DYNAMO/AMIE S-PolKa Scientist Summaries - January 2012
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OTHER MONTHS:
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| The positive velocity potential
anomaly continues to spread eastward across the Indian
Ocean, suggesting large-scale subsidence over the northern
part of the basin (Figure 1). This
large-scale subsidence likely contributed to the very low
relative humidity conditions across the northern Indian
Ocean, although convection continued to be active in the
southern Bay of Bengal (Figure 2). A
tropical disturbance is also festering SW of Diego Garcia.
Tropical Cyclone Benilde became a Category 2 storm, and it
continues to track southwestward away from Diego Garcia.
Although the winds along the equator at 200 hPa remain
predominately easterly from Sumatra westward to 80 E, at
Gan the winds have turned southerly as they curve into the
flow of a deep trough over India, creating a zone of
diffluence west Gan (Figure 3). A
mid-level cyclonic circulation is building off of the
southern tip of India, with northwesterly winds over Gan.
Gan is also in the exit region of a low-level westerly
wind jet that is part of the circulation around Tropical
Cylone Benilde. Gan had ample amounts of precipitable water, but most of this moisture was confined to low-levels since the middle levels were extremely dry (Figure 4). The Revelle was in even drier conditions. Diego Garcia was on the edge of a midlevel moisture gradient, and was receiving a stream of water vapor from the NW (Figure 5). Towards the end of the day, the Diego Garcia sounding was very moist up until 800 hPa, but had several dry and stable layers above. The Revelle left station today as it returns to port (pink line, Figure 5). An 1800 UTC Revelle sounding shows the extermely dry air above 800 hPa. Most of the precipitation over Gan was shallow and in the morning hours, although a stream of thick high cloud passed over about 1200 UTC (Figure 6). Mostly shallow cumuli were seen throughout the day, although the thicker upper-level cloud blanketed the sky near sunset (Figure 7). The day began with the layer below 650 hPa being very moist, but with very dry and stable air above that level (Figure 8). At 0000 UTC, the last pulse of the convective lines discussed yesterday went through the domain, with shallow convection that was mostly below the melting layer, relatively high wind speeds, and very little ice production (top panels, Figure 9). After the cluster of morning convective cells cleared out, the layer between 650-900 hPa began to dry out, creating a thick stable layer by 1200 UTC (Figure 8). Throughout the rest of the day, very shallow lines of cumulus clouds went through the radar domain, with some of the cells occasionally precipitating (lower panels, Figure 9). The 850-1000 hPa shear was ~5 m/s throughout the day (Figure 8). One persistent radar artifact was third-trip echo from intense convection SE of the radar domain (Figure 10). This feature had low reflectivity and a strongly negative velocity signature in an otherwise positive velocity field. However, the linear depolarization ration (LDR), which generally becomes positive in second-trip echo, remained negative. This may be the reason why the particle identification algorithm misidentified this misplaced echo as cloud and drizzle hydrometeors. |
| Figure 1. CPC 200 hPa velocity potential
anomaly overlaid infrared imagery for 31 December. |
| Figure 2. Visible satellite imagery
overlaid with WWLN lightning data at 0700 UTC 1
January. |
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| Figure 3. IMD WRF-ARW 0000 UTC analyses
for 0000 UTC 1 January at 200 hPa, 500 hPa, and 850 hPa. |
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| Figure 4. CIMSS MIMIC total precipitable
water for 0400 UTC, and Meteo-France ARPEGE for 0000
UTC 1 January analyses at 500 and 850 hPa. *Note that
there is no instrumentation in the SE corner of the
depicted array at this time. |
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| Figure 5. Water vapor imagery and the
Diego Garcia and soundings for 1800 UTC 1 January. |
| Figure 6. ARM KAZR reflectivity data
for 1 January. |
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| Figure 7. Photos looking SSE at 0400 UTC,
ESE at 0700 UTC, and ESE at 1200 UTC 1 January. |
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| Figure 8. Gan soundings for 0000-1800 UTC 1
January. |
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| Figure 9. S-Polka reflectivity PPI and
reflectivity, velocity, and PID RHI for 0000 UTC and
1716 UTC 1 January. |
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| Figure 10. Infrared satellite imagery at
0800 UTC and S-PolKa reflectivity (top), velocity, linear
depolarization ration, and PID RHI (bottom) for 0816 UTC 1
January. |
| Flow over the central Indian
ocean was very disorganized today as small localized
features modified the wind fields at all three corners of
the triangular DYNAMO array (Figure
1). Tropical Cyclone Benilde, with estimated maximum
sustained winds of 75 knots at 0600 UTC, passed due south
of Diego Garcia as it continued its southerly turn away
from the equator. The Joint Typhoon Warning Center
is forecasting the storm to weaken in the next 24 h.
Low-level clockwise flow on the northwestern side of the
storm merged with another cyclone positioned to the
southwest of Diego Garcia. The low-level convergence at
the centers of these two cyclones was associated with
large distinct areas of deep convection in the satellite
IR measurements (Figure 2). A small
region of convective activity was also apparent east of
Gan, in the former position of the Revelle. This
feature was collocated with an area of 850 hPa velocity
speed convergence in the 0600 UTC analysis from the French
ARPEGE model (left panel, Figure
1). At 500 hPa a counterclockwise circulation
enveloped the entire array with light and variable winds
in the center (Figure
1, center), a feature that was reflected in the
soundings from Gan which showed a noisy transition from
low-level westerlies to upper-level easterlies between 625
hPa and 500 hPa (Figure 3).
