics for
14-16 December. |
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| Figure 13. S-PolKa RHI particle ID
statistics for 14-16 December. |
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| Figure 14. S-PolKa hourly rainfall for
14-16 December. |
| Convection remained active in much
of the central and eastern part of the Indian Ocean today,
although the southern part of the Arabian Sea has quieted
down considerably (Figure 1). Electrical
activity was mostly concentrated in the lines NE of Gan as
well as the line that extends through the center of the
DYNAMO array and across Diego Garcia. The array remains
under a zone of 200 hPa diffluence, with the low
geopotential heights associated with a trough to the far
south extending into the northern hemisphere
(Figure 2). Upper-level conditions
continue to be more strongly consistent with broad-scale
upward over the past several days as the 200 hPa
velocity potential anomaly is now negative over the
entire northern Indian Ocean (Figure
3). A weakly-cyclonic 500 hPa gyre was positioned
over the southern DYNAMO array, with a zone
of confluence just west of Gan between southerlies south
of the equator and westerlies along the equator (Figure 2). North of the equator, the
low-level confluence continued today between intensified
low-level westerlies and northeasterlies stretching from
Gan to Sri Lanka. Most of the air between 10 N and 10 S
remained moist overall, although a dry tongue was visible
at 800 hPa oriented NW-SE over Gan that was co-located
with the 500 hPa confluence region
(Figure 3,
Figure 4). This dry tongue was visible over Gan for most of the day, with very little precipitation through the daylight hours (Figure 5), and a zone of low relative humidities in the soundings between 700-925 hPa (Figure 6). The moisture has been building up over the course of the past week. The upper and lower levels were dry while the mid-levels remained moist, with deep-layer winds with a westerly component (Figure 7). The low-level northwesterlies were as high as 10 m/s between the surface and 800 hPa. Large areas of altocumulus were visible during the morning and early evening hours, although the sky was only partly cloudy with mostly shallow cumulus during the middle of the day (Figure 8). These nonprecipitating clouds were visible as lines of weak reflectivity at the 1.5 degree elevation of S-PolKa, as well as the vertically-pointing KAZR, for much of the day (Figure 9, Figure 10). These lines appeared to be oriented along the low-level shear vector or along outflow boundaries. The situation was very different during the day at Diego Garcia, which saw active convective lines just to its north for most of the daylight hours that dissipated during the evening (Figure 5). Much of the day was very moist there up to 250 hPa, although dry air began to appear in the upper-levels of the soundings later in the evening (Figure 6). Later in the evening, a cell that was part of a long NW-SE convective line deposited heavy rain over Gan (Figure 11). This line had trailing collapsed convection and anvil streaming behind it, although this stratiform was once again weak and short-lived (Figure 11). This convection was not remarkably deep (Figure 11) and seemed to produce more irregular and horizontally-oriented crystals than aggregates and graupel. Overall, echo tops were modest for the day, and the tendency to produce irregular ice crystals was consistent for most of the day (Figure 12). Most of the rainfall was convective, with very little stratiform component. After the passage of the line at ~1400 UTC, convection became more isolated, although it still appeared to form somewhat along lines of shear or outflow boundaries, and the sounding below 400 hPa was considerably moister (Figure 7, Figure 9). At 2100 UTC, convection that left behind somewhat more robust regions of stratiform began to enter the radar domain, which will be discussed in tomorrow's summary. |
| Figure 1. Infrared satellite imagery
overlaid with WWLN lightning data for 0600 UTC 17
December. |
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| Figure 2. IMD WRF-ARW model analyses at
0000 UTC 17 December for 200 hPa, 500 hPa, 700 hPa and 850
hPa. |
| Figure 3. CPC 200 hPa velocity potential
overlaid infrared satellite imagery for 16 December. |
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| Figure 4. CIMSS MIMIC total precipitable
water at 0600 UTC (left), and Meteo-France ARPEGE 850 hPa
wind and relative humidity model analyses for 0000 UTC 17
December. |
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| Figure 5. S-PolKa reflectivity overlaid
visible satellite imagery (top) and infrared satellite
imagery (bottom) from 0300-2100 UTC 17 December. |
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| Figure 6. Diego Garcia (left) and Gan
(right) weekly sounding series from 11-17 December. |
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| Figure 7. Gan soundings for 0300-2100 UTC
17 December. |
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| Figure 8. Photos looking east at 0401 UTC,
east 0600 UTC, SE at 0900 UTC, and south at 1100 UTC 17
December. |
| Figure 9. ARM KAZR reflectivity for 17
December. |
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| Figure 10. S-PolKa reflectivity PPI for
0501 UTC, 08312 UTC, and 2131 UTC 17 December. |
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| Figure 11. S-PolKa reflectivity PPI and
reflectivity, velocity, and PID RHI for 1400-1543 UTC 17
December. |
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| Figure 12. S-PolKa echo top, particle
type, and hourly rainfall statistics for 17 December. |
| Conditions over the Indian Ocean
continue to grow more favorable for convection as the
negative velocity potential, proportional to upper-level
divergence, continues to intensify over the center and
eastern part of the basin (Figure 1). The
DYNAMO array continues to be under a region of 200 hPa
diffluence, although geopotential heights are beginning to
increase again in the western Indian Ocean (Figure
2). There is a discontinuity in the 200 hPa flow in
that area where the winds turn sharply from southeasterly
to northerly within a very short distance. The 500 hPa
flow continues to be moist and relatively weak, although
the ARPEGE and IMD WRF-ARW model analyses do not agree
about the strength and the direction of the flow over and
around the DYNAMO array (Figure 2, Figure 3); in general, however, the flow
is weak and disorganized between 10 N and 10 S. They have
much better agreement at 850 hPa, where the winds over the
array are strongly westerly. The 850 hPa flow into Gan
continues to be in a confluent region between cyclonic
gyres lying south of the Arabian Sea and the Bay of
Bengal, with a tongue of dry air protruding southward from
the southern tip of India. Today marked the return of the Revelle to its station, where it took soundings into a very moist column containing winds with a strong northerly component up to 800 hPa, a strong westerly component up to 550 hPa, and then a strong easterly component above 500 hPa (Figure 4). Dryer portions of the column were below 700 hPa and around 400 hPa. This moisture and wind pattern generally repeats itself at Gan. At Diego Garcia the moisture in the column is similar, although while the low-level winds were westerly earlier in the day (not shown), the winds are strongly southerly by 1200 UTC. At 0600 UTC the Revelle observed widespread convection with cold cloud tops and intense reflectivities, which dissipated over the course of the day (Figure 5). At about the same time, Diego Garcia also had convection with cold cloud tops very nearby. The convection at Gan, however, continues to have relatively low cloud tops that has some organization into lines oriented NW-SE. As mentioned in yesterday's summary, the convection overnight showed a tendency to have more robust stratiform regions (Figure 6, Figure 7). These stratiform regions, like the ones yesterday, had a tendency to get separated from the convection that generated them and dissipate in place. Most of the contribution to the total rainfall by the stratiform precipitation was during the morning hours (Figure 8). The convective and stratiform precipitation shown in Figure 6 released an outflow that was both highly reflective and had a high differential reflectivity signal (Figure 7). These constantly merging outflow boundaries seemed to be the main characteristic of the organization of today's convective initiation. As these lines passed overhead, the skies alternated between being sunny with scattered high clouds to lines of cumulus and cumulonimbus (Figure 9). Distinctive outflow boundaries set up to the northeast of a convectively-active region SW of the radar (Figure 10). This convective region was oriented NW-SE, in the same direction as the low-level wind shear at the time. Pulses of the outflow boundaries merged and stretched into a line, along which a line of convective cells eventually formed as the original convective region dissipated. Outflow boundaries released from cells to the NW and SE of the radar merged with a second line of convection that formed northeast of the first. Eventually the two lines merged with each other. One of the intense cells resulting from the merger had 45 dBZ echo reaching to 6 km and weak echo tops reaching about 12 km at its peak. The irregular ice crystals it produced were displaced towards the northwest (top panels, Figure 11). Through the rest of the day, more convective cells continued to initiate along outflow boundaries, some of which left behind patches of dissipating melting ice (middle panels, Figure 11). Throughout the day, the stratiform precipitation contribution to the rainfall only amounted to 18% (Figure 8). The stronger cells had echo tops that were deep enough to be caught in the upper-level easterlies (lower panels, Figure 11). In general, the frequency of taller stratiform-classified echo top heights has increased substantially since yesterday, which is consistent with the increased stratiform rainfall contribution (Figure 12). The convective echo heights increased in frequency overall, but only the less frequent 20 and 30 dBZ echoes increased in depth. |
| Figure 1. CPC 200 hPa velocity potential
overlaid infrared satellite imagery for 17 December. |
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| Figure 2. IMD WRF-ARW analyses for 0000
UTC 18 December at 200 hPa, 500 hPa, and 850 hPa. |
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| Figure 3. Meteo-France ARPEGE analyses for
0000 UTC 18 December at 500 hPa and 850 hPa. |
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| Figure 4. Gan, Revelle, and Diego
Garcia soundings for 1200 UTC 18 December. |
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| Figure 5. S-PolKa and Revelle reflectivity
PPI overlaid infrared satellite imagery for 0000-1800 UTC
18 December. |
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| Figure 6. S-PolKa reflectivity overlaid
infrared satellite imagery and the convective/stratiform
separation for 2216-2346 UTC 17 December. |
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| Figure 7. S-PolKa reflectivity (top) and
differential reflectivity (bottom) PPI for 2216-2346 UTC
18 December. |
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| Figure 8. S-PolKa rainfall statistics for
18 December. |
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| Figure 9. Photos looking SSE at 0500 UTC,
SE at 0800 UTC, and SSE at 1100 UTC 18 December. |
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| Figure 10. S-PolKa reflectivity PP from
0431-0801 UTC 18 December. |
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| Figure 11. Reflectivity PPI and
