DYNAMO/AMIE S-PolKa Scientist Summaries - October 2011
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OTHER MONTHS:
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| The central Indian Ocean seems to be going
into a dry phase. Both ECMWF and NCEP indicate this trend
(See Figure 1). Several days ago it
was overcast and showery here. At the present time,
satellite imagery indicates cloudiness is located just
south of our area (See Figure 2).
We will now be looking for how the atmosphere humidifies
during the buildup toward the next MJO disturbance. The
humidity is consistently high in the boundary layer here.
Note the high values everywhere. Figure 3
is a typical water vapor channel satellite image over the
Indian Ocean. The humidity at low levels is so high
that it attenuates the radar beam at a rate of 0.5 db per
km. The variability in humidity with phase of MJO is
primarily at mid-to-upper levels. We'll be looking for the
occurrence of dry layers at midlevels and for the
processes that lead to their moistening. Large-scale moisture convergence is small or zero in midlevels during MJO buildup over the Indian Ocean. If that's true, then humidification on the cloud scale may be an important factor. So we need to pay close attention to the cloud structures that may be producing the humidification. Today we saw a nice example of an isolated cumulonimbus with an anvil at 8-12 km altitude. The images in Figures 4-7 show the anvil at both S-band and Ka-band. The PPIs are at 11 degree elevation. This anvil had a bright band and was associated with shearing between low-level westerlies and higher-altitude easterlies. Note how the attenuation in the melting layer completely eliminates the Ka-band signal above the bright band (cf. Figures 5 and 7). The anvil had a smooth streaky appearance in the PPI, likely associated with the strong shear. This type of anvil behavior may be involved in humidification in midlevels. Note that shear may be involved in the humidification process if this type of cloud is a key player. The rings seen below the anvil are echoes produced layers of strong humidity gradient and are further discussed in the summaries for 6 November and 29 November. |
| Figure 1.
ECMWF OLR combined analysis and forecast. Forecast is
after the horizontal white line. |
| Figure 2. IR image over Indian Ocean. |
| Figure 3.
METEOSAT image for Indian Ocean. |
| Figure 4.
S-band 11 degree elevation reflectivity PPI for
0550 UTC 2 October 2011. |
| Figure 5. S-band RHI of reflectivity at 0550 UTC 2 October 2011. |
| Figure 6. Ka-band 11 degree elevation reflectivity PPI for 0550 UTC 2 October 2011. |
| Figure
7. S-band RHI of PID from S-PolKa. 0958 UTC 3
October 2011. |
| Satellite data over the Indian Ocean showed
us to be in a cloudier pattern today (Figure
1). Midlevels were quite dry (Figures 2-3). The clouds that formed
were from a field of cumulus in which some individual
towers pentrated through the dry layer and reached about
16 km. The anvils were thin and exhibited mammatus (Figures 4-5). As the anvils
aged they extended laterally great distances, but the
mammatus structures dissappeared (Figure
6). The remains of the anvil formed a long streamer
of cirrostratus (Figure 7). On
radar, the cloud shown in the previous photos formed an
extremely narrow cell of reflectivity from the surface to
about 15 km (Figures 8-9). On radar
the anvil had a smooth echo structure with values as high
as 25 dbZ in the lower portion and lower rather uniform
values at higher levels (Figure 10).
The particle ID algorithm (Figure 11)
shows heavy rain in the lower-level core echo. The anvil
consists entirely of low reflective ice particles. In the
lower anvil, the particles have sufficiently high ZDR to
indicate that they are horizontally oriented (light blue).
The remainder of the anvil is low-reflectivity ice with no
preferred orientation. The radial velocity data in the
anvil (Figure 12) show that the
anvil is occuring a layer of strong and complex shear. The
reversals of direction correspond well to those seen in
the sounding in Figure 3. |
| Figure 1. IR image over Indian Ocean. |
| Figure 2.
Gan sounding 2330 UTC 2 October 2011 |
| Figure 3.
Gan Sounding 0550 UTC 3 October 2011. |
| Figure 4.
Photo looking SE of S-PolKa, 0846UTC 3 October
2011 |
| Figure 5.
Photo looking SE of S-PolKa, 0847UTC 3 October
2011. Note mammatus |
| Figure 6.
Photo from S-Polka 0340 UTC 4 October 2011.
Looking overhead and slightly south at anvil streaming
westward |
| Figure 7.
Photo looking SE of S-PolKa, 1005UTC 3 October
2011. Remains of anvil. |
| Figure 8.
S-band RHI at 0943 UTC 3 October 2011. Azimuth
122 deg. |
| Figure 9. S-band RHI at 0943 UTC 3 October 2011. Azimuth 124 deg. (The "worm echo.") |
| Figure 10.
S-band RHI of reflectivity from S-PolKa. 0958
UTC 3 October 2011. |
| Figure 11.