Strong easterlies were present at 200 hPa over the
northern portion of the array with weaker easterlies south
over Diego Garcia (right panel, Figure
1). Most of the north-central Indian Ocean remained
under a sizable region of positive 200 hPa velocity
potential anomalies, implying upper-level subsidence (Figure 4). The soundings from Gan showed the layer above 900 hPa to be extremely dry throughout the day, with sporadic moist layers aloft appearing only after sunset ( Figure 3). In contrast with yesterday, the surface mixed layer below 900 hPa was also relatively dry. Only a small 900 hPa moist layer existed throughout the day. The time series of soundings over Gan shows that the layer below 700 hPa has lost the modest gains in moisture that we had seen in the previous few days (Figure 5). The integrated result of all this drying was a column integrated precipitable water estimate slightly above 40 mm (Figure 6). Dry conditions were manifested as very clear skies over Gan, with only scattered shallow cumulus clouds, presumably from the 900 hPa moist layer, early in the afternoon (top row of Figure 7). These clouds were mostly gone by 1200 UTC when the 900 hPa layer had dried out slightly compared to its morning values (bottom row, Figure 7). The ARM KAZR cloud radar was remarkably echo-free for the entire day (not shown). Winds in the lowest 4 km over Gan showed a persistent backing signature throughout the day (Figure 3). Speed shear in the lowest 2 km was less than 5 m/s. S-PolKa observed no measurable precipitation today. The lines of non-precipitating clouds that were frequent over the past several days were no longer present. Despite the lack of weather echo there was a handful of interesting radar echoes. Strong moisture gradients near the surface resulted in some humidity rings generated by Bragg scattering (Figure 8). The cross-section through these rings shows the strongest scattering between 1-2 km. This layer is consistent with the small amount of moisture at 900 hPa that appeared in the soundings through the day (Figure 3). In addition to the humidity-gradient rings, overnight the SMART-R radar on Gan detected unusual ground clutter from the small atoll 100 km north of Gan (Figure 9). This was likely caused by anomalous propagation (AP) of the radar beam as it interacted with the strong low-level moisture gradients and possibly the nearly isothermal layer between 900 and 800 hPa. AP clutter was not observed in the S-PolKa data. |
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| Figure 1. 0000 UTC analysis from the
French ARPEGE model at 850 hPa, 500 hPa, and 200
hPa. (Note: The array drawn on these maps no longer
reflects the true position of the Revelle or the Mirai as
these ships have left station). |
| Figure 2. Satellite IR measurements with
lightning activity (purple hatches) at 1000 UTC. |
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| Figure 3. Soundings from Gan at ~0000 UTC
(5 am), ~0600 UTC (11 am), ~1200 UTC (5 pm), and ~1800 UTC
(11 pm). |
| Figure 4. CPC 200 hPa velocity potential
anomaly overlaid with infrared satellite imagery. |
| Figure 5. Time series of the upper-air
soundings from Gan from 12/25 through 1/2. |
| Figure 6. CIMMS MIMIC Total Precipitable
Water (TPW) product at 1000 UTC. |
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| Figure 7. Cloud photos looking N at 11 am
(top left), SE at 11 am (top right), N at 5 pm (bottom
left), and SE at 5 pm (bottom right). |
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| Figure 8. Humidity rings in the radar PPI
scans (left) and in RHI scans (right). |
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| Figure 9. AP ground clutter observed
overnight (~11 pm) by the SMART-R radar (left), but not
S-PolKa (right). |
| Gan remains in a very dry region
while a long band of convection arcs over Diego Garcia (Figure 1). Lightning remains active along
portions of that convective band, within the dissipating
remnants of Tropical Cyclone Benilde, and the persistent
convective region to the east of Gan. The convective line
over Diego Garcia lies in a region of weak easterly 200
hPa flow (Figure 2). The 500 hPa winds
over Diego Garcia were very weak and variable. The
northwesterly flow that is circulating around the deep 700
hPa trough brought ample amounts of moist air to Diego
Garcia. The mid-level relative humidity was lower than on
previous days (Figure 3). The Diego Garcia soundings reflect the activity of the convective line (Figure 4). Throughout the day, the layer below 750 hPa remained very moist. At 0000 UTC, there was a dry layer between 350-750 hPa that was relatively stable, but the layer from 300-400 hPa was moist. By 0600 UTC, this upper moist layer dried substantially as convection over the island cleared. This upper layer began to moisten again by 1200 UTC as new convection began to initiate to the SE of Diego Garcia. At Gan, the prevailing winds decelerated over the island at all of the levels shown (Figure 2). Gan continued to sit in a region of relatively low total precipitable water and low relative humidities at 500 hPa (Figure 3). At 850 hPa, Gan was on the dry side of the gradient of high humidities to the south. The skies over S-PolKa mostly contained shallow cumulus clouds throughout the day (Figure 5), most of which were in lines propagating southward but oriented NW-SE with the low-level winds (Figure 6). None of these clouds were deep enough to register on the KAZR radar, and no precipitation fell. Low-level shear was weak and mostly directional, and the upper-level layer of easterlies now extends down to 650 hPa (Figure 7). The rings of Bragg scatter in the humidity layers were particularly intense in the early morning hours, when numerous sharp humidity gradients were apparent in the sounding. By 1200 UTC, several of these Bragg scattering layers disappeared as the humidity gradients became less distinct. |
| Figure 1. Visible satellite imagery
overlaid with WWLN lightning data for 0700 2 January.
(*Note that the Revelle is currently not on station at the NE
point.) |
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| Figure 2. IMD WRF-ARW 0000 UTC analyses
for 3 January at 200 hPa, 500 hPa, 700 hPa, and 850
hPa. The star indicates the approximate location of Gan. |
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| Figure 3. CIMSS MIMIC total precipitable
water for 0000 UTC, and Meteo-France ARPEGE 0000
UTC analyses at 500 hPa and 850 hPa for 3 January.
*Note that there is no instrumentation in the NE
or SE corner of the depicted array at this time. |
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| Figure 4. Infrared satellite imagery (top
row) and Diego Garcia soundings (bottom row) for 0000-1800
UTC 3 January. |
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| Figure 5. Photos looking NNE at 0400 UTC,
ESE at 0700 UTC, and SE at 1200 UTC 3 January. |
| Figure 6. S-PolKa reflectivity PPI overlaid
visible satellite imagery for 1000 UTC 3 January. |
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| Figure 7. Gan sounding and S-PolKa
reflectivity PPI and RHI for 0000 UTC and 1200 UTC 3
January. |
| Gan remained in a dry region with
column integrated precipitable water less than 40 mm, RH
less than 30% and 50% at 850 hPa and 500 hPa, respectively
(Figure
1). The increase of convection and lightning
activity over the southern Bay of Bengal was significant
(Figure
2). At 850 hPa, Gan was in a region of
northwesterlies where return flow over the north Indian
Ocean was joined by the flow at the northern edge of
cyclonic circulation driven by the Tropical Cyclone
Benilde (left panel
of Figure
3). Easterly to southeasterly winds prevailed near
Gan at mid-levels (middle
panel, Figure
3). At 200 hPa, the flow was southeasterly over Gan
(right
panel, Figure
3). The Gan soundings showed surface northeasterlies, low-level northwesterlies, mid-level easterlies, and upper-level southeasterlies (Figure 4). The levels between these four layers where the wind direction switched roughly corresponded to three stable layers. The column of air from 450 to 600 hPa and from 800 to 850 hPa was slightly moistened through the day. Nonprecipitating cumuli stretched in long NW-SE oriented lines across the radar domain and were both in the S-band reflectivity and in visible satellite imagery (upper row, Figure 5). These lines of non-precipitating shallow cumuli were all had tops lower than 4 km, consistent with the dryness between 600 to 900 hPa (lower row, Figure 5). Later in the day (~1700 and ~2300 UTC), shallow precipitating convective cells developed propagating southward on the west side of the radar (Figure 6). These cells were very small and short lived, but developed reflectivity up to 40 dBZ. Soundings taken at Diego Garcia showed drying above 600 hPa and a change of wind direction from southwesterly to southeasterly at 500 hPa (Figure 7). An arcing band of convection was positioned over Diego Garcia for most of the day. The band was more intense in the early part of the day and became less active into the evening and overnight hours (Figure 2). Figure 8 shows a sequence of intensification of convection lying within this band from 55°E, 0°S to 80°E 15°S. The intensification started at about 57°E around 2100 UTC on 3 January, propagated to 70°E at 0500 UTC, and reached 75°E by 2100 UTC on 4 January. This westward propagating signal could have been associated with wave activity. |
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| Figure 1. CIMSS MIMIC total precipitable
water for 0000 UTC, and Meteo-France ARPEGE 0000 UTC
analyses at 850 and 500 hPa for 4 January. *Note that
there is no instrumentation in the NE and SE corners of
the depicted array at this time. |
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| Figure 2. Infrared satellite imagery
overlaid with WWLN lightning data for 0000 UTC and 2300
UTC 4 January. |
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| Figure 3. IMD WRF-ARW 0000 UTC
analyses for 4 January at 850 hPa, 500 hPa, and 200 hPa. |
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| Figure 4. Gan soundings for 0000-1800 UTC 4
January. |
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| Figure 5. Upper panels: S-PolKa PPI
reflectivity over satellite visible imagery for 0407,
0607, and 1007 UTC. Bottom panels: photographs looking SE,
SE and NE at 0400, 0600 and 1000 UTC 4 January. |
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| Figure 6. S-PolKa PPI reflectivity for 1722
and 2352 UTC 4 January. |
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| Figure 7. Diego Garcia at 0000, 1200, and
1800 UTC for 4 January. |
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| Figure 8. Satellite IR imagery for 2100 UTC
3 January, 0100, 0500, 0900, 1600, and 2100 UTC for 4
January. |
| The air over
Gan remained abnormally dry today, with integrated total
precipitable water values hovering around 40 mm
throughout the day (Figure 1). A
pocket of dry air straddled the equator across the
entire Indian Ocean, sandwiched between a large area of
contiguous moisture to the south and a more broken but
equally moist region along 5ºN. The 850 hPa flow
was westerly south of Gan and through the center of the
DYNAMO array (left panel, Figure
2). Over the atoll and areas north the low-level
flow was easterly with velocities increasing northward
into the line of moisture along 5ºN. The transition
from easterly to westerly winds west of Gan resulted in
the a low-level boundary that stretched NW to SE from
0ºN,65ºE to 10ºS, 75ºE. The region
south of this boundary
had ample moisture that extended upward into the 500 hPa
level, while the area on the northern side was
comparatively dry. Convergence, confluence, or deformation
along the boundary was associated with a long and
persistent line of clouds and precipitation stretching NW
to SE and eventually terminating at the western side of
the DYNAMO array (Figure 3). In the
afternoon, a lone precipitating cell within range of the
radar formed along the line, giving S-PolKa its only view
of precipitation for the day (Figure 4).