reflectivity, velocity, and PID RHI for 0700 UTC, 1200
UTC, and 1616 UTC 18 December. |
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| Figure 12. S-PolKa echo top statistics for
18 December. |
| Today marked some of the deepest
convection we have seen at S-PolKa in a few weeks, as well
as the first electrification seen at S-PolKa since October
(Figure 1; see the 24
October summary for the last instance). Conditions
remain favorable for convection over the Indian Ocean as a
negative velocity potential anomaly continues to build
over the center and eastern part of the basin, and ample
moisture continues to cover most of the region (Figure 2). The 200 hPa flow continued to
split from easterlies along the equator to southerlies
over India, but flow over the array itself was weak and
not strongly diffluent as it has been over the past few
days (Figure 3). Gan is underneath a
region of confluence from 500 hPa downward. At 500 hPa,
the confluence is from the flow around two cyclonic gyres
to the north and west of the array. Closer to the surface,
the confluence is between intense westerlies along the
equator and the northerlies coming around a cyclonic
region centered SE of Sri Lanka. The most intense portion
of these easterlies has moved further east than in
previous days, and a dry tongue at 850 hPa continues to
penetrate further south towards the array
(Figure 2,
Figure 3). Skies were overcast for much of the day (Figure 4), although the skies did clear somewhat during a dry spell between ~1200-1800 UTC (Figure 5). The day began with large amounts of deep, intense convection that left behind large areas of stratiform precipitation (Figure 6). As in the previous two days, the convection mostly traveled towards the SE, although this convection only minimally organized itself into lines. Winds were primarily out of the NW up to 400-500 hPa and easterly above that (Figure 7). The layer below 900 hPa was somewhat unstable beginning at 1800 UTC 18 December, but by 0000 UTC that layer become dry adiabatic. The capping stable layer seen around 500 hPa at 0000 UTC had disappeared by 0600 UTC, although by that point the layer below 700 hPa began to stabilize. As mentioned earlier, these storms were the first observed to contain lightning since October. Some indication of particle separation conducive to charge separation appeared in some of the convective cells, with heavy rain, graupel, aggregates, horizontally-oriented pristine crystals, and irregular ice crystals all stacked above each other (Figure 8). Some questions has been raised as to the more common presence of high upper-level differential reflectivity signatures overall, as well as the associated oriented ice crystal classification in the particle identification. Some of these convective cells reached up to 16 km, which is the deepest convection seen in a few weeks (Figure 9). These particularly deep cells had very intense updraft and upper-level divergence signatures and large amounts of aggregate and graupel production within the convective cores. As these cells collapsed, they left behind very robust stratiform regions, which had distinctive brightbands and fallstreaks, mid-level convergence signatures, and occasionally graupel signatures at the top and bottom of the melting ice layer (upper row of Figure 10). As the stratiform region in Figure 10 began to dissipate, the aggregates settled and melted, and the mid-level convergence signature weakened. This stratiform region, which began to collect NE of the radar around 0600 UTC, took 6 hours to dissipate completely. After the dissolution of the stratiform region around 1200 UTC, the column stabilized briefly (Figure 7). Over the next 4 hours, the radar was relatively quiet, with the occasional scattered convection arising out of lines of cloud lines along the wind or at the edges of cold pools (Figure 11). However, between 1300-1500 UTC the soundings began to show signs of destabilizing below 900 hPa, and by 2100 UTC the stable layers at 900 hPa, 800 hPa, and 500 hPa cooled and the sounding was once again highly unstable (Figure 7). This time the sounding was drier than it was at 0000 UTC. Beginning around 1900 UTC a wide band of isolated convective cells moved over the radar. Although some large areas of well-formed stratiform precipitation occurred during the first half of the day, The strong convective elements dominated the total rainfall for the day (Figure 12). The stratiform rain fraction for the day was about 20%. Compared with 18 December, convective-classified echoes were more frequently deep and intense today (Figure 13). The higher frequency of 20 dBZ echo around the melting level in the stratiform-classified echo indicates the stronger tendency for brightband signatures on this day. The particle identification statistics show the more frequent occurrence of oriented ice crystals and graupel on 19 December, especially at higher altitudes and near the melting level (Figure 14). The distribution of particles between the two days, however, seem rather similar, although with a greater occurrence on 19 December. |
| Figure 1. Infrared satellite imagery
overlaid with WWLN lightning data for 0600 UTC 19
December. |
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| Figure 2. CPC 200 hPa velocity
potential overlaid infrared satellite imagery for 18
December, and the Meteo-France ARPEGE analyses for 850 hPa
for 0000 UTC 19 December. |
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| Figure 3. IMD WRF-ARW analyses for 0000
UTC 19 December at 200 hPa, 500 hPa, 850 hPa, and 925 hPa. |
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| Figure 4. Photos looking ESE at 0500 UTC,
SSE at 0700 UTC, WSW at 0900 UTC, and SW at 1200 UTC 19
December. |
| Figure 5. ARM KAZR reflectivity data for
19 December. |
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| Figure 6. S-PolKa reflectivity PPI
overlaid infrared and satellite imagery for 0000-0900 UTC
19 December. |
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| Figure 7. Gan soundings for 19 December. |
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| Figure 8. S-PolKa reflectivity PPI and
reflectivity, differential reflectivity, and PID RHI for
0200 UTC 19 December. |
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| Figure 9. S-PolKa reflectivity PPI and
reflectivity, velocity, and PID RHI for 0246 UTC 19
December. |
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| Figure 10. S-PolKa reflectivity PPI and
reflectivity, velocity, and PID RHI for 0716 UTC and 0816
UTC 19 December. |
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| Figure 11. S-PolKa reflectivity PPI for
1316-2246 UTC 19 December. |
| Figure 12. S-PolKa hourly rainfall
statistics for 19 December. |
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| Figure 13. S-PolKa echo top
statistics for 18-19 December. |
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| Figure 14. S-PolKa particle identification by height and daily RHI summary statistics for 18-19 December. |
| The enhanced convective activity
continues in the central and eastern Indian Ocean as part
of a large-scale area of convective activity that has been
moving westward since about 8 December (Figure
1). This feature has since stalled and has not moved
zonally since 16 December. Easterlies at 200 hPa remain
strong around the equator, although just west of Gan the
winds slow down somewhat, suggesting a localized region of
upper-level convergence west of the DYNAMO array
(Figure 2). The 500 hPa winds
along the equator have become more steadily westerly from 70 E
eastward, with a region of confluence to the NE of Gan
between two cyclonic gyres to the west and north of the
array. A dry slot is working its way into the array south
of Gan. At 850 hPa, the westerlies have once again become
very intense across a W-E geopotential height gradient.
The 850 hPa tongue of dry air mentioned in previous
summaries seems to have moistened somewhat, and now the
main region of confluence has shifted eastward more
towards the Revelle. The day started with most of the convective activity in the southern part of the DYNAMO array (note that there is no longer a ship at point SE), with less convective activity near Gan and the Revelle (Figure 3). However, by the end of the day the convective activity in the northern part of the array picked up considerably while the convection to the south lost intensity. At Diego Garcia, the day began with very moist air up to 500 hPa and dry air aloft and easterlies down to 800 hPa, but by 1800 UTC an extremely dry air mass had moved into the lower levels and the wind field became much less coherent (Figure 4). Although the change was much less drastic, the Revelle began the day with dryer air from 500 hPa downward that moistened over the course of the day, coinciding with a deepening of the westerlies from 600 hPa to 350 hPa. At Gan, the skies were full of cumulus, cumulonimbus and their debris as differing periods of convection moved through the area (Figure 5). At 450 hPa the winds began to shift from the lower-level westerlies to the upper-level easterlies; the air above 450 hPa remained relatively dry throughout the day (Figure 6). At the beginning of the day, the air was relatively dry below that level, but this layer moistened considerably by the end of the day. The column alternated between being highly unstable to relatively stable in the lowest levels, likely corresponding to the 4 periods of convective activity during the day. Strongly stable and dry layers suggestive of subsidence were present between 400 and 300 hPa at 0600 and 1200 UTC. This first period of convection occurred near 0000 UTC as a series of convective cells streamed across the radar domain, depositing streamers of collapsing convection in their wake (top panels, Figure 7). This convection was intense but not very deep, which limited the amount of ice produced. At 0000 UTC there was about 15 m/s of shear between the surface and 800 hPa, but above this level the winds were mostly ~10 m/s (Figure 6). The transitional layer from westerlies to easterlies was from 450 hPa to ~300 hPa. The column showed signs of moistening at ~650 hPa and 900 hPa after this initial wave of activity, and stabilized briefly after this period; during this time, lines of nonprecipitating clouds were visible in the low-level reflectivity (Figure 8)--some along the wind, others at cold pool boundaries. These nonprecipitating clouds blossomed into a stream of isolated convective cells that did not leave behind streamers of rain behind at about 0900 UTC (bottom panels, Figure 7); these more isolated cells were also relatively shallow but intense enough to produce graupel and aggregates. This period of 0600-1200 UTC was when the Gan soundings showed drying and stable layers above the 400 hPa level (Figure 6). The third stage of convective activity was distinguished by stratiform beginning to mix in with the convective cells around 1200 UTC (upper panels, Figure 9). The low-level shear had now shifted to being about 5 m/s, the winds had also picked up to 15 m/s up to 550 hPa, and the dry stable layers above the 400 hPa level were gone (Figure 6). At this point the cells were mostly embedded within the stratiform and still remained relatively shallow. The regions had large amounts of aggregates but relatively little amounts of irregular ice; most of the action appeared to occur below 10 km. The final stage was when these large regions of convection transitioned to solid line oriented approximately parallel to the lower tropospheric wind around 2000 UTC (lower panels, Figure 9). Different from lines earlier in the day, which moved with an eastward