S-band RHI of PID from S-PolKa. 0958 UTC 3
October 2011. |
| Figure 12. S-band RHI of radial velocity from S-PolKa. 0958 UTC 3 October 2011. |
| METEOSAT IR imagery (Figure
1) showed a swath of small convective clouds across
Addu Atoll. The convection across the region is oriented
in SW-NE lines, more or less along the shear. The 3 hourly
Gan soundings showed moistening throughout the day.
Again today we saw thin deep cored convective cells
reaching about 15 km height and streaming westward (Figures 2-5).
The photo series in Figures 6-9
shows a panorama from east-overhead-west showing the anvil
of one of these cells streaming overhead. Today the cells
formed into lines. One was located north of S-PolKa and
oriented SW-NE. Figure 10 shows
this line of cells as seen from the S-PolKa site. Note
that it had the same kind of convective-scale cells with
thin but robust high-level anvils as we have seen in
isolated cells. Figure 11 zooms in
on one of the thin anvils in a young stage. Note that,
like yesterday, the anvils exhibited mammatus. Figure 12 shows the SW-NE oriented
line seen in the photos as it appeared on the S-PolKa
S-band radar at about 08 UTC. The yellow line shows the
location of a high-resolution RHI scan, which is
illustrated in Figures 13-15. Figure 13 shows the reflectivity core
of a convective cell penetrating into the anvil of another
cell. Figure 14 shows its
microphysical structure of both the lower portion of the
active convective cell and and the anvil as determined by
the particle ID (PID) algorithm of Vivekanandan et al.
(1999). Heavy rain (red) and possibly some graupel (dark
green) are identified in the lower portion of the
cell. The anvil consists of low reflectivity ice
particles, some of which are horizontally oriented (light
blue) and some of which show no orientation (pink). The
radial velocity data (Figure 15)
show that the anvil lies in a layer of strong shear
between southwesterly wind components below and
northeasterly components above. Figure
16 shows a view of the convective line, and Figure 17 shows a vertical cross
section of the PID through one of the cells. The dark
green in the upper part of the cell and also near the
melting level indicates that graupel was likely present.
The dark green and blue near the melting level,
however, might also indicate melting
and non-melting aggregates at that level. Figures 18-19 show the passage of
the line of convection over the S-PolKa site as seen
in S-band reflectivity PPIs. A "fineline" marked the
edge of the approaching gust front. The fineline was on
the north edge of the atoll at 0946 UTC and beyond the
south edge of the atoll at 1101 UTC. The convective line
moved by a discrete jump between these two times. The
older line weakened and new cells formed to the south.
S-PolKa lay between the new cells to the southeast and the
old line to the northwest. Figure 20
shows the roll cloud at the fineline as see from the
S-PolKa site. Figure 21 shows two
cells forming SE of S-PolKa, where the line was reforming
by discrete propagation. |
| Figure 1. IR image over Indian Ocean. |
| Figure 2.
Gan sounding 0228 UTC 4 October 2011 |
| Figure 3.
Gan Sounding 0528 UTC 4 October 2011. |
| Figure 4.
Gan Sounding 0834 UTC 4 October 2011. |
| Figure 5.
Gan Sounding 1134 UTC 4 October 2011. |
| Figure 6.
Photo looking east from S-Polka 0340 UTC 4
October 2011 |
| Figure 7.
Photo looking southeast from S-Polka 0340 UTC 4
October 2011. Note anvil streaming westward |
| Figure 8.
Photo from S-Polka 0340 UTC 4 October 2011.
Looking overhead and slightly south at anvil streaming
westward |
| Figure 9.
Photo from S-Polka 0340 UTC 4 October 2011.
Looking southwestward at anvil streaming westward |
| Figure 10.
Photo from S-Polka 0435 UTC 4 October 2011.
Looking north from the S-PolKa site. |
| Figure 11.
Photo from S-Polka 0436 UTC 4 October 2011.
Zoomed in look at anvil with mammatus. |
| Figure 12.
PPI of S-Polka S-band reflectivity at 0809 UTC 4
October 2011. |
| Figure 13. RHI of S-Polka S-band reflectivity at 0809 UTC 4 October 2011, along yellow line in previous figure. |
| Figure 14. Same as previous figure except for polarimetric particle identification. |
| Figure 15. Same as previous figure except for radial velocity. |
| Figure 16.
PPI of S-Polka S-band reflectivity at 0707 UTC 4
October 2011. |
| Figure 17.
RHI of S-Polka S-band PID at 0722 UTC 4 October
2011. |
| Figure 18.
PPI of S-Polka S-band reflectivity at 0946 UTC 4
October 2011. Note fineline north of the atoll. |
| Figure 19.
PPI of S-Polka S-band reflectivity at 1101 UTC 4
October 2011. Note fineline is now south of the atoll. |
| Figure 20.