Winds at 500 hPa were dominated by a large anticyclone
centered over the Arabian Sea in the northern hemisphere,
stretching from the Horn of Africa across India and into
southeast Asia (center panel, Figure
2). In the southern hemisphere a smaller anticyclone
was centered near 18ºS and 90ºE. At 200 hPa
winds over the array were strong southeasterlies, which
increased in speed over Gan (right panel, Figure 2). Upper-air soundings from Gan showed many of the same patterns that have characterized our persistent dry spell (Figure 5). Humidity was low through most of the column, with the exception of intermittent moistening of the layers centered around 900 hPa and between 600 and 700 hPa. By the evening any minor gains in low-level moisture had been wiped out by dry air within the 700-900 hPa layer (right panel, Figure 5). The layer between the surface and 800 hPa showed a noisy backing of the winds from northeasterlies to westerlies. Weak easterlies descended from 750 to 850 hPa between 0000 UTC on 5 January and 0000 UTC on 6 January. All of the soundings captured the strong southeasterly winds between 200 and 300 hPa. Very shallow cumulus were scattered about during the day with a gradual clearing through the afternoon and into the evening (Figure 6). The dry air above the shallow cloud-bearing layer was readily apparent via its impact on the cloud field. Any individual cumulus with enough buoyancy (or bravery) to rise higher than the mean depth of the field was quickly stamped out by the entrainment of dryer air (Figure 7). With the exception of the lone storm that cropped up off of the surface boundary to our west (Figure 4), S-PolKa observed no measurable precipitation within its maximum unambiguous range. Overnight, tall storms to our far north and northeast were observed by the radar in the form of multi-trip echo (Figure 8). The storms were far enough away that their radar returns were up to 3rd (between 300 and 450 km) and 4th (between 450 and 600 km) trip echos. These distances were reflected in the LDR values associated with these echos (right panel, Figure 9). Because S-Pol uses alternating H and V polarized waves, second trip echo will appear as anomalously high linear depolarization ratio (LDR) values--the radar algorithms indicate that almost all of the original outgoing wave has been depolarized into the other channel. The 3rd trip echo will have normal values well below 0. The LDR measured in these storms had areas of high values closer to the radar adjacent to lower values further away. This likely means the storm was on the border between the 2nd and 3rd trip returns, a location that is confirmed in the satellite imagery (top row, Figure 9). |
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| Figure 1. CIMMS MIMIC Total Precipitable
Water (TPW) product for 0400 UTC on 5 January. |
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| Figure 2. 5 January 0600 UTC analysis from
the French ARPEGE model at 850 hPa (left), 500 hPa
(center), and 200 hPa (right). (Note: The array
drawn on these maps no longer reflects the true position
of the Revelle or the Mirai as these ships have left
station). |
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| Figure 3. Visible satellite imagery at
0600 (left), 1000 (center), and 1300 (right) UTC on 5
January, with S-PolKa's 150 km range ring in the center. |
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| Figure 4. PPI of S-PolKa's reflectivity
field at 1000 UTC (left) and a cross-section through the
lone precipitating cell of the day (right). |
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| Figure 5. Upper-air soundings from Gan at
2335 UTC on 4 January (left), and 1135 & 2335 UTC on 5
January (center and right). |
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| Figure 6. Cloud photos looking west (top)
and southeast (bottom) at 0400 UTC (left), 0800 UTC
(center), and 1200 UTC (right) on 5 January. |
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| Figure 7. Photos taken 3 minutes apart of
a relatively buoyant cumulus cloud rising into a layer of
very dry air at 1030 UTC on 5 January. |
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| Figure 8. S-Pol radar reflectivity
overlaid on IR imagery (left) and zoomed in to within the
150 km maximum unambiguous range (right). |
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| Figure 9. S-Pol radar reflectivity with
IR imagery (top), reflectivity within 150 km (bottom left)
and linear depolarization ratio (bottom right) at 1901
UTC. |
| Gan remained under suppressed
conditions with intense positive velocity potential at 200
hPa (Figure 1). The
positive velocity potential over the northern and western
Indian Ocean has stayed in this area since 31 December
(not shown). The column integrated precipitable water
indicated dryness along the equator from the the coast of
Africa to the middle of the Indian Ocean
(Figure
2). At 850 hPa, the region around Gan was in the
center of a gyre that dominated the whole Indian Ocean
basin, while Diego Garcia was in the region of strong
westerlies on the southern boundary of the gyre (left
panel, Figure
3). The 850 hPa winds over Gan varied through the in
association with the circulation around the gyre
(Figure
4). Strong easterlies prevailed over the northern
Indian Ocean at 500 hPa, and over a large portion of the
Indian Ocean at 200 hPa (middle and right
panels, Figure
3). The winds over Gan had mostly easterly
components at all levels by 1500 UTC
(Figure
4). The entire column was persistently dry. At Diego
Garcia, the layer between 850 hPa and 650 hPa stayed dry
during the day, and the drying above 400 hPa was
significant
(Figure
5). Winds at 500 hPa changed from southwesterlies at
the beginning of the day to southeasterlies by 1800
UTC. Weekly sounding time series from Gan show a decrease of RH to less than 40% through the entire column, except in the boundary layer and several discrete upper layers (left panel, Figure 6). The wind speed below 500 hPa at Gan peaked on 2 January, and has gradually weakened since. At Gan the easterly wind components have been steadily descending; at the beginning of the period shown in the left panel Figure 6, the easterlies were only at levels higher than 300 hPa. At the present time the easterly layer extends all the way down to about 700 hPa. The upper-levels over Diego Garcia have moistened during the second half of the week (right panel, Figure 6), and the descent of easterlies did not occur there. The band of convection along 5°N extended westward to 65°E by 0000 UTC 6 January, with extensive lightning activities (left panel, Figure 7). Part of this band (from 65° E to 80° E) dissipated by 1900 UTC (middle panel, Figure 7). To the south of Diego Garcia, convection continued to develop along 10°S from 50°E to 85°E (right panel, Figure 7). There was no measurable precipitation near Gan today. Figure 8 shows zonally-oriented lines of non-precipitating shallow cumuli observed by S-PolKa in the late afternoon. The clouds were propagating southward. Photographs show cloud lines to the south and southeast of S-PolKa (upper middle and upper right panels, Figure 8). The RHI reflectivity observations indicate that the cloud tops were no higher than 3 km (bottom right panel, Figure 8). The RHI scan also shows distinct humidity-gradient layers at 2 and 3 km, corresponding to the stable layers at the same level in the Gan 1200 UTC sounding (upper right panel, Figure 4). |
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| Figure 1. CPC 200 hPa velocity potential
anomaly overlaid infrared imagery for 5 January. |
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| Figure 2. CIMSS MIMIC total precipitable
water for 0000 UTC 6 January. |
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| Figure 3. IMD WRF-ARW 0000 UTC
analyses for 6 January at 850 hPa, 500 hPa, and 200 hPa. |
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| Figure 4. Gan soundings for 0000-2100 UTC 6
January. |
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| Figure 5. Diego Garcia soundings for 0000
UTC and 1800 UTC 6 January. |
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| Figure 6. Weekly sounding time series for
Gan (left) and Diego Garcia (right) from 29 December to 5
January. |
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| Figure 7. Infrared satellite imagery
overlaid with WWLN lightning data for 0700 UTC, 1900 UTC
and 2300 UTC 6 January. |
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| Figure 8. Upper panels: S-PolKa PPI
reflectivity over satellite visible imagery for 1207 UTC,
photographs looking SE and S for 1200 UTC 6 January.