component, this line moved toward the southwest across the low-level northwesterly wind, with a stratiform region trailing behind to its northeast; it moved partly by discrete propagation with new cells forming along a fine line cold pool boundary lying parallel to but southwest of the active convective line. The low-level shear became about 10 m/s between the surface and 900 hPa by 1800 UTC; mid-level winds remained between 10-15 m/s (Figure 6). The transitional layer from westerlies to easterlies was much narrower, being only from 450 hPa to 350 hPa. During this period, the convective regions were comparatively broad, and the stratiform region had ample aggregates and a strong melting signature, but the brightbands were poorly resolved because they were far from the radar (lower panels, Figure 9). The convective line shown in the lower panels of Figure 9 persisted long after the stratiform region behind it dissipated (Figure 10). This line continued to produce a highly reflective outflow, which at about 2300 UTC was visible in both the PPI and RHI scans, and which had a velocity signature indicating its flow towards the radar. As noted above, new intense convective cells initiated along this outflow boundary then joined the propagating convective line, giving the line its component of discrete propagation. The stratiform contribution to the rainfall was 26%, which is greater than it has been in previous days but is nonetheless a relatively low contribution overall (Figure 11). The short but intense bursts of mostly convective precipitation are apparent in the ARM precipitation data, which showed mostly stepwise increases in precipitation accumulation. The restricted heights of the convective echoes shows up in the convective echo top statistics, which indicate very modest heights of the 20 and 30 dBZ echoes, but little change in the most intense and weakest echoes since yesterday (Figure 12). The stratiform echoes had a more modest decrease in height from yesterday. The relative amount of the particle ID statistics remained the same since yesterday, but the amount of pixels that that contained hydrometeors overall increased, likely because of the overall increase in echo coverage and precipitation on this day. |
| Figure 1. ECMWF Hovmoller for 19
October-18 December, plus 10 day forecast to 28 December. |
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| Figure 2. IMD WRF-ARW analyses at 200 hPa,
500 hPa, and 850 hPa, and the Meteo-France AREPEGE
analyses for 500 hPa and 850 hPa for 0000 UTC 20 December. |
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| Figure 3. Infrared satellite imagery for
0000 UTC and 1800 UTC 20 December. |
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| Figure 4. Diego Garcia and Revelle soundings for
0000 UTC and 1800 UTC 20 December. |
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| Figure 5. Photos looking ESE at 0400 UTC,
SE at 0700 UTC, north at 1000 UTC, and SE at 1140 UTC for
20 December. |
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| Figure 6. Gan soundings for 0000-1800 UTC
20 December. |
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| Figure 7. S-PolKa reflectivity PPI and
reflectivity, velocity, and PID RHI for 0146 UTC and 0946
UTC 20 December. |
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| Figure 8. S-PolKa reflectivity PPI for
0531-0916 UTC 20 December. |
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| Figure 9. S-Polka reflectivity PPI and
reflectivity, velocity, and PID RHI for 1401 UTC and 2016
UTC 20 December. |
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| Figure 10. S-PolKa reflectivity and
velocity PPI, and reflectivity and velocity RHI for
2316-2331 UTC 20 December. |
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| Figure 11. S-PolKa hourly rainfall estimate
(left) and ARM precipitation estimate (right) for 20
December. |
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| Figure 12. S-PolKa echo top (left) and RHI
PID summary (right) for 19-20 December. |
| Large areas of convection continue
to cover the central and eastern Indian Ocean today,
although interestingly this convection is much less
electrically active than in previous days
(Figure 1). Much of the action is
in the very moist northern and central portions of the
DYNAMO array, while Diego Garcia continues to be in a
dry region (Figure 2). The upper
level winds near Gan are still weakly easterly, but the
200 hPa winds near the Revelle have switched around to being
southeasterly (Figure 3).
A cyclonic gyre continues to strengthen at 500 hPa just
south of the southern tip of India, and strong confluence
near Gan continues between this cyclone's circulation and
the westerlies along the equator. The strong low-level
westerlies continued today, along with the geopotential
height gradient in the central Indian Ocean. The 850 hPa
flow into the westerly jet over the northern array is
becoming more coherent, with easterlies along 5-10 N and
10-15 S curving around at about 70 E and converging into
strong westerlies directly over the northern array. The Revelle saw the bulk of the most intense convection while S-PolKa observed its outer edges (Figure 4). Between the two radars, a large convective complex developed over the course of the day, with very cold cloud tops and broad cloud shields. As a result, both radars registered a mix of intense convective and broad stratiform regions. The column above Gan was very moist between 750-900 hPa and above 500 hPa, with some drier layers in between the moister layers (Figure 5). Low-level shear was low at the beginning of the day, but increased to being ~15 m/s between the surface and 800 hPa by 0600 UTC and maintained that strength most of the day. The winds with strong westerly components went up to 400 hPa, and the layer through which the winds switched from westerly to easterly narrowed throughout the day. The Revelle, in contrast, had drier air between 450-800 hPa and above 400 hPa at the beginning of the day. The column moistened between 0600-1200 UTC, but by 1800 UTC the upper- and lower layers dried out again. The winds, which began the day mostly northerly below 800 hPa, westerly between 400-800 hPa, and easterly above that level, were mostly weak and variable throughout the day. These winds returned to being more strongly westerly near the surface and easterly aloft by 1800 UTC. At Gan, the sky was mostly flat grey and overcast as thick mid- and upper-level clouds covered the sky most of the day (Figure 6). The day was marked by three main periods of convective activity represented by the panels in Figure 7: a period of groups of convective cells mixed with stratiform precipitation, a period of intense convection lines, and a period of broad stratiform precipitation that covered most of the radar domain. Another round of more isolated convection appeared very late in the day as well. As has been the case over the past several days, the convective and stratiform regions mostly move with each other. In the first period, the convective cells were sometimes more isolated and sometimes mixed with and/or embedded in stratiform precipitation (top panels, Figure 8). Some of these cells reached over 14 km, with deep updrafts, ample aggregate and melting ice signatures, and strong melting signatures. In the second period, convection grew into an intense line of deep convection around 0700 UTC (bottom panels, Figure 8), which had a very distinctive low-level convergence and upper-level divergence signature. This larger line was oriented NW-SE, in the same direction as the environmental flow, and remained mostly stationary as convection propagated along its length. This line collapsed into stratiform by 0900 UTC, with many settling and melting aggregates as well as several graupel signatures below the melting level (upper panels, Figure 9). As the stratiform settled and dissipated, the velocity field became much more coherent, with a strong mid-level convergence signature (lower panels, Figure 9). This stratiform region took until 1500 UTC to fully dissipate, during which there was a brief spurt of shallow nonprecipitating clouds that erupted and organized into a line of very shallow convection in the SE portion of the radar domain (Figure 10). Convective cells likely had difficulty breaking through the low-level stable layer that established itself from 0600-1800 UTC (Figure 5). This stable layer was gone by 2100 UTC (not shown), and deep convection broke out again that continued through the night (Figure 7). These events will be discussed in tomorrow's summary. Because of the long-term presence of the broad stratiform regions, this is the first day in many that the stratiform rain contribution surpassed that of the convective contribution for about 4 hours (Figure 11). The overall stratiform contribution for the day, however, was 30%. The distribution of convective and stratiform echo tops is very similar to yesterday's. The distribution of hydrometeors is similar to yesterday's as well, except that the irregular ice contribution was double that of yesterday, likely because the amount of area that stratiform echo covered the radar domain was greater today (Figure 12). This tendency towards large amounts of irregular ice also likely is the reason for the increase in drizzle seen today. This day also had a greater tendency to be contaminated by second-trip echo, likely because of the intense precipitation between Gan and the Revelle; caution must therefore be used in interpreting the low-elevation scans. |
| Figure 1. Infrared satellite imagery
overlaid with WWLN lightning data for 0400 UTC 21
December. |
| Figure 2. CIMSS MIMIC total precipitable
water for 1800 UTC 21 December. |
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| Figure 3. IMD WRF-ARW analyses for 0000
UTC 21 December at 200 hPa, 500 hPa, and 850 hPa. |
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| Figure 4. Infrared satellite imagery
overlaid with the S-PolKa, Revelle, and SMART-R reflectivity
PPI for 0000-1800 UTC 21 December. |
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| Figure 5. Gan and Revelle soundings for 0000-1800 UTC 21
December. |
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| Figure 6. Photo looking ENE at 0600 UTC,
and ARM KAZR reflectivity for 21 December. |
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| Figure 7. S-PolKa reflectivity PPI and
convective/stratiform seperation for 0400 UTC, 1000 UTC,
and 2100 UTC 21 December. |
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| Figure 8. S-PolKa reflectivity PPI and
reflectivity, velocity, and PID RHI for 0246 UTC and 0716
UTC 21 December. |
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| Figure 9. S-PolKa reflectivity PPI and
reflectivity, velocity, and PID RHI for 0931 UTC and 1101
UTC 21 December. The upper left panel is reflectivity PPI
overlaid visible satellite imagery. |
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| Figure 10. S-PolKa reflectivity PPI from
1001-1116 UTC 21 December. |
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| Figure 11. S-PolKa reflectivity echo top
and hourly rainfall statistics for 21 December. |
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| Figure 12. S-PolKa PID by height and total
summary statistics for 21 December. |
| Large-scale upper-level divergence
continues over the Indian Ocean as a negative velocity
potential anomaly at 200 hPa has strengthened and expanded
to cover the entire basin (Figure 1).
Active convection remains mostly concentrated over the
central and eastern portions of the Indian Ocean, although
most of the lightning activity is limited to the portion
just south of the Bay of Bengal (Figure 2).