Photo at 1034 UTC 4 October 2011 looking ENE of
S-PolKa. It shows the roll cloud at the gust front as the
convective line neared S-PolKa. The roll cloud probably
corresponds to the fineline seen in the previous figures. |
| Figure 21. Photo at 1053 UTC 4 October 2011 looking ESE of S-PolKa. It shows two new cells where the convective line is reforming by discrete propagation to the SE of the atoll. |
| The conditions over the Indian Ocean in
general have not changed much, but the region surrounding
Gan is free of high cloudiness
(Figure 1).
The sounding showed dryness a all levels (Figure 2). |
| Figure 1.
IR image over Indian Ocean. Cloud activity is located
east and west of Gan. |
| Figure 2.
Gan sounding 0600 UTC 5 October 2011 |
| The Gan sounding shows a deep layer of dry
air (Figure 1). IR satellite imagery
shows the DYNAMO array to be in a cloudless void with
scattered deep convection to the east, south, and west (Figure 2). Although the
Indian Ocean is convectively suppressed in general, the
deep cells that are occurring are robust enough to be
producing lightning (crosses). Later in the day the cloudy
area to the east had moved somewhat westward, almost
encroaching on the DYNAMO array (Figure
3). The Revelle C-band radar was picking up echo at
long range from the this cloudy area (Figure
4). At the S-PolKa site the clouds were very small
cumuli in the morning and into midday (Figure
5). In the afternoon, they increased in size
somewhat and formed into coherent lines (Figure
6). These lines were evident in the S-PolKa S-band
reflectivity as cloud lines (Figure 7).
These lines were the locus of new cell development (Figure 8). |
| Figure 1.
Gan sounding 2330 UTC 2 October 2011 |
| Figure 2.
IR image over Indian Ocean, 0100 UTC 6 October. WLLN
lightning shown by crosses |
| Figure 3.
IR image over Indian Ocean, 1100 UTC 6 October. WLLN
lightning shown by crosses |
| Figure 4.
Revelle C-band radar surveillance scan image, 1059 UTC 6
October 2011. |
| Figure 5.
Photo looking NE of S-PolKa, 0647UTC 3 October
2011 |
| Figure 6.
Photo looking SE of S-PolKa, 0925UTC 3 October
2011. Note mammatus |
| Figure 7.
S-band PPI at 1001 UTC 6 October 2011. |
| Figure 8.
S-band PPI at 1116 UTC 6 October 2011. |
| Today I'm looking at the large-scale pattern
surrounding S-PolKa and especially the dry suppressed
conditions now affecting the region. Figure
1 shows the relative humidity time-height series for
the first week of the project. Moist conditions at upper
levels on 3 October is consistent with the anvils describe
in the 3 October S-PolKa Science Summary, and the deep
moist conditions on 4 October are consistent with
convective line activity describe in the 4 October
Summary. After 4 October, severe drying in midlevels has
come into play, consistent with all the summaries from 5
October onward. Figures 2 and 3
show the latitudinal and longitudal cross sections of the
circulation over the Indian Ocean and West Pacific during
21-27 September (a little out of date, but shows the
general idea of what is going on). Note how there is very
little midlevel convergence over the Indian Ocean. This
fact suggests that humidification in midlevels is likely
associated with cloud-scale processes. The near-surface
wind is weak westerly (Figure 4). At
850 mb the wind is westerly but a little stronger (Figure 5). At 700 mb the wind is
northwesterly and dry air is intruding over Gan (Figure 6). The flow over Gan is very
weak at 500 mb (Figure 7). At 200 mb
the flow has reversed to easterly (Figure 8).
The sounding at Gan shows a deep layer of dry air (Figure 9). Figure
10 shows the IR image with WWLN lightning flashes
(crosses) superimposed. The S-PolKa radar lies in a region
free of high clouds in the IR images, and the WWLN data
indicates that where even small cells of deep clouds occur
they are becoming electrified, which indicates graupel
production. They likely have anvils like those we saw on
3-4 October. The METEOSAT water vapor channel shows
areas of medium brightness around the cores of cells as
they age; see for example the cell near -5 S and -55 E at
the two times in Figures 11 and 12. It
is not clear how much this signal represents water vapor
enhancement around the anvils or thin cloud. In the
vicinity of S-PolKa, the clouds early in the day were
small scattered cumulus (Figure 13).
Later in the day, some larger buildups could be seen in
the distance (Figure 14). These
buildups continued until evening and produced some small
patches of anvil (Figure 15). |
| Figure 1.
Interpolated relative humidity from Gan DOE soundings.
Provided by Sally McFarlane. |
| Figure 2. Pressure-longitude cross section for week of 21-27 September. |
| Figure 3.
Pressure-latitude cross section for week of 21-27
September. |
| Figure 4.
NCEP GFS 10 m winds and precipitation for 0100 UTC 7
October 2011. |
| Figure 5. NCEP GFS 850 mb winds and precipitation for 0100 UTC 7 October 2011. |
| Figure 6. NCEP GFS 700 mb winds and precipitation for 0100 UTC 7 October 2011. |
| Figure 7. NCEP GFS 500 mb winds and precipitation for 0100 UTC 7 October 2011. |
| Figure 8. NCEP GFS 200 mb winds and precipitation for 0100 UTC 7 October 2011. |
| Figure 9.