Bottom panels: S-PolKa PPI reflectivity, velocity, and RHI
reflectivity for 1207 UTC 6 January. |
| Dry air
continued to dominate the weather picture around Gan as
the column-integrated total precipitable water estimate
remained below 40 mm (Figure 1).
Moisture in the western and central Indian Ocean was
separated into two bands on either side of the equator,
with the northern half of the DYNAMO array lying in the
equatorial dry region, and the southern half in the
moister area along the southern convective band. This
pattern is contrasts with the larger amount of moisture
that is evenly distributed from 15ºN to 15ºS
over the entire maritime continent. The 0600 UTC model
analysis from the French ARPEGE GCM shows the 50% RH
contour extending vertically all the way into the 500 hPa
level in both the northern and southern bands across the
Indian Ocean (center panel, Figure
2). The low-level counterclockwise gyre mentioned
over the past few days continued its slow westward
propagation today, putting Gan in a region of weak
low-level southerlies (left panel, Figure 2). Flow at 500 hPa was
southeasterly through the center of the array. Winds
transitioned to moderate easterlies west of Gan, such that
weak mid-level divergence was apparently located over the
atoll (center panel, Figure 2).
Over the equator strong easterly winds at 200 hPa
stretching across the entire basin (right panel, Figure 2) and flowing between
anticyclonic centers on both sides of the equator. The
western Indian Ocean remains under a sizable region of 200
hPa positive velocity potential anomalies, indicative of
large scale subsidence aloft (Figure
3). With the exception of transient moistening and drying at the top of the surface mixed layer, soundings from Gan showed very little variation through the day (Figure 4). A deep layer of easterly winds with maximum velocities just above 300 hPa dominated the profile between 650 and 200 hPa. Between 800 and 700 hPa winds veered smoothly from southerly to easterly, while in the boundary layer they were light and variable in all of the soundings. The descent of easterlies mentioned in yesterday's summary is still evident (Figure 5). Figure 4 shows that at least some easterly wind components were found intermittently through the whole troposphere throughout the day. Today marked the first day since Christmas in which the entire column of air was devoid of any persistent westerly winds. The deep layer of moisture that characterized the last MJO event and dissipated quickly after Christmas has yet to recover ( Precipitation in the central Indian Ocean was concentrated in two symmetric bands at 10ºN and 10ºS (Figure 8). Late in the day, two large regions of convective activity organized in both bands near 60ºE (right panel, Figure 8). Convective activity in the long line stretching from Sri Lanka to the windward shore of Sumatra decreased significantly during the day. S-PolKa was once again confined to observing only scattered non-precipitating shallow clouds and humidity gradient features. In contrast to the previous days, the long lines of cumulus oriented with the wind were noticeably absent (Figure 9). One possible explanation for this change is the lack of structure in the boundary layer winds, which would favor boundary layer rolls or other forms of linear cloud patterns. The time-lapse motion of the cumulus clouds observed by the radar showed a very slight counterclockwise rotation, consistent with the influence of the low-level gyre to our southwest. With the exception of a brief period at the end of 2011, no precipitation has been observed by S-PolKa for the better part of two weeks (Figure 10). |
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| Figure 1. CIMMS MIMIC Total Precipitable
Water (TPW) product for 0600 UTC on 7 January. |
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| Figure 2. 7 January 0600 UTC analysis from
the French ARPEGE model at 850 hPa (left), 500 hPa
(center), and 200 hPa (right). (Note: The array
drawn on these maps no longer reflects the true position
of the Revelle or the Mirai as these ships have left
station.) |
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| Figure 3. CPC 200 hPa velocity potential
anomaly overlaid on infrared satellite imagery for 6
January. |
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| Figure 4. Upper-air soundings from Gan at
2336 UTC on 6 January (left), and 0835 UTC (center) and
1735 UTC (right) on 7 January. |
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| Figure 5. Vertical profile of moisture
and winds over Gan from 8 December 2011 through 7 January
2012. |
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| Figure 6. Cloud photos looking E at 1100
UTC (left), 1200 UTC (center), and 1300 UTC (right) on 7
January. |
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| Figure 7. Upper-air soundings from Diego
Garcia at 2317 UTC on 6 January (left), and 1114 UTC
(center) and 2334 UTC (right) on 7 January. |
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| Figure 8. Satellite IR measurements and
lightning activity indicators (purple hatches) at 1800 UTC
on 6 January (left) and 1800 UTC on 7 January (right). |
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| Figure 9. PPI views of S-Pol radar
reflectivity at 1000 UTC (left), 1100 UTC (center), and
1200 UTC (right) on 7 January. |
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| Figure 10. Daily rainfall accumulations
recorded by S-PolKa from the beginning of the project
through 7 January 2012. |
| The large convection band in
the northern Indian Ocean continued to propagate
westward across the southern Arabian Sea with large
amounts of lightning activity
(Figure
1). The cyclone in the Mozambique Channel has been
named Tropical Cyclone 05S, or Chanda, which moved
southeastward during the day, and weakened after making
landfall in western Madagascar (right panel, Figure 1).
Meanwhile, the convection band positioned near Diego
Garcia intensified during the day. A large portion of Indian Ocean remained under suppressed conditions. There was a sharp gradient of 200 hPa velocity potential stretching NE-SW, from the Bay of Bengal to the south-central part of the basin (Figure 2). The dryness at the equator across the Indian Ocean remained, with precipitable water less than 40 mm near Gan (Figure 3). At 850 hPa, Gan was on the northeastern side of the gyre that dominated the western Indian Ocean, straddling the equator (left panel, Figure 4). The 850 hPa winds over Gan were mostly southwesterly, but varied substantially below this level through the day (Figure 5). Above 850 hPa, easterlies were dominant over Gan. Winds with significant southerly and northerly components were observed at 550 hPa and above 350 hPa respectively (Figure 5). From the African coast to the middle of Indian Ocean, strong easterlies were dominant at 500 hPa (center panel, Figure 4). At 200 hPa, three distinguishable rotational centers defined the wind field over the Indian Ocean (right panel, Figure 4). The flow from the gyre in the northern Indian Ocean to the one in the southern Indian Ocean contributed to the northerly component of the upper-level winds over Gan and Diego Garcia. Diego Garcia remained very moist below 700 hPa (Figure 6). The S-PolKa PPI velocity field captured the change of wind direction from southeasterlies to northeasterlies at low-levels (bottom panels, Figure 7). The skies were mostly clear at S-PolKa during the daytime, with only very shallow clouds to the north (Figure 8). Lines of shallow clouds aligned with the low-level wind direction started to form overnight, propagating southwestward (Figure 7). The tops of these cloud lines reached up to about 3 km (top right panel, Figure 7). The orientation of the cloud lines was different from the zonally (east-west) oriented cloud lines observed over the past few days (6 January S-PolKa report). |
 |
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|
|
| Figure 1. Infrared satellite imagery
overlaid with WWLN lightning data for 0000 UTC (left)
and 2300 UTC (center) on 8 January. Meteo-France
ARPEGE 0000 UTC 850 hPa analysis for 8 January
(right). Note the location of the cyclone near
Mozambique. |
 |
| Figure 2.: CPC 200 hPa velocity potential
anomaly overlaid infrared imagery for 7 January. |
 |
| Figure 3.: CIMSS MIMIC total precipitable
water for 0000 UTC. |
 |
 |
 |
| Figure 4. IMD WRF-ARW 0000 UTC
analyses for 8 January at 850 hPa (left), 500 hPa
(center), and 200 hPa (right). |
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 |
 |
| Figure 5. Gan soundings for 0000-1800 UTC
on 8 January. |
 |
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| Figure 6. Soundings from Diego Garcia at
0600 UTC (left) and 1800 UTC (right) on 8 January. |
|
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|
 |
| Figure 7. S-PolKa PPI reflectivity (upper
panels) and velocity (bottom panels) for 0007 UTC and 2052
UTC on 8 January. |
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|
|
|
|
|
|
| Figure 8. Panoramic view looking SW, W,
NW, N, NE, E, SE (left to right) at S-PolKa at 1100 UTC on
8 January. |
|
Large-scale atmospheric conditions for precipitation
continued to be unfavorable as the northern part of the
DYNAMO array went another day without rain. A deep layer
of dry air above the boundary layer combined with
subsidence at 200 hPa to suppress any activity other
than very shallow clouds (Figure
1). Water vapor in the western Indian Ocean remained
concentrated in two bands straddling the equator,
gradually merging into a more contiguous area east of
90ºE (Figure 2). At 200
hPa a northern shift in the easterly winds was centered
due south from the tip of India (right
panel, Figure 3). Moisture
south of this feature filled in the eastern side of the
old DYNAMO array (Figure 2).