Diffluence has returned to being directly over the
northern part of the triangular array, associated with the
winds turning away from the equator but also with a sudden
increase in wind speed between 75-80 E
(Figure 3). Easterlies at the 500
hPa level continue to intensify, and the mid-level
vorticity is increasing to the north of the array where
a cyclonic gyre continues to intensify, possibly
partially in response to top heavy heating connected to
the mesoscale convection in that general area
(Figure 2). At 850 hPa, two very
distinctive cyclonic gyres continue to organize and
align with each other to the north and south of the
equator. The 850 hPa winds between the gyres continue to
intensify as they flow across the geopotential height
gradient, and strong confluence continues in the
northern portion of the array. Moisture is plentiful
across the northern part of the array in the low-levels,
but at 500 hPa dry air is wrapping around the western
side of the cyclonic gyre to the north, introducing dry
air into the region near Gan (Figure
4). The moist layer was very deep at the Revelle, but very
dry mid- and low-level air impinged upon Diego Garcia
(Figure 4,
Figure 5). Strong convection
continued overnight in the cyclonic gyre
to the north of Gan and the Revelle, while the skies were very quiet
over Diego Garcia (Figure
6). The Revelle saw a mixture
of isolated convective and stratiform
precipitation. Figure 7 is
more detailed version of the Revelle radar data 0200 UTC, a time
between the first two panels of Figure
6. It shows a squall line located east of the ship
and moving northeastward with a stratiform region
positioned to the west of the ship. Over the next 12 hours
the stratiform region expanded as the whole system moved
off to the NE; the system can be seen superimposed on the
satellite infrared image at 0600 and 1200 UTC in the upper
right and lower left panels of Figure
6. Gan remained on the fringes of the main region of high cloudiness, but received much of the high cloud blowing towards the west, with a variety of lower clouds below (Figure 8). The cloud cover alternated between being partly broken and overcast as a series of convective lines passed over S-PolKa, some of which trailed stratiform precipitation and thick anvil in their wakes (Figure 9). The day began with lines of convection that were oriented NW-SE along the northwesterly surface winds (Figure 10, Figure 11). As the day progressed, there was a greater tendency for the convection to merge into lines perpendicular to the surface winds (Figure 10); eventually, these convective lines became squall lines that moved with the surface winds at ~14 m/s (estimated from the radar time-lapse images). More isolated convection dominated between episodes of convective lines (Figure 12). In the left hand panel it is especially clear that the isolated precipitating cells were arising out of nonprecipitating cloud lines parallel to the northwesterly low-level wind. Winds with strong westerly components extended up to ~400 hPa at the beginning of the day (Figure 11). The westerly layer deepened to above 300 hPa by 2100 UTC, and the dry stable layers between 700-800 hPa completely moistened. Low-level shear was 10-15 m/s throughout the day, and the mid-level westerlies were 15-20 m/s. These stronger mid-level westerlies extended up to ~400 hPa by 2100 UTC. The first squall line merged into a coherent line at 0700 UTC, darkening the skies and bringing strong enough winds to kick up the coral sand by the radar (Figure 13). This squall line had a sharp discontinuity in the velocity fields and had winds measured up to ~22 m/s (Figure 14). At first, the convection was mostly below 8 km, with very limited aggregate and graupel production. As the line developed, some of the convection grew up to 14 km as the leading cells overtook convective cells ahead of the line. As the convective cells grew, the stratiform regions became more extensive but were relatively inhomogeneous (not shown). The second squall line organized at about 1800 UTC (Figure 15). This line was even less organized than the first and had weaker winds overall, but was much longer in length. The leading convection in this line reached depths of up to 12 km and produced ample amounts of aggregates, graupel, and irregular ice hydrometeors. The stratiform regions had a strong melting signature with fall streaks, a descending rear inflow, settling aggregates, and graupel signatures mixed in with the melting snow layer. Echo tops were relatively moderate today, with the higher frequencies of the 20-30 dBZ convective echoes being between 7-8 km (Figure 16). 30 dBZ echoes extending above 10 km remained relatively rare. The heights of the 20 dBZ stratiform echoes were even more modest, with echoes very infrequently getting above 10 km. Although the general distribution of hydrometeors still has not changed much from the past several days, the amount of ice hydrometeors has once again dropped to relatively low levels. This drop is consistent with the generally low stratiform precipitation contribution to the total rainfall (24%; not shown), which has consistently occurred throughout this latest peak in precipitation (Figure 17). |
| Figure 1. CPC 200 hPa velocity potential
overlaid infrared satellite imagery for 21 December. |
| Figure 2. Infrared satellite imagery
overlaid with the WWLN lightning data for 0700 UTC 22
December. |
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| Figure 3. IMD WRF-ARW model analyses for
0000 UTC 22 December at 200 hPa, 500 hPa, and 850 hPa. |
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| Figure 4. Meteo-France ARPEGE analyses at
500 hPa and 850 hPa, and the CIMMS MIMIC total
precipitable water for 0000 UTC 22 December. |
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| Figure 5. Diego Garcia and Revelle soundings for
1800 UTC 22 December. |
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| Figure 6. S-PolKa and Revelle reflectivity
PPI overlaid the infrared satellite imagery for 0000-1800
UTC 22 December. |
| Figure 7. Revelle PPI for 0159 UTC 22 December. |
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| Figure 8. Photos looking ESE at 0400 UTC,
0700 UTC, and 1000 UTC. |
| Figure 9. ARM KAZR reflectivity data for
22 December. |
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| Figure 10. S-PolKa reflectivity overlaid
infrared satellite imagery (top) and convective/stratiform
separation (bottom) for 0200-1900 UTC 22 December. |
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| Figure 11. Gan soundings for 0300-2100 UTC
22 December. |
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| Figure 12. S-PolKa reflectivity PPI for
0300-2100 UTC 22 December. |
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| Figure 13. Photos looking NW, east, and
south at 0630 UTC 22 December. |
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| Figure 14. S-PolKa reflectivity and
velocity PPI (top) and reflectivity, velocity, and PID RHI
(bottom) for 0700 UTC 22 December. |
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| Figure 15. S-PolKa reflectivity and
velocity PPI (top) and reflectivity, velocity, and PID RHI
(bottom) for 2000 UTC 22 December. |
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| Figure 16. S-PolKa echo top and PID RHI
summary statistics for 22 December. |
| Figure 17. S-PolKa daily rainfall
accumulation from 5 October - 22 December. |
| Gan is in the middle of a band of
clouds feeding into the large convective system far to our
east, which has been building in intensity over the past
few days (Figure 1). These convective
cells traveled with the intense westerly winds that extend
from at least 500 hPa downward (Figure 2).
The 200 hPa winds over the DYNAMO array were mostly weak,
but the area just north of the array remains in a zone of
strong diffluence. The two low-level cyclonic gyres
mentioned yesterday continued to have the strong westerly
jet between them. The northern gyre also has intense
easterlies on its northern side. This northern gyre has
become an area of interest for the Joint Typhoon Warning
Center, who have given the disturbance a medium chance of
developing into a tropical cyclone. The 500 hPa
geopotential heights are now showing a depression just
east of the array, while the low-level geopotential height
gradient mentioned in past days now extends from 700 hPa
downward. At middle latitudes in the northern hemisphere,
a strong trough is moving across the nothern Arabian Sea,
with mid-to-upper-level southwesterlies across the
Himalayas. Low-level moisture is plentiful and has a plume-like signature pointing extending east-west across the Indian Ocean, likely as a result of the strong westerly winds (Figure 3). At 500 hPa, regions of dry air are beginning to infiltrate near Gan and Diego Garcia, but the Revelle remains in plentiful moisture. At Gan, the deep layer of westerlies extended up to 300 hPa at 0000 UTC, but by the end of the day these westerlies were up to nearly 200 hPa (Figure 4). Air was dryer in the mid-levels, especially at 0600 UTC, but the sounding was mostly very moist throughout the day. The low-level shear magnitude was ~10 m/s between the surface and 900 hPa. Westerlies of 15 m/s were observed up to 400 hPa. The day began with scattered isolated convection around S-PolKa (Figure 5), with a highly variable sky of cumulus and cumulonimbus, debris from various convective showers, altocumulus, and cirrus blowing off the tops of deeper cells (Figure 6). At 0000 UTC the sounding had stable layers near 600 hPa and 900 hPa (Figure 4). The low-level shear was evident in the forward tilting of some of the shallower convective clouds. A few lightning strikes were observed in the convection near Gan, likely in a convective cell that reached ~14 km and had graupel, aggregate, and irregular ice production (Figure 7). By 0600 UTC these more isolated convective cells began to form multicellular groups, and the amount of ice and anvil production greatly increased as the convective regions broadened (Figure 8). At this time, the sounding was significantly more unstable, with a disappearance of the stable layers as well as intense daytime heating, but the layer around 500 hPa was very dry (Figure 4). The deep layer of westerlies is evident in the radial velocity RHI in Figure 8. The rest of the day was full of squall lines both large and small (last four panels, Figure 5). Four distinctive squall lines passed directly over Gan, all with thick trailing anvils but only the last two had significant stratiform precipitation regions (Figure 9). The first two squall lines that went by the radar near 1100 UTC and 1600 UTC were more poorly organized and did not produce much trailing stratiform (Figure 10). The convection was relatively deep and capable of transferring momentum from the mid-levels