Gan sounding 0300 UTC 7 October 2011. |
| Figure 10.
METEOSAT IR image for 0730 UTC and previous 6 h of WWLN
lightning strikes (+'s). |
| Figure 11.
METEOSAT IR image for 0730 UTC and previous 6 h of WWLN
lightning strikes (+'s). |
| Figure 12.
METEOSAT IR image for 0730 UTC and previous 6 h of WWLN
lightning strikes (+'s). |
| Figure 13.
Cloud photo looking NE of S-PolKa at 0615 UTC 7 October
2011. |
| Figure 14. Cloud photo looking E of S-PolKa at 0740 UTC 7 October 2011. |
| Figure 15. Cloud photo looking SSW of S-PolKa at 1303 UTC 7 October 2011. |
| Dry conditions aloft over Gan are continuing
(Figure 1). IR imagery shows that
S-PolKa is far from any major convection (Figure
2). However, visible images show that there are a
lot of smaller cumulus clouds in the moist layer all over
the Indian Ocean (Figure 3). A zoomed
in view of the visible image of the region around S-PolKa
(Figure 4) shows numerous small
cumulus clouds, somewhat organized intpo SW-NE lines.
Fragments of these clouds can sometimes be seen
evaporating (Figure 5). Some clouds
were growing to moderate heights (~8 km) and producing
small anvils (Figure 6). This anvil
detached and was left aloft and dropping virga into the
dry air below (Figure 7). One of the
taller cells reaching nearly 10 km is seen in Figures 8 and 9. One strong rain
cell SE of S-PolKa (Figure 10)
produced a sharply defined cold pool (Figure
11). New cells were triggering on the boundary of
the cold pool. The cold pool was clearly evident in the
ZDR field, evidently as a result of drying of the air,
eliminating the occurrence of Bragg scattering at S-band (Figure 12). |
| Figure 1.
Gan sounding for 0300 UTC 8 October 2011. |
| Figure 2.
METEOSAT IR image for 0830 UTC 8 October 2011. |
| Figure 3. METEOSAT visible image for 0830 UTC 8 October 2011. |
| Figure 4. METEOSAT visible image for 0830 UTC 8 October 2011. |
| Figure 5. Photo looking south from S-PolKa site 0705 UTC 8 October 2011. |
| Figure 6. Photo looking NE from S-PolKa site 0706 UTC 8 October 2011. |
| Figure 7. Photo looking NE from S-PolKa site 0751 UTC 8 October 2011. |
| Figure 8.
S-PolKa S-band PPI at 1.5 deg elevation at 0731 UTC 8
October 2011. |
| Figure 9. S-PolKa S-band RHI at 60 deg azimuth at 0725 UTC 8 October 2011. |
| Figure 10. S-PolKa S-band reflectivity PPI at 0.5 deg elevation at 0602 UTC 8 October 2011. |
| Figure 11. S-PolKa S-band reflectivity PPI at 0.5 deg elevation at 0832 UTC 8 October 2011. |
| Figure 12. S-PolKa S-band PPI of ZDR at 0.5 deg elevation at 0832 UTC 8 October 2011. |
| Over the region of Addu Atoll (1S, 73E), the
large-scale circulation is generally weak at upper levels.
The 200 and 500 hPa flow is variable in direction (Figures 1 and 2).
At 700 hPa, the winds are weak westerly (Figure
3). The flow is clearly westerly at 925 hPa (Figure 4). The winds in the Gan
sounding for 0300 UTC 9 October 2011 are consistent with
the patterns seen on the maps (Figure 5).
The sounding is again very dry above 850 hPa, but the
layer below 850 mb is slightly moister than yesterday. The
water vapor channel image shows Gan to be still in the
very dry zone north of the ITCZ (Figure 6).
The IR channel shows no high cloudiness in the vicinity of
Gan (Figure 7). However, the visible
image in Figure 8 shows a band of
cumulus and/or isolated cumulonimbus extending over Gan.
From about 0830 UTC until later in the day, several of
these clouds were seen from the S-PolKa site, visually and
on radar, to be developing vertically and occasionally
producing isolated heavy rainshowers. A good example is
shown in a sequence of cloud photos in Figures
9 and 10. Radar data
from this storm are shown in the next few figures. The
yellow line in the PPI in Figure 11
shows the location of an RHI cross section through three
cells associated with this storm. The reflectivity cross
section shows that although these reflectivity cells were
reaching only about 8 km, they exhibited reflectivities of
up to ~60 dBZ (Figure 12). The cross
sections of radial velocity show divergence near the tops
of the cells (Figure 13). The ZDR
cross section shows positive values (yellow) indicating
large raindrops a in the middle levels of the cells and
near the surface (Figure 14). The
LDR cross section shows a subtle indication of graupel in
the middle cell between the 3 andd 5 km levels, where
slightly less negative values were occurring (Figure 15). The RhoHV
(horizontal-vertical correlation cofficient is less
positive in the same region, indicating a mix of rain and
graupel (Figure 16). The particle
identification based on all the polarimetric variables
indicates graupel as likely at the 5 km level (light green
in Figure 17). |
| Figure 1.