Moisture was displaced away from the equator on the
northern side. These water vapor patterns were evident
in the basin-wide distribution of cloudiness
(Figure 4). Storm intensity,
as measured by IR temperatures and lightning frequency,
increased virtually everywhere through the course of the
day. Diego Garcia was in the middle of the southern band
of precipitation. The upper-air soundings from Gan exhibited many of the patterns that have characterized the last week of dry weather, but also showed subtle changes (Figure 5). Dry air filled the column above the boundary layer, with easterly winds above 600 hPa. However, upper-level moisture slightly increased in the latter part of the day, particularly between 600 and 400 hPa (right panel, Figure 5). In the boundary layer, low-level northeasterlies in the morning eventually backed more than 100º counterclockwise, ending the day with a strong westerly component. The net result by 0000 UTC on 10 January was a 3 km deep layer of westerlies, with a peak velocity near 10 m/s around 800 hPa (right panel, Figure 5). These winds late in the day marked the first appearance of a westerly wind layer more than a few hundred meters deep in almost a week (Figure 6). The dominant signal in the soundings from Diego Garcia was the aforementioned southern band of moisture, with the layer of low-level westerlies also deepening from 4 to 5 km by the end of the day (Figure 7). Despite no precipitating storms to observe, S-PolKa did have plenty of non-precipitating targets, especially before noon. A large field of low-level cumulus, aligned in long lines oriented with the prevailing wind, started to form after sunset on 8 January (S-PolKa Science Summary, 8 January). Overnight these lines filled much of theobservable area east of the radar. The lines were typically a few hundred meters wide, 1-2 km deep, anywhere from 25 to 75 km long, and spaced no more than 10 km apart (Figure 8). Clouds forming in the lines propagated from NE to SW through 0300 UTC on the 9th, at which point they began moving more from north to south. A photo of these clouds lines, taken between 0330 and 0400 UTC, is shown in Figure 9. Based purely on the appearance of the cloud lines it seems like they were forming in the upward branch of boundary layer rolls. A closer look at the soundings from Gan provided a lot of evidence that this was the case (Figure 10). Between 1800 and 2100 UTC on 8 January, when the lines first became clear, the soundings showed a strongly dry-adiabatic shallow boundary layer that is moist at the top and capped by shallow 1-2 K temperature inversion (first and second panel, Figure 10a & 10b). At 2100 UTC the winds in the boundary layer were purely northeasterly (they showed no directional shear) and had a maximum velocity of 5 m/s a few hundred meters beneath the inversion (second panel, Figure 10b). This pattern continued past 0000 UTC on the 9th as the lines become more clearly defined and started to appear in the area north of the radar (third panel, Figure 10c). The 0300 UTC sounding, approximately 2 hours after sunrise, showed a deeper boundary layer that was dryer at the top, as well the first appearance of directional wind shear, from NNE to NNW, just below the inversion (fourth panel, Figure 10d). These trends continued in the 0600 UTC sounding as the cloud lines became less distinct in the radar reflectivity (fifth panel, Figure 10e). By 0900 UTC only two lines of cloud remained to the southeast of the radar and they were completely gone by 1200 UTC (not shown). One could hypothesize that a combination of a deepening boundary layer and directional shear after sunrise made the boundary layer roll circulations harder to maintain, eventually leading to the breakup of the cloud lines. Cumulus clouds became more scattered in the early afternoon and the sky was clear by sunset (Figure 11). |
 |
| Figure 1. CPC 200 hPa velocity potential
anomaly overlaid on infrared satellite imagery for 8
January. |
 |
| Figure 2. CIMMS MIMIC Total Precipitable
Water (TPW) product for 1200 UTC on 9 January. |
 |
 |
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| Figure 3. 9 January 0600 UTC analysis from
the French ARPEGE model at 850 hPa (left), 500 hPa
(center), and 200 hPa (right). (Note: The array
drawn on these maps no longer reflects the true position
of the Revelle or the Mirai as these ships have left
station.) |
 |
 |
| Figure 4. Satellite IR measurements and
lightning activity indicators (purple hatches) at 1200 UTC
on 9 January (left) and 0000 UTC on 10 January (right). |
 |
 |
 |
| Figure 5. Upper-air soundings from Gan
at 2335 UTC on 8 January (left), and 1135 UTC (center)
and 2335 UTC (right) on 9 January. |
 |
| Figure 6. Vertical profile of moisture
and winds over Gan from 2 January through 9 January 2012. |
 |
 |
 |
| Figure 7. Upper-air soundings from Diego
Garcia at 2314 UTC on 8 January (left), and 1231 UTC
(center) and 2318 UTC (right) on 9 January. |
 |
 |
| Figure 8. PPI and RHI of S-Pol radar
reflectivity at 0000 UTC on 9 January. |
 |
| Figure 9. Cloud photos facing east (far
left) through southwest (far right) sometime between 0330
and 0400 UTC. (Photo credit: Wen Chau Lee) |
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 |
 |
 |
| Figure 10. PPI view of S-PolKa
reflectivity (top) and the upper-air sounding from Gan
(bottom) from 1800 UTC on 8 January (far left) through
0600 UTC on 9 January (far right) in three hour
intervals (a: 8 Jan/1800, b: 8 Jan/2100, c: 9 Jan/0000,
d: 9 Jan/0300, e: 9 Jan/0600). |
 |
 |
 |
| Figure 11. Cloud photos facing east at
0600 UTC (1100 local time, left), 0900 UTC (1400 local
time, center), and 1200 UTC (1700 local time, right). |
| Gan remained in the dry region
covering the equatorial western and central Indian Ocean
(Figure
1). The 200 hPa velocity potential anomalies showed
continuing suppressed conditions over a large portion of
the Indian Ocean
(Figure 2). The
divergence over the maritime continent at 200 hPa has
intensified over the past several days, resulting in a
strong upper-level east-west oriented velocity potential