down to the surface. However, because of the somewhat broken nature of the lines, a coherent velocity discontinuity was not clear except in the very center of the reflectivity arc. The radial velocity RHI in the lower right panel of Figure 10 shows strong outbound velocity at the base of the convective cell and a downward sloping rear inflow jet in the small trailing stratiform region. Between 1200-1500 UTC, the sounding was somewhat drier at mid-levels but moist overall, and remained so for the rest of the day (Figure 4). Beginning at about 1800 UTC the layer between 400-700 hPa cooled somewhat while the lowest levels remained warm, increasing the available convective energy in the sounding. The third convective line, which did not have the distinctive bow appearance or a contiguous convective line, crossed over the radar at about 1800 UTC (Figure 5). This line did have leading convective cells, copious amounts of trailing stratiform from ample aggregate and irregular ice production, and a descending rear inflow jet of powerful winds of near 20 m/s (Figure 11, upper panels). The final and most coherent squall line passed over Gan near 2200 UTC, and had an undulating but contiguous line of convective cells (Figure 11, lower panels). This line also had large areas of stratiform precipitation trailing behind it, with large amounts of aggregates and irregular ice crystals. Both lines had graupel in their convective cores as well as graupel signatures along the melting level in the stratiform regions. The first squall line near 1100 UTC dropped the temperature 3 deg C and increased the wind speeds by 5 m/s (Figure 12). The latter two had the most significant rainfall, with rain rates over 80 mm/hr measured by tipping bucket. The last squall line briefly increased the wind speeds by 7 m/s and had a rain rate over 100 mm/hr. Interestingly, the largest accumulated rainfall measured by S-PolKa occurred near 1700 UTC (Figure 13), when the third squall line was approaching the radar. Most of the stratiform contribution came late in the day with the second and third squall lines, making the stratiform precipitation 28% of the day's total. |
| Figure 1. Infrared satellite image overlaid
with WWLN lightning data for 0700 UTC 23 December. |
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| Figure 2. IMD WRF-ARW analyses at 0000 UTC
23 December for 200 hPa, 500 hPa, 700 hPa, and 850 hPa. |
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| Figure 3. Meteo-France ARPEGE analyses at
0000 UTC 23 December for 500 hPa, and the CIMSS MIMIC
total precipitable water for 0700 UTC 23 December. |
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| Figure 4. Gan soundings for 23 December. |
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| Figure 5. S-PolKa reflectivity PPI for
0300-2100 UTC 23 December. |
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| Figure 6. Photos looking east at 0200 UTC,
ESE at 0402 UTC, NE at 0536 UTC, ESE at 0701 UTC (top);
ESE at 0805 UTC, SW at 1005 UTC, north at 1045 UTC, and
SW at 1105 UTC 23 December. |
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| Figure 7. S-PolKa reflectivity PPI and
infrared satellite data overlaid with WWLN lightning data
(top) at 0246 UTC; S-PolKa reflectivity, velocity, and PID
RHI at 0245 UTC 23 December. |
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| Figure 8. S-PolKa reflectivity PPI and
reflectivity, velocity, and PID RHI for 0846 UTC 23 December. |
| Figure 9. ARM KAZR reflectivity for 23
December. |
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| Figure 10. S-PolKa reflectivity and velocity PPI,
and reflectivity and velocity RHI for 1116 UTC and 1616
UTC 23 December. |
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| Figure 11. S-PolKa reflectivity PPI, and
reflectivity, velocity, and PID RHI for 1901 UTC and 2201
UTC 23 December. |
| Figure 12. ARM Gan meteogram for 23
December. |
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| Figure 13. S-PolKa hourly rainfall
statistics and ARM Gan accumulated rainfall for 23
December. |
| The upper levels continue to be
favorable to upward motion as the negative velocity
potential anomaly over the central Indian Ocean continues
to strengthen (Figure 1). Most of the
convection is concentrated in the eastern Indian Ocean (Figure 2), associated with two cyclonic
gyres north and south of the equator (Figure
3). The northern gyre still is considered to have a
medium chance of developing into a tropical cyclone by the
Joint Typhoon Warning Center. A band of convection
persists along the equator and across the entirety of the
basin (Figure 2), embedded within the
intense westerlies seen from 500 hPa downward (Figure 3). Upper-level winds were still
weak and variable at 200 hPa over the northern part of the
triangular array, while the southern portion of the array
has moderate easterly winds. Low-level moisture continues
to be plentiful across the array (Figure 4).
The southern sector still has dry mid-level air impinging
upon it while the northern array remains moist. The band of convection along the equator brought variable skies to the northern array, with occasional bands of convection moving through both the S-PolKa and Revelle radar domains as they travel eastward, while the skies around Diego Garcia remained mostly clear (Figure 5). The While the soundings at Gan and the Revelle were mostly relatively moist with deep westerlies during the day, the Diego Garcia soundings were very dry from 650 hPa upward and had winds predominantly with strong easterly components (Figure 6). Low-level shear was about 10 m/s in magnitude at both Gan and the Revelle, which gained an increasingly directional component during the day. In the morning, the remnant stratiform of yesterday's convective activity filled the southeastern sky (first panel, Figure 7; Figure 8). This morning stratiform reached up to 12 km, and even several hours after its generation continued to have large amounts of settling aggregates and a strong melting signature, although the brightband was poorly resolved because of the distance from the radar (Figure 9, upper panels). Much of the middle part of the day was marked by lines of shallow cumulus clouds that began to precipitate at 0900 UTC (Figure 7, Figure 10). Convection remained rather shallow through the day until larger convective lines moved in around 1600 UTC (Figure 11). By this point, the winds below 1.5 km were mostly northwesterly while the winds above continued to be westerly, and the layer of 15 m/s winds had dropped down to 600 hPa (Figure 6). These deeper cells thus had a distinctive tilt as their lower portions outpaced their upper portions (Figure 11, upper panels). The stratiform regions that resulted from this burst of convection was less intense and homogeneous than the stratiform in the morning, but still had a distinctive melting signature (Figure 9, lower panels). This stratiform also had a large amount of oriented pristine crystals in its anvil. The day ended with isolated shallow convective cells that appeared to be oriented and stretched along the low-level shear (Figure 11, lower panels). These cells were too shallow to produce much ice, but were efficient in producing large amounts of heavy rain. The statistics also bear out how shallow most of the convection was, with the 30 dBZ echoes never reaching above 10 km (Figure 12). There is an interesting split between the heights of the stronger and weaker echo of the convective cells for the less frequent convective echoes, but the more frequent echos had a very modest height. One large change from previous days is that the stratiform precipitation made up 50% of the precipitation for the day, most of which was contributed during the early morning hours. The microphysics were different from the previous day in two ways: first, that the graupel on 23 December reached substantial heights of 12 km, which did not occur on 24 December; the second, that oriented ice crystals made up a larger percentage of the upper-level ice hydrometeors on 24 December (Figure 13). |
| Figure 1. CPC 200 hPa velocity potential
anomaly overlaid infrared satellite imagery for 23
December. |
| Figure 2. Infrared satellite imagery
overlaid with the WWLN lightning data for 0700 UTC 24
December. |
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| Figure 3. IMD WRF-ARW analyses at 0000 UTC
24 December. |
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| Figure 4. Meteo-France ARPEGE analysis at 0000 UTC
24 December, and CIMSS MIMIC total precipitable water for
0000 UTC 24 December. |
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| Figure 5. Visible satellite imagery at 0400
UTC and 1130 UTC, and infrared satellite imagery at 1700
UTC 24 December, overlaid with S-PolKa reflectivity PPI
data. Second row shows Revelle
PPIs similar times. |
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| Figure 6. Gan soundings at 0600 UTC and
1800 UTC, Revelle
soundings for 0900 UTC and 1800 UTC, and Diego Garcia
soundings for 0600 UTC and 1800 UTC 24 December. |
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| Figure 7, Photos looking SE at 0600 UTC,
north at 0800 UTC and ESE at 1000 UTC. |
| Figure 8. ARM KAZR reflectivity data for 24
December. |
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| Figure 9. S-PolKa reflectivity PPI and
reflectivity, velocity, and PID RHI for 0400 UTC and 1900
UTC 24 December. |
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| Figure 10. S-PolKa reflectivity PPI from
0831-0916 UTC 24 December. |
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| Figure 11. S-PolKa reflectivity PPI and
reflectivity, velocity, and PID RHI for 1646 UTC and 2331
UTC 24 December. |
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| Figure 12. S-PolKa echo top and hourly
rainfall statistics for 24 December. |
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| Figure 13. S-PolKa particle ID by altitude
statistics for 23-24 December. |
| Despite continued upper-level
support for enhanced convection over the Indian Ocean (Figure 1), conditions quieted down
considerably at Gan as the band of convection stretching
across the basin moved to the south (Figure 2).
The tropical disturbance in the Bay of Bengal was upgraded
to Tropical Cyclone 06B, and is forecast to reach near
hurricane strength before making landfall near Chennai,
India. The DYNAMO triangle array is in the saddle region
between the two anticyclonic gyres to the SW and SE and
the diffluent flow north of the equator (Figure
3). The mid-level monsoon circulation appears to be
reorganizing, with a ridge over the Arabian Peninsula and
Cyclone 06B embedded in the monsoon trough. The region of
mid-level moisture has become increasingly constricted as
the associated dry northerly flow from over India impinges
on Gan and the Revelle.