200 hPa winds for 0000 UTC 9 October 2011. |
| Figure 2.
500 hPa winds for 0000 UTC 9 October 2011. |
| Figure 3.
700 hPa winds for 0000 UTC 9 October 2011. |
| Figure 4.
925 hPa winds for 0000 UTC 9 October 2011. |
| Figure 5.
Gan sounding for 0300 UTC 8 October 2011. |
| Figure 6.
METEOSAT 10 micron water vapor image for 1130 UTC 9
October 2011. |
| Figure 7. METEOSAT IR image for 1130 UTC 9 October 2011. |
| Figure 8. METEOSAT visible image for 1130 UTC 9 October 2011. |
| Figure 9.Photo looking ESE from S-PolKa site 1036 UTC 9 October 2011. |
| Figure 10. Photo looking ESE from S-PolKa site 1108 UTC 9 October 2011. |
| Figure 11. S-PolKa S-band PPI for 1058 UTC 9 October 2011. |
| Figure 12. S-PolKa S-band reflectivity RHI for 1058 UTC 9 October 2011. |
| Figure 13. S-PolKa S-band radial velocity RHI for 1058 UTC 9 October 2011. |
| Figure 14. S-PolKa S-band RHI of ZDR for 1058 UTC 9 October 2011. |
| Figure 15. S-PolKa S-band RHI of LDR for 1058 UTC 9 October 2011. |
| Figure 16. S-PolKa S-band RHI of RhoHV 1058 UTC 9 October 2011. |
| Figure 17. S-PolKa S-band RHI of RhoHV 1058 UTC 9 October 2011. |
| The Gan sounding today still shows very dry
conditions in midlevels above a moist layer below 800 hPa
(Figure 1). The METEOSAT water vapor
channel image also shows Gan in a very dry zone (Figure 2). The IR image shows all major
deep convection areas to be far distant from Gan (Figure 3). The visible image, however,
shows and interesting band of lower-topped isolated
cumulus and/or cumulonimbus extending across the Gan area
(Figure 4). The behavior of the clouds
near the S-PolKa site were probably typical of this belt
of clouds. The photo in Figure 5 shows
a typical example of the cumulus cloud field midday. The
clouds were numerous and building actively, though mostly
not reaching great heights. But we observed several clouds
growing into cumulonimbus towers with rainshowers. Figure 6 shows a typical example as
it appeared to us at the S-PolKa site. Figure
7 shows the radar echo from an example of one of
these cumulonimbus clouds in a PPI of the S-band S-PolKa.
An RHI of the reflectivity of this cell in Figure 8 shows the cell reaching 16
km. This height is greater than we've seen in convection
in this region in the last few days. The RHI of
polarimetrically determined particle type in Figure 9 shows they distribution of
hydrometeor type that is typical of the cumulonimbus cells
that we've seen so far in the project: heavy rain at low
levels (red) graupel just above the 0 deg C level (light
green) and probably smaller graupel above (light blue)
encased in smaller ice particle (pink). After the
cumulonimbus of this type that formed intermittently
through the day died out, they left behind debris in the
form of flattened stratus, altostratus, and cirrostratus
fragments (Figure 10). At around
sundown, we saw line of these cumulonimbus cells to the
north (Figure 11). During the night,
deep convective cells continued to form in a complex
pattern of variously oriented lines--see the PPIs from
S-PolKa in Figures 12-17. Heavy
rain occurred at Gan from one of these cells (Figure 16). Judging by the formation
of these cells along enhanced cloud lines seen in this
sequence of PPIs, it appears that the line orientations
were determined primarily by cold pool boundaries. The
cold pools can be seen as holes in the background
reflectivity in the various PPIs in Figures
12-17. The PPI in Figure 14
shows ZDR instead of reflectivity. The reds indicate
positive values of ZDR. The reds in the general background
"noise" stand out at night and may be a result of an
enhancement of sea clutter (echoes from ocean waves) owing
to changing bending of the radar beam in the boundary
layer at night. The strong reds in the fineline SW of
S-PolKa might be due to birds flocking along the fineline.
These ZDR behavior in the fineline and background noise
are new observations and these possible explanations are
speculative. |
| Figure 1.
Gan sounding for 0300 UTC 10 October 2011. |
| Figure 2. METEOSAT 10 micron water vapor image for 0900 UTC 10 October 2011. |
| Figure 3.