gradient. At 850 hPa, both Gan and Diego Garcia were inside the region of westerlies which prevailed over the southern Indian Ocean (left panel, Figure 3). The northern Indian Ocean was mostly dominated by easterlies stretching all the way from the maritime continent to Africa. Winds over Gan at 500 hPa oscillated between easterly and southeasterly during the day (Figure 4). The occurrence of southeasterly winds at 500 hPa coincided with the change of winds from southeasterly to almost southerly above 300 hPa (upper right panel, Figure 4). At 500 hPa winds northeasterlies and southeasterlies merged into easterlies on the equator at about 75°E (middle panel, Figure 3). Diego Garcia remained very moist from the boundary layer up to 200 hPa, except for the slightly drier layer in between 400 hPa and 500 hPa (Figure 5). Winds with an easterly component descended from 450 hPa to 650 hPa by 1200 UTC. The convective band positioned over Diego Garcia yesterday had weakened considerably by the end of the day, leaving only some high clouds drifting toward the northwest (Figure 6). During the later hours of the day, both KAZR and S-PolKa observed remnants of the storm to the south of Gan at about 9 km (Figure 7; bottom panels of Figure 8). After 1100 UTC, high clouds were spotted to the south of S-PolKa (right panel, Figure 9). These high clouds coincided with moistening above 300 hPa after 1500 UTC (bottom panel, Figure 4). Through the day, the clouds around Gan were mostly extremely shallow (left panel, Figure 9). After 0800 UTC, stronger and more extensive echoes began to appear in the northeast sector of the S-PolKa PPI scan (upper panel, Figure 8). On average these echoes lasted over an hour, with their maximum height up to about 3 km (upper right panel, Figure 8). A line of shallow cumuli formed over S-PolKa near 0400 UTC (Figure 10). The cloud line stayed close to the coastline of the atoll until it finally dissipated by 1000 UTC. From 0400 UTC to 0800 UTC, it seemed the clouds moved back and forth in a direction perpendicular to the line itself, forming a wave like structure. This oscillation may be associated with land-sea interactions during the day. |
 |
| Figure 1. CIMSS MIMIC total precipitable
water for 0000 UTC 10 January. |
 |
| Figure 2. CPC 200 hPa velocity potential
anomaly overlaid on infrared imagery for 9 January. |
 |
 |
 |
| Figure 3. IMD WRF-ARW 0000 UTC
analyses for 10 January at 850 hPa (left), 500 hPa
(center), and 200 hPa (right). |
 |
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 |
 |
| Figure 4. Gan soundings for 0000-2100 UTC
on 10 January. |
 |
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| Figure 5. Soundings from Diego Garcia at
0000 UTC (left) and 1200 UTC (right) on 10 January. |
 |
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| Figure 6. Infrared satellite imagery
overlaid with WWLN lightning data for 0000 UTC (left)
and 2300 UTC (center) on 10 January. |
 |
| Figure 7. Vertically pointing ARM KAZR
reflectivity data over Gan for 10 January. |
 |
 |
 |
 |
| Figure 8. S-PolKa PPI (left panels) and
RHI (right panels) reflectivity for 0813 UTC and 2228 UTC
on 10 January. |
 |
|
| Figure 9. Photographs looking south at
0400 UTC and 1100 UTC on 10 January. |
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 |
 |
| Figure 10. Photographs looking SE at 0400
UTC, 0500 UTC, 0600 UTC, 0700 UTC, and 0800 UTC on 10
January. |
| The positive velocity potential
anomaly over the DYNAMO array has weakened
(Figure 1). The
column integrated precipitable water estimates indicated
that Gan remained in the east-west elongated region that
was relatively drier than the surrounding area, but was
more humid compared to yesterday
(Figure
2). The 850 hPa analyses showed increased relative
humidity over Gan during the day
(Figure
3). The moistening below 600 hPa from 1200 UTC to
1800 UTC was notable in the Gan soundings, while the
layer above 350 hPa became drier as the high clouds from
the south dissipated overnight
(Figure
4, Figure
5). At 850 hPa, the wind field over northern Indian Ocean has lost its coherence. The easterly wind components from the maritime continent weakened before reaching the southern tip of India (Figure 3). The northeasterlies from the Arabian Sea contributed to the northwest side of the circulation that dominated the equatorial western Indian Ocean. The circulation at 500 hPa was quite similar to 10 January, except that the area of confluence by northeasterlies and southeasterlies on the equator has slightly moved to the west (left panel, Figure 6), causing the winds over Gan at this level to be more variable throughout the day compared to yesterday (Figure 4). At 200 hPa easterlies continued to prevail over the equatorial region across the Indian Ocean (right panel, Figure 6). The winds over Diego Garcia were mostly westerly at the low-levels and easterly above 500 hPa (Figure 7). Between 600 hPa and 700 hPa, the zonal component of the winds over Diego Garcia varied during the day. The convective band positioned over Diego Garcia that dissipated yesterday (10 January S-PolKa Science Summary) has reformed on the north side of its previous position, just to the south of Gan (Figure 8). This relocation corresponded to the northern extension of the 850 hPa westerly region through the day (Figure 3). The sky over S-PolKa was mostly clear with remnants of the overnight high clouds until 0600 UTC (Figure 9). After 0900 UTC, the cumulus began to form lines that appeared on S-PolKa PPI scans as NW-SE oriented moderate echo lines (Figure 10). These cloud lines persisted and slowly propagated toward the southeast with the low-level wind until about 2000 UTC. Although the amount of clouds increased during the day, the cloud tops remained mostly below 2 km (Figure 9; Figure 10). |
 |
| Figure 1. CPC 200 hPa velocity potential
anomaly overlaid on infrared imagery for 10 January. |
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| Figure 2. CIMSS MIMIC total precipitable
water for 0000 UTC 11 January. |
 |
 |
| Figure 3.