Winds with strong westerly components remain intense from
500 hPa downward along the equator, although closer to the
surface the winds have become more northwesterly. Low-level moisture remains plentiful across the northern portion of the triangle array, but dryer air continues to impact the air over Diego Garcia (Figure 4). At Gan, the influx of dry air from the north is most visible between 300-500 hPa, with northwesterly winds up to 700 hPa and between 400-500 hPa, westerlies between 500-700 hPa, and southerlies above 250 hPa. Winds were very weak at the surface but increased to 10 m/s by 900 hPa. At the Revelle conditions were dry from 800 hPa upward (Figure 5), and the winds were westerly up to 250 hPa. At Diego Garcia, the air was extraordinarily dry above 700 hPa with weak and variable winds. By the end of the day, conditions at Gan and the Revelle continued to dry out, but the mid-levels moistened considerably at Diego Garcia. The skies were quiet and hazy over Gan today, with a some high cirrus and cirrostratus, occasional altocumulus, and a few shallow cumulus clouds (Figure 6). The ARM KAZR data captured the early morning precipitation received at Gan that transitioned to a mixture of high and mid-level cloud (Figure 7). The early morning hours marked the departure of the large convective line as it moved southward, with convective cells that occasionally reached 12-14 km and broken regions of collapsed convection (Figure 8). These convective cells did not seem to have the robust ice production of the convection of the past few days, but the collapsed cells nonetheless had a signature of melting ice particles. After the morning convection dissipated, the radar showed mostly nonprecipitating and lightly-precipitating clouds that were oriented in lines from NW to SE (Figure 9). One notable precipitation seen today was a convective line that developed ~1000 UTC and had deep convection up to 14 km (Figure 10). This convection had a strong upper-level divergence signature and ample aggregate and ice crystal production, as well as some graupel. However, these convective cells dissipated without having any sustained remnant stratiform precipitation. After this breakout of convection, the skies returned to nonprecipitating lines of cumulus (Figure 9). By 1800 UTC, stable layers set up along with three distinctive dry layers (Figure 5). The sharp humidity gradients at 2 and 3 km show up distinctly in the Bragg scatter rings (wreaths?) close to the radar (Figure 11). |
| Figure 1. CPC 200 hPa velocity potential
anomaly overlaid infrared imagery for 24 December. |
| Figure 2. Visible satellite imagery
overlaid with WWLN lightning data for 0700 UTC 25
December. |
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 |
| Figure 3. Meteo-France ARPEGE 0000 UTC
analyses at 200 hPa, 500 hPa, and 850 hPa for 25
December. *Note that there is no instrumentation in
the SE corner of the depicted array at this time. |
| Figure 4. CIMSS MIMIC total precipitable
water for 0600 UTC 25 December. |
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|
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 |
| Figure 5. Gan and Diego Garcia for 0000 UTC
and 1800 UTC, and Revelle
soundings for 0600 UTC and 1800 UTC 25 December. |
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| Figure 6. Photos looking east at 0600
UTC, NE at 0712 UTC, south at 1006 UTC, and NE at 1200
UTC 25 December. |
| Figure 7. ARM KAZR reflectivity data for
25 December. |
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 |
| Figure 8. S-PolKa reflectivity PPI and
reflectivity, velocity, and PID RHI for 0201 UTC 25
December. |
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| Figure 9. S-PolKa reflectivity PPI for 0546
UTC, 1601 UTC, and 2046 UTC 25 December. |
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| Figure 10. S-PolKa reflectivity PPI and
reflectivity, velocity, and PID RHI for 1016 UTC 25
December. |
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| Figure 11. S-PolKa reflectivity PPI and
RHI for 1750 UTC 25 December. |
| The skies over Gan and the Revelle cleared
substantially today as the large convective band across
the Indian Ocean continued to move further south (Figure 1). This band of convection
coincides with the band of intense low-level westerlies
funneling between two cyclonic gyres: the first that is
remaining mostly stationary in the Bay of Bengal, the
other further south off the coast of Sumatra
(Figure 2). The cyclone in the
Bay of Bengal has been named
Tropical Cyclone Thane (06B), and is forecast to slowly
move towards the northwest as a minimal hurricane strength
storm within the next few days. Low-level winds over Gan
and the Revelle have
weakened somewhat and turned northwesterly as the jet of
intense mid- and low-level westerlies moves further south.
The 200 hPa winds remain strongly easterly in the northern
portion of the triangle array, but are variable and weak
near Diego Garcia. The dryness of the air within the northern portion of the triangle array is apparent in the decrease in total precipitable water over Gan and the Revelle, while Diego Garcia is now fairly moist (Figure 3). At Diego Garcia, the layer between 600-950 hPa and above 350 hPa is dryer than the layer between 350-550 hPa (Figure 4). At Gan, three distinct dry and stable layers exist between 625-925 hPa, 500-625 hPa, and from 300-500 hPa. At the Revelle, the dry and stable layers were from 500-925 hPa and from 300-500 hPa. Winds at all three places had strong westerly components below 500 hPa, were light and variable between 300-500 hPa, and had strong easterly components above 300 hPa. At Gan, the winds between 300-500 hPa had a stronger northerly component, while at the Revelle the winds had a stronger westerly component. Coincident with the larger amount of moisture, Diego Garcia had convection move through the area throughout the day (Figure 1). In contrast, the dryer conditions at the Revelle were reflected in the relative lack of convection nearby; the Revelle only saw a few bands isolated convective oriented E-W across the southern domain of the radar (e.g., Figure 5). The skies were clear, very still and hazy at Gan today, with some shallow cumulus and high cirrus and cirrostratus (Figure 6). The haze was likely resulting from the burning of garbage at the nearby dump. ARM's KAZR observed the very high clouds that were visible during the middle part of the day (Figure 7), but most of the convective clouds were apparently too shallow to be observed by KAZR though they were mapped by S-PolKa. The example in Figure 8, for 0530 UTC, shows the cloud population consisting of lines of shallow cumulus and high cirrus clouds. The weak reflectivities near 12 km were likely from the cirrus aloft. The sharp humidity gradients at 1 km, just below 2 km, and at 4 km, were visible in several Bragg scattering rings in the S-PolKa reflectivity data. The only notable precipitation of the day was when some of the shallow cumulus clouds grew into slightly deeper cumulonimbus clouds, with heights below 5 km and all warm hydrometeors (Figure 9). The string of precipitation events that occurred over the past couple of weeks have been notable because of the general lack of contribution from stratiform precipitation (Figure 10). Most of the echo top heights have been modest, with the mode of the weakest echoes being mostly below 10 km (Figure 11). Even in the outlier distribution, the weaker echo tops have not strayed much above 15 km. |
| Figure 1. Visible satellite imagery
overlaid with the WWLN lightning data for 0700 UTC 26
December. |
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 |
| Figure 2. IMD WRF-ARW analyses at 0000 UTC
26 December for 200 hPa, 500 hPa, and 850 hPa. |
| Figure 3. CIMSS MIMIC total precipitable
water for 0700 UTC 26 December. |
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 |
| Figure 4. Gan, Revelle, and Diego Garcia soundings at
0600 UTC 26 December. |
| Figure 5. Revelle radar PPI for 1700 UTC 26
December. |
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| Figure 6. Photos looking north at 0402 UTC, north at
0500 UTC, NE at 0900 UTC, and SE at 1100 UTC 26 December. |
| Figure 7. ARM KAZR reflectivity data for 26
December. |
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| Figure 8. Infrared satellite imagery,
S-PolKa reflectivity PPI at 0.5 and 11 degree elevation,
and S-PolKa reflectivity RHI for 0530 UTC 26 December. |
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 |
| Figure 9. S-PolKa reflectivity PPI with the
reflectivity and PID RHI for 1045 UTC 26 December. |
| Figure 10. Daily rainfall accumulation
from 5 October to 26 December. |
| Figure 11. Convective echo top statistics
from 18-26 December. |
| Upper-level support for convection
is shrinking in area and concentrating more in the eastern
part of the Indian Ocean (Figure 1).
Enhanced 200 hPa divergence remains over Cyclone Thane as
it slowly moves westward towards the eastern Indian coast
(Figure 2), as well as over the southern
cyclonic gyre that has persisted over the past several
days (Figure 3). The low-level westerly
jet mentioned over previous days has moved even further
south and is beginning to weaken in its center portion.
The 200-500 hPa flow over India has become highly zonal,
and the 500 hPa anticyclone over the Arabian Sea continues
to pull dry air over India and towards the DYNAMO array (Figure 4). Dry air at both 500 hPa and 850
hPa has fully infiltrated the northern portion of the
triangle array, while Diego Garcia remains in moister air
with some convective activity (Figure 2).
This infiltration of dry air has been occurring in the
upper levels above Gan and the Revelle since Christmas as both
stations remain in a deep layer of winds with strong
westerly components up to 200 hPa (Figure 5).
Diego Garcia's period of higher relative humidities began
about the same time, coincident with a switch to deeper
westerly winds. The skies over Gan were bright, sunny, with some scattered shallow cumulus clouds. Conditions were less hazy than the last two days (Figure 6). The S-PolKa radar and visible satellite imagery showed that most of the lines of cumulus clouds were organized along lines stretching WNW-ESE (Figure 7). These clouds were part of a larger line of shallow clouds stretching from Cyclone Thane near Sri Lanka to the African coast (Figure 2). In the early morning hours a deeper line of cumulus clouds passed through the area (Figure 8). No rainfall was recorded by S-PolKa on today. The dry layer between 700-900 hPa had a stable lapse rate at its base, and several of the sharp humidity gradients within and around the dry layers in the sounding co-located with Bragg scattering layers were evident in the S-PolKa reflectivity data (Figure 9). Later in the evening, the nonprecipitating became more numerous, with the clouds traveling along their lines towards the SE (Figure 10). |
| Figure 1. CPC 200 hPa velocity potential
overlaid infrared satellite imagery for 26 December. |
| Figure 2. Infrared satellite imagery
overlaid with WWLN lightning data for 0700 UTC 27
December. |
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| Figure 3. IMD WRF-ARW analyses at 0000 UTC
27 December for 200 hPa, 500 hPa, and 850 hPa. |
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| Figure 4. CIMSS MIMIC
total precipitable water for 0000 UTC, and Meteo-France
ARPEGE analyses at 500 hPa and 850 hPa for 1200 UTC 27
December. |
|
|
|
| Figure 5. Gan, Revelle, and Diego Garcia weekly
sounding series for 21-28 December. |
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| Figure 6. Photos looking north at 0500 UTC,
ESE at 0700 UTC, south at 1000 UTC, and ESE at 1200 UTC. |
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| Figure 7. S-PolKa reflectivity PPI overlaid
visible satellite imagery for 0246 UTC and 0746 UTC 27
December. |
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| Figure 8. S-PolKa reflectivity PPI and RHI
for 0100 UTC 27 December. |
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| Figure 9. Gan sounding at 0835 UTC, and
S-PolKa reflectivity PPI and RHI for 0850 UTC 27 December. |
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| Figure 10. S-PolKa reflectivity and
velocity PPI for 1746 UTC 27 December. |
| Conditions remain quiet at Gan and
the Revelle as
they continue to be in a region of dry air between the
inflow into Tropical Cyclone Thane and a large convective
line in the middle of the DYNAMO triangle array (Figure 1). Cyclone Thane continues to
intensify over the Bay of Bengal as it moves very slowly
westward, and is expected to continue intensifying before
it makes landfall over Chennai, India. Meanwhile, Tropical
Cyclone 4S, about 2100 km southeast of S-PolKa, also
continues to intensify slowly and is forecast to gain
hurricane strength as it slowly moves towards the west.