METEOSAT IR image for 0900 UTC 10 October 2011. |
| Figure 4. METEOSAT visible image for 0900 UTC 10 October 2011. |
| Figure 5. Photo looking NE from S-PolKa site 0934 UTC 10 October 2011. |
| Figure 6. Photo looking SE from S-PolKa site 0833 UTC 10 October 2011. |
| Figure 7. S-PolKa S-band PPI for 0945 UTC 10 October 2011. |
| Figure 8. S-PolKa S-band reflectivity RHI for 0941 UTC 10 October 2011. |
| Figure 9. S-PolKa S-band PID at 0941 UTC 10 October 2011. |
| Figure 10. Photo looking ESE from S-PolKa site 1207 UTC 10 October 2011. |
| Figure 11. Photo looking ESE from S-PolKa site 1253 UTC 10 October 2011. |
| Figure 12. S-PolKa S-band PPI for 1201 UTC 10 October 2011. |
| Figure 13. S-PolKa S-band PPI for 1701 UTC 10 October 2011. |
| Figure 14. S-PolKa S-band PPI of ZDR for 1701 UTC 10 October 2011. |
| Figure 15. S-PolKa S-band PPI for 1831 UTC 10 9 October 2011. |
| Figure 16. S-PolKa S-band PPI for 1906 UTC 10 October 2011. |
| Figure 17. S-PolKa S-band PPI for 2306 UTC 10 October 2011. |
| The Gan sounding continues to show a moist
layer at low levels, now extending up to nearly 700 mb (Figure 1). The METEOSAT water vapor
channel now shows a weak to moderate level of signal over
the array over and to the east and south of Gan (Figure 2). This signal may be increased
precipitable water, but may also be influenced by the thin
veil of cirrus overhead. The IR image does not show any
significant area of high cloudiness over the region of Gan
and the NE corner of the array (Figure 3).
The visible imagery shows isolated convective cells all
around the region (Figure 4).
The cirrus is evidently too thin to see in this image. The
sky around the S-PolKa site was clear this morning,
seemingly cleared out as a result of yesterday's slightly
elevated convective activity (Figure 5.
Note the patch of high cirrus in the picture. The cirrus
is generally hard to see in photos, but it was there.
Later in the day, small cumulus started to form in the
moist layer, and the cirrus remained evident at high
levels (Figure 6). As the day wore
on, some of the small cumulus grew into small cumulonimbus
clouds (Figure 7). Figures
8-10 show how this small cumulonimbus appeared on
the S-PolKa S-band radar. The reflectivity cells were
topping out at about 8 km (Figure 9).
Like other cumulonimbus we've seen, these cells showed
enhanced ice particle presence just above the 0 deg C
level (Figure 10), but they were too
week to be producing graupel. The conditions may have
remained suppressed by the after effects of the convection
of yesterday, and possibly by the cirrus shielding of the
daytime solar heating. As we have noted in previous
discussions, the low-level boundary-layer echo is
suppressed by convective showers; the cold pools of the
convective-scale downdrafts are seen as "holes" in the
background echo at low elevation angles. Figures 11 and 12
show a cross section through one of these boundary layer
holes, seen at 70-80 km range. In Figure
12, note the complete absence of low-level echo in
the hole. Also note enhancement of echo at the outer edges
and somewhat above the hole, which are likely due to Bragg
scattering off nonprecipitating clouds enhanced at the
boundaries of the cold pool. |
| Figure 1.
Gan sounding for 0300 UTC 11 October 2011. |
| Figure 2.
METEOSAT 10 micron water vapor image for 0830 UTC 11
October 2011. |
| Figure 3. METEOSAT IR image for 0830 UTC 11 October 2011. |
| Figure 4. METEOSAT visible image for 0830 UTC 11 October 2011. |
| Figure 5. Photo looking NE from S-PolKa site 0400 UTC 11 October 2011. |
| Figure 6. Photo looking ESE from S-PolKa site 0620 UTC 11 October 2011. |
| Figure 7. Photo looking ESE from S-PolKa site 0908 UTC 11 October 2011. |
| Figure 8. S-PolKa S-band reflectivity PPI for 1116 UTC 11 October 2011. |
| Figure 9. S-PolKa S-band reflectivity RHI for 1127 UTC 11 October 2011. |
| Figure 10. S-PolKa S-band PID at 1127 UTC 11 October 2011. |
| Figure 11. S-PolKa S-band reflectivity PPI for 1216 UTC 11 October 2011. |
| Figure 12. S-PolKa S-band reflectivity RHI for 1222 UTC 11 October 2011. |
| The large-scale conditions in which
the DYNAMO/AMIE radars are operating are summarized by
the maps in Figure 1. Note
the anticyclonic gyres at 200 hPa to the north and south
of the experimental array. Gan lies under the easterlies
between the two gyres. At lower levels (e.g. 925 hPa)
the flow is moderate southwesterly between two cyclonic
gyres. The humidity profile over Gan since the beginning
of October is shown in Figures
2 and 3. The
moist layer below about 1.5 km is ever present. Perhaps
the most interesting new result is the appearance of a
persistent moist layer at about 4 km. The anomaly
pattern in Figure 3 shows
that the excessively dry conditions seen in midlevels
for the past week or so is moderating. The Gan sounding
shows the lower moist layer extending to about 600 hPa (Figure 4). The METEOSAT water
vapor channel is consistent with an increasing amount of
moisture, with Gan lying in the moderate values of total
precipitable water (Figure 5). The
infrared image shows patchy high clouds over the S-PolKa
domain, with no major regions of wide-spread
high cloud tops, while the more disturbed region
associated with the ITCZ remains about 350 km to the
south of Gan. (Figure 6). The
visible image shows a band of isolated convective cells
extending across the Gan region, with the ITCZ remaining
south of Gan but north of Diego Garcia
(Figure 7). When we arrived at the
S-PolKa site, we could see cirrostratus to the south
from the convection far to the south
(Figure 8). During most of the day
we could see thin cirrus and cirrostratus overhead
(Figures 9
and 10>).