Meteo-France ARPEGE 0000 UTC (left) and 1800 UTC (right)
analyses at 850 hPa for 11 January. *Note that
there is no instrumentation in the NE and SE corner of
the depicted array at this time. |
 |
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 |
 |
| Figure 4. Gan soundings for 0000-1800 UTC
on 11 January. |
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| Figure 5. Vertically pointing ARM KAZR
reflectivity data over Gan for 11 January. |
|
|
| Figure 6. IMD WRF-ARW 0000 UTC
analyses for 11 January at 500 hPa (left) and 200 hPa
(right). |
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 |
 |
| Figure 7. Soundings from Diego Garcia at
0000 UTC (left), 1200 UTC (middle) and 1800 UTC (right) on
11 January. |
 |
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| Figure 8. Infrared satellite imagery
overlaid with WWLN lightning data for 0000 UTC (left)
and 2300 UTC (right) on 11 January. |
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 |
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| Figure 9. S-PolKa PPI and RHI
reflectivity (upper panels), photographs looking N and SE
at 0500 UTC on 11 January. |
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| Figure 10. S-PolKa PPI and RHI
reflectivity (upper panels), photographs looking N and SE
at 0900 UTC 11 January. |
| The slow
convergence of moisture from the north and south continued
as Gan finished the day with a column-integrated
precipitable water estimate slightly less than 50 mm (Figure 1). The two bands of
moisture on either side of the equator, present for the
past several days, finally merged in the western Indian
Ocean. A slight separation was still apparent from
70ºE to 90ºE. Analysis from the Meteo-France
ARPEGE GCM had the low-level moisture to the south of the
equator being advected by strong westerlies stretching
from 60ºE into the maritime continent (left panel, Figure 2). North of the
equator, light northerly winds continued to bring moisture
closer to the DYNAMO array. RH values at 500 hPa showed
the southern line of moisture to be deeper than its
northern counterpart (center panel, Figure 2). Westerly winds at
500 hPa formed a long line across the central and eastern
Indian Ocean at 5ºS but were less coherent through
the center of the array. Easterly winds along and north of
the equator dominated at 200 hPa, including over the
northern half of the DYNAMO array (right panel, Figure 2). Upper-level support
for convection also became more favorable, as indicated by
200 hPa velocity potential anomalies being near neutral
along the equator, with slightly positive (convergence)
values to the north and slightly negative (divergence)
values to the south (Figure
3). Over Gan, the layer below 700 hPa moistened considerably during the day (Figure 4). At 800 hPa, RH values crossed the 60% threshold for the first time in a week (Figure 5). Late in the day cirrus clouds from the convection to the north and south of Gan were associated with sizable moistening above 300 hPa (right panel, Figure 4). The air between 700 and 300 hPa remained quite dry. Boundary layer winds backed from northwesterly in the morning to northerlies by 2100 UTC. Winds above the surface layer were predominately westerly up to 500 hPa and easterly above 375 hPa, with a relatively smooth transition in between. In contrast with the trend of deepening low-level westerlies over the past few days, the depth of the westerly winds remained relatively constant through the day. The column over Diego Garcia continued to be quite moist compared to Gan (Figure 6). Winds transitioned from low-level westerlies to upper-level easterlies at 700 hPa early in the day and at 600 hPa in the late afternoon. Convective activity in the central Indian Ocean was concentrated in a band stretching across the basin between 5ºS and 10ºS, and in a less active line extending southwestward from the southern tip of India (left panel, Figure 7). The radar observed mostly scattered cumulus clouds that increased in number through the day (Figure 8; Figure 9). Thin cirrus generated in each of the cloud bands between 5ºS and 10ºS and in lines approached S-PolKa late in the afternoon (right panel, Figure 7; bottom right panel, Figure 10). With the exception of two lines that formed north and northeast of the radar, the clouds did not show any persistent organization (lower left panel, Figure 8). The aforementioned low-level moistening through the day coincided with cumulus eventually rising above the boundary layer to depths approaching 2.5-3 km late in the afternoon (bottom row, Figure 10). A large cluster of storms to the northwest of Gan led to sporadic multi-trip echo overnight (Figure 11). Negative measurements of the linear depolarization ratio (LDR) suggests these echoes were likely third trip, between 300 and 450 km away (right panel, Figure 11). |
 |
 |
| Figure 1. CIMMS MIMIC Total Precipitable
Water (TPW) product for 0000 UTC on 12 January (left) and
13 January (right). |
 |
 |
 |
| Figure 2. 12 January 0600 UTC analysis
from the French ARPEGE model at 850 hPa (left), 500 hPa
(center), and 200 hPa (right). (Note: The array
drawn on these maps no longer reflects the true position
of the Revelle or the Mirai as these ships have left
station.) |
 |
| Figure 3. CPC 200 hPa velocity potential
anomaly overlaid on infrared satellite imagery for 11
January. |
 |
 |
 |
| Figure 4. Upper-air soundings from Gan
at 0235 UTC (left), 1135 UTC (center,) and 2035 UTC
(right) on 12 January. |
 |
| Figure 5. Weekly time series of the
soundings from Gan from 5 January through 12 January. |
 |
 |
| Figure 6. Upper-air soundings from Diego
Garcia at 2317 UTC on 11 January (left) and 1134 UTC on
12 January (right). |
 |
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| Figure 7. Satellite IR measurements
and lightning activity indicators (purple hatches) at 0000
UTC (left) and 1200 UTC (right) on 12 January. |
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 |
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| Figure 8. S-Pol radar reflectivity from
0300 UTC (upper left) to 1800 UTC (lower right) in three
hour increments. |
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| Figure 9. Photos looking east at 0400 UTC
(0900 LT, right), 0800 UTC (1300 LT, center), and 1200 UTC
(1700 LT, right). |
 |
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 |
 |
| Figure 10. S-Pol radar reflectivity in
viewed in PPI (left column) and RHI (right column) format
at 0600 UTC (top) and 1500 UTC (bottom) on 12 January. |
 |
 |
 |
| Figure 11. IR satellite measurements
(left), S-Pol radar reflectivity (center), and S-Pol LDR
(right) at 1500 UTC on 12 January. |
| Conditions
on Addu Atoll changed rapidly as convective lines from the
north and south brought very moist air toward the equator.
Total precipitable water values over Gan, which had been
hovering around 40 mm for the past two weeks, climbed over
50 mm and were continuing to rise at 1800 UTC
(Figure 1). The western Indian
Ocean filled with very moist air from the equator to
10ºS. The buildup of moisture, particularly between
700 and 500 hPa, was captured in the upper-air soundings
from Gan (left panel, Figure
2). High RH above 300 hPa was associated with a
large amount of cirrus clouds streaming in from the
convection to the north and south of
S-PolKa. The layer between 500 and 300 hPa remained
comparatively dry. Westerly winds continued to increase in
depth, growing from 500 hPa to 400 hPa during the course
of the day. This feature was seen clearer in the weekly
time-series of RH and winds from Gan
(Figure 3). Boundary-layer
winds were generally out of the north, but ended the day
northeasterly (right
panel, Figure
2). Upper-level support for convection as indicated
by the 200 hPa velocity potential field remained near
neutral (Figure 4). A long line of strong low-level westerly winds, stretching from the northern tip of Madagascar all the way into the maritime continent, accompanied the buildup of moisture along and south of the equator (left panel, Figure 5). This pattern was reflected to some degree in the 500 hPa wind analysis, with the exception that the winds in the far western Indian Ocean were less coherent (center panel, Figure 5). Winds at 850 hPa had a confluent signature over the northwest corner of the DYNAMO array where westerlies and northwesterlies merged. At Gan, surface westerlies gradually increased in velocity from a morning value less than 2 m/s to an evening peak of 4 m/s, then weakened into the night (Figure 6). At 200 hPa, winds along the equator were easterly in the western side of the basin and southeasterly in the eastern side (right panel, Figure 5). Winds in the northern hemisphere were much stronger than in the southern hemisphere. The two approaching bands of deep convection had merged in the western Indian Ocean by the end of the day (top row, Figure 7). The convection intensified when the two lines joined, resulting in very cold cloud tops and increased frequency of lightning. A wide line of very intense storms stretched northwest to southeast through the DYNAMO array by 0000 UTC on 14 January (right column, Figure 7). Although deep convection moved closer to Gan than at any point this year, the deepest echoes stayed outside of S-PolKa's maximum unambiguous range (bottom row, Figure 7). The tall and intense storms nearby did occasionally fill the radar display with second-trip echo (Figure 8). While most of the radar observations were non-precipitating clouds and multi-trip echo, S-PolKa did record measurable precipitation for the first time in more than 10 days. Rain came from scattered small cells that generally lasted for about an hour (left column, Figure 8). The most frequent 10 dBZ echo top height measured by S-PolKa was just under 5 km (slightly above the freezing level), with the deepest storms topping out between 8 and 10 km (left panel, Figure 9). Precipitation was generally light rain and drizzle (right panel, Figure 9). Moderate and heavy rain was observed, but far less frequent. The precipitating cells closest to the radar during the day conformed to these broad statistics very well (Figure 10). S-PolKa observed two clear cases of convection forming along a propagating outflow boundary. The first occurred near midday, with the boundary moving in from the north and shallow storms forming in a line oriented east to west (Figure 11). Convection in this event was completely gone by 1100 UTC. The second event began at about 1500 UTC as another line of clouds formed along an outflow boundary again moving toward the radar from the north (Figure 12). In this case the cells forming along the boundary never really strengthened--maximum reflectivity was <15 dBZ. Lesser surface instability at night and stronger directional wind shear between 1 and 2 km may have contributed to the differences in intensity between the two cases (left panel, Figure 2). |