The negative velocity potential anomaly continues to move
further eastward (Figure 2), with large
amounts of deep convection over the Maritime Continent.
This tendency appears in the RMM MJO phase diagram (Figure 3). Over the past several days, the
signal wandered back out of the center circle and began to
show eastern propagation into the eastern Maritime
Continent. An unanswered question is whether this
technically is a new MJO or a continuation of the one that
initiated in the Indian Ocean in November. Today's DYNAMO
briefing forecaster expressed the opinion that it is the
former rather than the latter; however, we are reserving
judgement. The triangle array remains in the region of 200 hPa easterlies; however, the flow becomes more variable directly to the array's west and northwest (Figure 4). The 500 hPa flow over the array turns southerly between the anticyclonic gyre to our west and the cyclonic flow associated with Cyclone 4S. This southerly flow meets the northerly flow coming around the western side of Cyclone Thane. The monsoon anticyclone over the Arabian Sea continues to drive dry midlevel air south of the equator, creating a significant moisture gradient across the western Indian Ocean (Figure 5). The strong low-level westerlies have returned to being over the array, with Gan and the Revelle on the jet's dry northern edge, and Diego Garcia on its far southern edge (Figure 4). Conditions over the northern portion of the triangle array remain dry at 850 hPa, whereas Diego Garcia still has plentiful moisture (Figure 5). The temperature and moisture profiles at Gan and the Revelle were similar, with extremely dry layers above 500 hPa and between 700-900 hPa, and a less dry layer between 550-700 hPa (Figure 6). At Gan, the winds below 700 hPa were westerly with moderate low-level shear, but the winds above 700 hPa had strong southerly components up to 250 hPa. The Revelle had deep westerlies up to 300 hPa with somewhat less low-level shear that at Gan, and easterlies above 300 hPa. The cloud cover above Gan was scattered to broken and the visibility was reduced by haze all day. Persistent lines of cumulus clouds were oriented along the low-level shear (Figure 7). The Bragg scattering layers were more fuzzy and indistinct compared to yesterday, possibly because of the less sharp moisture gradients. Both the S-PolKa and Revelle radars registered only very weak echo the entire day. Diego Garcia had a change in conditions with the passing of the large convective band stretching from Cyclone 4S to Madagascar (Figure 1). When the line was north of Diego Garcia, the winds were mostly southwesterly up to 300 hPa (Figure 8). The profile was relatively moist compared to the northern stations, but not saturated. By 1200 UTC, the upper levels became drier as the line to the north intensified, perhaps suggesting compensating subsidence, while the low-level conditions remained the same. The layer of upper-level southeasterlies also began to lower down to 400 hPa. By 1800 UTC, this northern line had dissipated, but another line to the south strengthened. The column was more moist by this point, and the upper-level winds became more fully southerly down to 550 hPa. This line eventually passed directly over Diego Garcia as it began to move north. |
| Figure 1. Infrared satellite imagery
overlaid with the WWLN data for 0300 UTC 28 December. |
| Figure 2. CPC 200 hPa velocity potential
anomaly overlaid infrared satellite imagery for 27
December. |
| Figure 3. CPC RMM MJO Phase diagram from 18
November to 27 December. |
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| Figure 4. IMD WRF-ARW analyses for 0000 UTC
28 December at 200 hPa, 500 hPa, and 850 hPa. |
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| Figure 5. CIMSS MIMIC total precipitable
water, and the Meteo-France ARPEGE analyses for 500 hPa
and 850 hPa, at 0000 UTC 28 December. |
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 |
| Figure 6. Gan and Revelle soundings for
0600 UTC 28 December. |
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 |
| Figure 7. Photo looking ESE at 0700 UTC,
and S-PolKa reflectivity PPI for 0016 UTC, 1016 UTC, and
2016 UTC 28 December. |
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| Figure 8. Infrared satellite data overlaid
with S-PolKa and Revelle
radar PPI (top), and Diego Garcia soundings (bottom) for
0000 UTC, 1200 UTC, and 1800 UTC 28 December. |
| Although conditions continued to
be dry at Gan and the Revelle, convective activity increased
in the central Indian Ocean between two tropical
cyclones and the large band of convection that arcs
through the center of the DYNAMO triangle array
(Figure 1). Tropical
Cyclone Thane made landfall over Chennai, India today as a
Category 1 storm. Tropical Cyclone 4S was upgraded to
Tropical Storm Benilde as it continued to strengthen and
organize. The negative velocity potential anomaly is now
completely focused over the eastern Indian Ocean and
Maritime Continent, while the positive velocity potential
anomaly over Africa has begun to filter into the western
Indian Ocean (Figure 2). The easterly 200
hPa flow splits just east of the Revelle as well as Diego Garcia,
resulting in a zone of diffluence just east of the array
and southerly winds over Gan (Figure 3).
The southwesterly flow over the array at 500 hPa continues
between the anticyclonic gyre to the west of Gan and the
cyclonic circulation of Tropical Storm Benilde. Mid- and
low-level westerly winds continue to be intense in the
zone of confluence between the circulations of the two
cyclones. Both Gan and the Revelle continue to be situated wtihin the dry mid- and low-level flow around the large convective line to the south, while Diego Garcia lies within the high moisture region, but near the sharp moisture gradient (Figure 4). At 0000 UTC, this convective line was directly over Diego Garcia, and the sounding was very moist through the entire column with southwesterlies up to 500 hPa and southerlies above that level (Figure 5). As this line moved further north, the column began to dry out, and the low-level winds briefly became southerly before switching back to southwesterly. The convective line sent a pulse of upper-level cloud towards Gan and the Revelle as the convection collapsed, but the convective line itself reformed in the same location. Beginning around 1200 UTC, the layer between 400-500 hPa began to dry out substantially at Diego Garcia, while the low levels remained moist. At Gan and the Revelle, distinct dry layers persisted between 650-850 hPa and 350-550 hPa (Figure 6). The air above 350 hPa at the Revelle was dry, while the upper-levels moistened somewhat at Gan. Both locations were in predominately westerly flow up to just above 400 hPa, and had winds with strong easterly components above that level. The low-levels were capped by a highly stable layer around 800 hPa. At Gan, the skies were sunny but hazy, with shallow cumulus and high cirrus cloud (Figure 7). The ARM KAZR radar sampled thin high clouds in the early part of the day (Figure 8). This upper-level cloud thickened substantially between 1200-1800 UTC. This thicker upper-level cloud was associated with the pulse of clouds from the collapse of the larger southern convective band (Figure 9). Otherwise, most of the clouds were shallow but distinct lines of cumulus clouds. The lines on radar could sometimes be seen to be portions of very long cloud lines ~1000 km in lenght (e.g. the northernmost line on radar in the left panel of Figure 9). Although the larger bands were aligned with the low-level wind, the convection within the bands began to orient itself at an angle to the low-level flow around 1600 UTC (Figure 10). These convective lines began to precipitate soon after, with shallow but very efficient rainfall. Precipitation was infrequent, and the 10 dBZ echo tops did not exceed 7 km the entire day (Figure 11). |
| Figure 1. Visible satellite imagery
overlaid with WWLN lightning data for 0700 UTC 29
December. |
| Figure 2. CPC 200 hPa velocity potential
anomaly overlaid infrared satellite imagery for 28
December. |
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| Figure 3. IMD WRF-ARW analyses at 0000 UTC
29 December for 200 hPa, 500 hPa, 700 hPa, and 850 hPa. |
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| Figure 4. CIMSS MIMIC total precipitable
water for 0400 UTC, and Meteo-France ARPEGE analyses at
0000 UTC 29 December for 500 hPa and 850 hPa. |
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 |
| Figure 5. Infrared satellite imagery
overlaid with S-PolKa and Revelle reflectivity PPI (top row) and
Diego Garcia soundings (bottom row) for 0000-1800 UTC 29
December. |
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| Figure 6. Gan and Revelle soundings for 1200 UTC 29
December. |
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| Figure 7. Photos looking ESE at 0400 UTC,
ESE at 0700 UTC, and north at 1000 UTC 29 December. |
| Figure 8. ARM KAZR reflectivity data for
29 December. |
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| Figure 9. S-PolKa reflectivity PPI overlaid
visible satellite imagery at 0300 UTC and 0900 UTC, and
infrared satellite imagery at 1500 UTC and 2300 UTC. |
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| Figure 10. S-PolKa reflectivity PPI and
reflectivity and velocity RHI for 1646 UTC 29 December. |
| Figure 11. S-PolKa echo top height
statistics for 29 December. |
| The large band of convection
mentioned in previous days continues to be bowed northward
towards Gan as Tropical Cyclone Benilde moves towards the
southwest (Figure 1). The storm organized
into a Category 1 storm by 1800 UTC 30 December, and is
forecast to its southwestern heading away from Diego
Garcia. The positive velocity potential anomaly continues
to cover the western Indian Ocean, but the negative
anomaly is not propagating eastward away from the Maritime
Continent (Figure 2). Instead, the region
of enhanced upper-level divergence is becoming smaller in
zonal extent. The array is in the exit region of the jet
of 200 hPa easterlies, with the flow north of the array
bending into southerlies over India, and the flow south of
the array becoming the northerly branch around a large
anticyclone (Figure 3). The array
continues to be in the entrance region of a westerly jet
of winds that extends from 500 hPa downwards. This region
of enhanced westerlies persists between the circulation of
Tropical Cyclone Benilde and the remnants of Tropical