Figure 10 also shows a typical
example of the cumulus on this day; they were growing
more vigorously than they have the last couple
days. Figure11 shows a closer view of
another example. Some of these developed into small
cumulonimbus (Figure 12).
During the day convection first developed to the north
of S-PolKa and then later to the
south. Figure 13
shows a time when cells
were present both north and south. During the night, the
convection to the north intensified and expanded
laterally--the first example of mesoscale organization
(albeit limited) that we've seen here
(Figure 14).
The RHI in Figure
15 shows reflectivity with
dBZ values in the high 50's and echo top at about 16 km
height. Polarimetry shows the lower levels to consist of
heavy rain with a column of graupel extending to 12 km
altitude (Figure 16).
The regions surrounding the intense cell formed a
melting layer evident in ZDR
(Figure 17). The radial velocity
data in Figure 18 show
a gust front where the low-level southwesterlies enter
the storm at a range of about 20 km (see the yellow
meeting green near the surface). Midlevel easterly
inflow is seen entering the storm at about the 8 km
level, and the southwesterlies are seen rising all
the way to the top of the convective cell, where there
was a strong divergence signature (the purple/red dipole
at the top of the echo). About an hour and a half later
the storm dissolved into a stratiform structure
(Figures
19, 20,
and 21). A weak bright
band is evident in the reflectivity pattern
(Figure 20), and
the polarimetric retrieval shows evidence of wet snow
(melting aggregates) at the bright band level
(Figure 21). A
seemingly chaotic pattern of convection continued
through the night over the region. The pattern of lines
of convection, however, was not really chaotic but
rather ordered by the cold pool boundaries appearing as
holes in the low-level echo
pattern. In Figure 22,
several of these holes
are pointed out, and time-lapse viewing of the radar
displays shows finelines of echo surrounding the pools
and the deeper convection arising out of the fine lines,
especially where they intersect. |
| Figure 1.
ECMWF analyses for 12 October 2011 1200UTC. |
| Figure 2.
Time series of water vapor density from DOE sounding at
Gan. Analysis by Scott Powell. |
| Figure 3.
Time series of water vapor density anomaly from DOE
sounding at Gan. Analysis by Scott Powell. |
| Figure 4.
DOE Gan sounding for 0300 UTC 12 October 2011. |
| Figure 5.
METEOSAT 10 micron water vapor image for 0830 UTC 12
October 2011. |
| Figure 6. METEOSAT IR image for 0830 UTC 12 October 2011. |
| Figure 7. METEOSAT visible image for 0830 UTC 12 October 2011. |
| Figure 8.Photo looking S from S-PolKa site at 0517 UTC 12 October 2011. |
| Figure 9.
Photo looking NE from S-PolKa site at 0753 UTC 12
October 2011. Photo by Brenda Dolan |
| Figure 10. Photo looking S from S-PolKa site at 0956 UTC 12 October 2011. |
| Figure 11.
Photo looking S from S-PolKa site
at 0755 UTC 12 October 2011. |
| Figure 12. Photo looking N from S-PolKa site at 0654 UTC 12 October 2011. |
| Figure 13. S-PolKa S-band reflectivity PPI for 1003 UTC 12 October 2011. |
| Figure 14. S-PolKa S-band reflectivity PPI for 1447 UTC 12 October 2011. |
| Figure 15. S-PolKa S-band reflectivity RHI for 1451 UTC 12 October 2011.. |
| Figure 16.
S-PolKa S-band RHI of polarimetric particle
type identificatiion for 1451 UTC 12 October 2011. |
| Figure 17. S-PolKa S-band ZDR RHI for 1451 UTC 12 October 2011. |
| Figure 18. S-PolKa S-band radial velocity RHI for 1451 UTC 12 October 2011. |
| Figure 19. S-PolKa S-band reflectivity PPI for 1601 UTC 12 October 2011. |
| Figure 20. S-PolKa S-band RHI of reflectivity for 1553 UTC 12 October 2011. |
| Figure 21.