 |
| Figure 1. CIMMS MIMIC Total Precipitable Water (TPW) product for 1800 UTC on 13 January. |
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 |
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| Figure 2. Upper-air soundings from Gan from 0235 UTC (far left) to 2035 UTC (far right) 13 January. |
 |
| Figure 3. Weekly time series of the
soundings from Gan from 6 January through 13 January. |
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| Figure 4. CPC 200 hPa velocity potential
anomaly overlaid on infrared satellite imagery for 12
January. |
 |
 |
 |
| Figure 5. 13 January 1800 UTC analysis
from the French ARPEGE model at 850 hPa (left), 500 hPa
(center), and 200 hPa (right). (Note: The array
drawn on these maps no longer reflects the true position
of the Revelle or the Mirai as these ships have left
station). |
 |
| Figure 6. Daily time-series of wind
speed (green line) and wind direction (blue line) from
Gan on 13 January. |
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 |
 |
 |
 |
| Figure 7. Top Row: Satellite IR
measurements and lightning activity indicators (purple
hatches) at 0000 UTC (left) and 1200 UTC (center) on 13
January, and 0030 UTC (right) on 14 January. Bottom Row:
Satellite IR Measurements with S-Pol radar reflectivity
with a 150 km range ring at the same times. |
 |
|
 |
 |
| Figure 8. S-Pol radar reflectivity alone
(left) and overlaid on satellite IR measurements (right)
at 0000 UTC (top) and 1800 UTC (bottom) on 13 January. |
 |
 |
| Figure 9. S-PolKa derived echotop heights
for convective (top left) and stratiform (bottom left)
precipitation. Particle type frequency at different
altitudes (right) for 13 January. |
 |
 |
 |
| Figure 10. PPI S-Pol radar reflectivity
(left), and RHI cross-sections of reflectivity (center)
and particle ID (right) near 2145 UTC on 13 January. |
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 |
 |
| Figure 11. S-PolKa radar reflectivity
observations of storms forming along an outflow boundary
north of the radar, from 0800 (far left) to 1030 UTC (far
right) 13 January in 30 minute increments. |
 |
 |
 |
 |
 |
| Figure 12. S-PolKa radar reflectivity
observations of storms forming along an outflow boundary
north of the radar, from 1500 (far left) to 1700 UTC (far
right) 13 January in 30 minute increments. |
| The northern part of the DYNAMO
array transitioned from shallow to deep convection
during the day. The convective band oriented west-east
across the basin has moved closer to the equator as the
200 hPa velocity potential weakened and moistening
occurred over the equatorial region of the Indian Ocean
(Figure
1; Figure
2). Gan was near the confluence region at 850 hPa
where northerly components on the western side of the
cyclonic circulation merged with the westerly jet
positioned just to the south of the equator (left
panel, Figure
3). At 500 hPa, Gan was in the region of
southwesterlies that extended from 70°E on the south
of equator to the Bay of Bengal (middle
panel,
Figure 3). Further aloft, at 200 hPa, the northern
Indian Ocean was dominated by divergence over the Bay of
Bengal and convergence over the Arabian Sea (right
panel, Figure
1). The 200 hPa winds over the central Indian Ocean
changed from southwesterlies to southeasterlies across
the equator (right
panel, Figure
3). Soundings from Gan showed upper-level moistening before 0900 UTC and winds changing from southerlies to southwesterlies (Figure 4). The air dried out slightly after 1500 UTC and had a reduced westerly wind component. Below 850 hPa, the winds over Gan were mostly northerlies and northeasterlies, with a maximum near-surface wind speed up to 10 m/s. Between 850 and 450 hPa there was a transition to westerlies and southwesterlies (Figure 4). The air between 500 hPa and 750 hPa was significantly moistened by 0900 UTC. The layer of easterlies from 450 to 300 hPa was collocated with very dry air. Diego Garcia continued to be quite moist (Figure 5). The winds through the entire column frequently changed direction. The region between the equator and 10ºS in the western Indian Ocean was filled with intense deep convection (Figure 6). These storms spilled over into the DYNAMO array, finally bringing long-awaited precipitation to Addu Atoll. S-PolKa recorded an area-averaged rainfall total of 12.3 mm for the day, of which approximately 70% was classified as convective and the rest stratiform precipitation (Figure 7). Most of the rainfall came in the latter part of the day. In contrast with yesterday, large regions of heavy and moderate rain were observed (Figure 8). Almost all storms rose well above the 0 deg C level, generating lots of ice crystals both regularly and irregularly shaped. Echo-top heights of 10 dBZ reached up to 15 km for both convective and stratiform classified cells (Figure 9). Echo tops between 10 and 15 km were commonly observed. Early morning precipitation moved in from the south and west of S-PolKa, while the area to the northeast was mostly covered by lines of non-precipitating clouds aligned NE-SW (Figure 10). These non-precipitating cloud lines were parallel to the low-level northeasterlies seen in the Gan soundings (Figure 4). Radial velocity estimates showed low-level winds with northerly components over most of the domain, but westerly winds on the far western edges of the domain (right panel, Figure 10). The radial velocities were thus consistent with model analysis for the day (left panel, Figure 3). A lull in the precipitation occurred around midday, followed by a gradual increase in isolated convection late in the afternoon (Figure 11). The deepest convection remained to our south during this time (bottom row, Figure 11). At 1800 UTC northerly winds continued to dominate most of the radar domain (left panel, Figure 12). A line of relatively shallow convection with a large trailing stratiform region approached S-PolKa from the north (Figure 12). Three hours later the winds were out of the northwest and the line from the north merged with a series of cells from the east to form a long line of intense convection stretching north-south across the entire radar domain (Figure 13). An RHI scan through a 14 km deep cell to our southeast showed a well-defined signature of leading convection and trailing stratiform (bottom left panel, Figure 13). The velocity cross-section was indicative of a descending rear-inflow jet with maximum velocity close to 20 m/s near the surface (bottom center panel, Figure 13). The stratiform region contained smaller ice up to heights of nearly 16 km, dry aggregates intermittently up to about 10 km, and a layer of wet aggregates dominating the melting layer (bottom right, Figure 13). Small amounts of graupel were evident in the upper and lower parts of the bright band, as we have seen previously in episodes of deep convection and stratiform precipitation. |
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| Figure 1. CPC 200 hPa velocity potential
anomaly overlaid on infrared imagery for 13 January. |
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| Figure 2. CIMSS MIMIC total precipitable
water for 2000 UTC 14 January. |
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| Figure 3. IMD WRF-ARW 0000 UTC
analyses for 14 January at 850 hPa (left), 500 hPa
(center), and 200 hPa (right). |
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| Figure 4. Gan soundings for 0300-2100 UTC
on 14 January. |
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| Figure 5. Soundings from Diego Garcia at
0000 UTC (left) and 1800 UTC (right) on 14 January. |
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| Figure 6. Infrared satellite imagery
overlaid with WWLN lightning data for 2200 UTC on 14
January. |
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| Figure 7. Hourly rainfall estimates from
S-Pol for 14 January. |
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| Figure 8. Particle ID statistics from
S-Pol for 14 January. |
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| Figure 9. Echotop height frequencies for
different intensities of convective (top) and stratiform
(bottom) precipitation on 14 January. |
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| Figure 10. S-Pol radar reflectivity over
laid on satellite IR imagery (left), radar reflectivity
(center), and radial velocity (right) near 0600 UTC
(1100 LT) on 14 January. |
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| Figure 11. S-Pol radar reflectivity at
0900 UTC (1400 LT, top left), 1200 UTC (1700 LT, top
center), and 1500 UTC (2000 LT, top right). S-Pol
reflectivity over laid on satellite IR imagery for the
same times (bottom). |
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| Figure 12. PPI views of radial velocity
(left) and radar reflectivity (center), and an RHI of
reflectivity (right) at 1800 UTC on 14 January. |
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| Figure 13. S-PolKa
PPI reflectivity (top left) and velocity (top center),
satellite infrared imagery overlaid with S-PolKa PPI
reflectivity (top right). RHI reflectivity, velocity
and particle ID (bottom row) for 2322 UTC on 14
January. |