Cyclone Thane. Further west, a cyclonic circulation has
appeared in the kink of the large convective band west of
Diego Garcia (Figure 1, Figure
3). This large band is in the region of confluence
between the circulation of Tropical Cyclone Benilde and
the westerly jet. The northern meridional moisture gradient has softened somewhat, but the Revelle remains in very dry air (Figure 4). The column above the Revelle had winds with strong westerly components up to 350 hPa and distinct dry layers between 700-850 hPa and 450-500 hPa (Figure 5). Above 350 hPa, the air was very dry with easterly winds. Diego Garcia is still within the thin rope of high 500 hPa moisture, and is in plentiful moisture at low levels (Figure 4). Diego Garcia had winds with strong southwesterly components up to 500 hPa (Figure 5). Above that layer, the winds became easterly up to 350 hPA, then southwesterly up to 250 hPa, and easterly above that. The upper levels alternated between being moist and dry throughout the day (not shown). At S-PolKa, the skies had scattered clouds during the day with shallow cumulus (Figure 6). Later in the afternoon, high clouds became visible overhead, which had enough signature to be observed by KAZR (Figure 7). In the beginning part of the day, stable layers existed with every dry layer, although the layer between 550-700 hPa was very moist (Figure 8). Conditions remained this way until 1200 UTC, after which the low levels began to moisten considerably. Convection was sparse and infrequent during the early part of the day, and was mostly shallow convection embedded within lines of nonprecipitating cumulus clouds (upper panels, Figure 9). Although these cells were highly efficient rain producers, they were not deep enough to have much ice production. Low-level shear between 850-1000 hPa was ~15 m/s (Figure 8). Around 0900 UTC convective cells became frequent enough to produce measurable rain across the radar domain (Figure 10), and they tracked from west to east (middle panels, Figure 9). These cells were larger in area, deeper, had more distinctive updraft and divergence signatures, and more ice production. The low-level shear remained ~15 m/s, and the stable layers persisted but narrowed. Around 1900 UTC a squall line passed over S-PolKa (lower panels, Figure 9). Convection within this line reached up to 12 km and had a substantial amount of ice, including aggregates, and graupel. This line was also capable of transporting a layer of stronger winds down to the surface. By 2100 UTC this low-level shear slackened to ~5 m/s, and only the stable layer at 700 hPa remained (Figure 8). By this point, the sounding was very moist up to just above 600 hPa. Rainfall today was mostly convective, with a very small amount of stratiform contribution towards the end of the day (Figure 10). Most of the echo tops were below 5 km, and none of the echo reached above 15 km (Figure 11). Likely because of the general shallowness of the convection, ice production was slight today, with most of the hydrometeors being of warm rain varieties. |
|
| Figure 1. Infrared
satellite imagery overlaid with WWLN lightning data for
1200 UTC 30 December. *Note that the Revelle is
currently not on station at the NE point. This point has
been included for reference to the approximate size of
the array. |
|
| Figure 2. CPC 200 hPa velocity potential
anomaly overlaid infrared imagery for 29 December. |
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 |
 |
| Figure 3. IMD WRF-ARW 0000 UTC analyses
for 30 December at 200 hPa, 500 hPa, and 850 hPa. |
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|
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| Figure 4. CIMSS MIMIC
total precipitable water for 0000 UTC, and Meteo-France
ARPEGE 0000 UTC analyses at 200 hPa, 500 hPa, and 850
hPa for 30 December. *Note that there is no
instrumentation in the SE corner of the
depicted array at this time. |
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|
| Figure 5. Diego Garcia at 0900 UTC and Revelle soundings
for 1200 UTC 30 December. |
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| Figure 6. Photos looking ESE at 0500 UTC
and 1000 UTC 30 December. |
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| Figure 7. ARM KAZR reflectivity data for 30
December. |
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| Figure 8. Gan soundings for 0600-2100 UTC
30 December. |
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 |
| Figure 9. S-PolKa reflectivity PPI and
reflectivity, velocity, and PID RHI for 0146 UTC, 1301
UTC, and 1931 UTC 30 December. |
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| Figure 10. S-PolKa hourly rainfall
statistics for 30 December. |
 |
 |
| Figure 11. S-PolKa echo top height
statistics and particle identification summary for 30
December. |
| The large band of convection to
the south of Gan mentioned in previous summaries
dissipated as the Tropical Cyclone Benilde continued to
move southwestward. By 1800 UTC, the northern part of the
array was inside a zone of smaller convection (right panel
of Figure 1). At
Diego Garcia Tropical Cyclone Benilde was to the island's
southeast and an area of intens convection was to the
southwest, and the air column was generally moist
(Figure
2). The 200 hPa positive velocity potential anomaly
extended east to Bay of Bengal in the northern Indian
Ocean, and the area of negative velocity potential
continued to move eastward across the Maritime Continent
(Figure 3). Low-level
westerlies prevailed over the array during most of the day
(Figure
4). At 500 hPa, the winds were part of the
circulation around Tropical Cyclone Benilde. At 0000 UTC,
the 500 hPa winds were northwesterly to the east of Gan,
and westerly to the west. The 500 hPa wind over Gan
switched to northwesterlies with strong northerly
component later in the day as the tropical cyclone moved
further west
(Figure
5). The upper-level winds were mostly easterly with
varying meridional component. The triangle array was in an area of modest precipitable water (Figure 2), but the air above the low levels was exceedingly dry (middle and right panel of Figure 2). In Gan, the day started with two stable layers at 700 hPa and 600 hPa (Figure 5). The layer below 700 hPa was very moist throughout the day. The air between 600 hPa to 700 hPa was moister by 1200 UTC, at which time several convective cells were breaking above the lowest stable layer (Figure 7). After this period, this middle layer became dryer through the rest of the day (Figure 5). The highest level of winds with westerly component descended from 400 hPa down to 450 hPa between 1200-2100 UTC. Low level shear between 850-1000 hPa was about 10 m/s or less. At S-PolKa, precipitating shallow cumuli were commonly observed during the day, with several convective cloud lines passing over Gan. Although in most cases the convective cells barely reached the melting level, they were able to produce surface rain rate of 30-50 mm/hr (bottom panel of Figure 7). From 0300-0900 UTC, groups of convective cells seen in visible satellite imagery just outside of the radar domain to the NE generated three distinct cold pools (upper panels of Figure 8). Another of these convective cells was within the radar domain around 0330 UTC, with echo tops just below 10 km and moderate ice production (lower panels of Figure 8). A series of convection lines went through the southern radar domain between 0400 and 1000 UTC (Figure 9). These convective cells reached up to ~12-13 km and produced heavy rain, ice, snow and aggregates. A squall line passed over Gan around 0800 UTC, creating a bow echo in the reflectivity PPI and a sharp discontinuity in the velocity field (Figure 10). Figure 11 shows photos of the squall line moving away from S-PolKa toward the east. Despite its heavy rain production, the convection was mostly confined below the melting level (Figure 10). There was a layer of high speed rear inflow between 300-800 m behind the leading convection. The low-level shear in front of the convection was evident in the RHI velocity field. The rainfall of the day consisted of more than 80% convective precipitation, although there were small amounts of stratiform rain throughout the day (Figure 12). During the entire day, there was no echo greater than 10 dBZ above 15 km (Figure 13). The more intense reflectivities were mainly below 9 km. The number of both convective and stratiform echoes between 4 and 8 km increased compared to yesterday, likely associated with the greater tendency for organized lines of convection. However, like yesterday most of the hydrometeors were of warm rain varieties. |
 |
| Figure 1. Infrared
satellite imagery overlaid with WWLN lightning data for
1100 UTC and1800 UTC 31 December. |
 |
 |
 |
| Figure 2. CIMSS MIMIC total precipitable
water for 0000 UTC, and Meteo-France ARPEGE 0000 UTC,
1800 UTC analyses at 500 hPa for 31 December. *Note
that there is no instrumentation in the SE corner of
the depicted array at this time. |
 |
| Figure 3. CPC 200 hPa velocity potential
anomaly overlaid infrared imagery for 31 December. |
 |
 |
|
| Figure 4. IMD WRF-ARW 0000 UTC analyses
for 31 December at 850 hPa, 500 hPa, and 200 hPa. |
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 |
| Figure 5. Gan soundings for 0300-2100 UTC
31 December. |
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| Figure 6. Diego Garcia at 1200 UTC and Revelle soundings
for 1200 UTC 31 December. |
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| Figure 7. ARM KAZR reflectivity data, and
surface measurements for 31 December. |
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| Figure 8. S-PolKa reflectivity PPI over
satellite visible image, and reflectivity, velocity, and
PID RHI for 0337 UTC 31 December. |
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| Figure 9. S-PolKa reflectivity PPI over
satellite visible image, and reflectivity, velocity, and
PID RHI for 0617 UTC 31 December. |
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| Figure 10. S-PolKa reflectivity and
velocity PPI, and reflectivity, velocity, and PID RHI for
0822 UTC 31 December. |
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| Figure 11. Photos looking east at 0930 UTC
31 December. |
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| Figure 12. S-PolKa hourly rainfall
statistics for 31 December. |
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| Figure 13. S-PolKa echo top height
statistics and particle identification summary for 31
December. |