S-PolKa S-band RHI of polarimetric particle
type identificatiion for 1553 UTC 12 October 2011. |
| Figure 22. S-PolKa S-band reflectivity PPI for 1901 UTC 12 October 2011. |
| Today featured a spectacular display of clouds viewed from the ground at the S-PolKa site, but rather moderate radar echo activity. Although the radar echoes were moderate, the humidity has remained relatively high through a deep layer, although the DOE Gan soundings show that it was somewhat drier in mid-to-upper levels during the middle of the day (Figure 1). The METEOSAT water vapor channel in Figure 2 shows Gan in a moderately moist zone, consistent with the soundings. The IR imagery shows some patchy high cloudiness over the region around Gan (Figure 3). The visible images show that this cloudiness was part of a band of isolated convective cells extending E-W across the region (Figure 4). As cumulonimbus built, they were highly sheared with elongated anvils (Figure 5). The sky was filled with middle and high level clouds (altocumulus, altostratus, cirrus, cirrostratus, and cirrocumulus), which seemed to be fragments left behind by yesterday's rather convectively active day. An example of how the sky looked most of the day is in Figure 6. The few convective clouds that developed had an appearance of vigorous growth, with sharply defined bubbles and turrets (e.g., Figure 7). However, the cells were not able to penetrate to great heights. A typical S-PolKa radar PPI pattern during the afternoon is in Figure 8. The RHI of reflectivity along the yellow line is shown in Figure 9. Cells at this time were reaching only 10 km. As we've seen throughout the project so far, the polarimetric retrieval from S-PolKa shows the cells containing a tower of ice particles (Figure 10). The green signal is probably small graupel, and the light blue is probably smaller or less concentrated graupel or other ice particles. The red is the heavy rain. By early evening (Figure 11) some cells were reaching up to about 12 km with anvil structures protruding from them (Figure 12). The hydrometeor pattern was similar to the early cumulonimbus (Figure 13). This scattered convective activity continued into the night. Figures 14, 15, and 16 show a cell about midnight exhibiting similar characteristics to those earlier in the day. Although the convection appears scattered at first glance in the PPIs of Figures 8, 11, and 14, close inspection of the low-elevation angle data reveals that the cells were occurring systematically on cold pool boundaries, and especially at intersections of such boundary. The cold pools can be seen as holes in the background echo patterns in all of those PPI displays. Figures 17 and 18 show a PPI of reflectivity dBZ paired with its corresponding PPI of differential reflectivity ZDR. The ZDR illustrates a behavior that we have noticed repeatedly in the S-PolKa observations of the tropical maritime boundary layer characterizing the central Indian Ocean region around Addu Atoll. The finelines surrounding the cold pools frequently, but not always, are marked by elevated values of ZDR (red in Figure 18). This behavior of the cold pool boundaries is most pronounced at night, although not all cold pool boundaries at night are highlighted with high ZDR values. Even in the same radar image some may be red in the ZDR images and some not. This observation is stimulating research on the boundary-layer cold pool behavior in this environment. |
| Figure 1.
DOE Gan soundings for 13 October 2011. |
| Figure 2.
METEOSAT 10 micron water vapor image for 1000 UTC 13
October 2011. |
| Figure 3. METEOSAT IR image for1000 UTC 13 October 2011. |
| Figure 4. METEOSAT visible image for 1000 UTC 13 October 2011. |
| Figure 5.Photos looking S from S-PolKa site at 0451 UTC 13 October 2011. |
| Figure 6.
Photo looking NE from S-PolKa site at 0627 UTC 13
October 2011. |
| Figure 7.
Photos looking NW from S-PolKa site at 0744 UTC 13
October 2011. |
| Figure 8. S-PolKa S-band reflectivity PPI for 1131 UTC 13 October 2011. |
| Figure 9. S-PolKa S-band reflectivity RHI for 1126 UTC 13 October 2011. |
| Figure 10.
S-PolKa S-band RHI of polarimetric particle
type identificatiion for 1142 UTC 13 October 2011. |
| Figure 11. S-PolKa S-band reflectivity PPI for 1446 UTC 13 October 2011. |
| Figure 12. S-PolKa S-band RHI of reflectivity for 1456 UTC 13 October 2011. |
| Figure 13.
S-PolKa S-band RHI of polarimetric particle
type identificatiion for 1456 UTC 13 October 2011. |
| Figure 14. S-PolKa S-band reflectivity PPI for 1901 UTC 13 October 2011. |
| Figure 15. S-PolKa S-band reflectivity PPI for 1906 UTC 13 October 2011. |
| Figure 16. S-PolKa S-band reflectivity PPI for 1906 UTC 13 October 2011. |
| Figure 17. S-PolKa S-band reflectivity PPI for 1631 UTC 13 October 2011. |
| Figure 18. S-PolKa S-band PPI of differential reflecitivity (ZDR) for 1631 UTC 13 October 2011. |
| This morning's DOE Gan sounding showed
moisture up to about 500 hPa and fairly dry conditions
above that level (Figure 1).
The METEOSAT water vapor showed moister conditions
surrounding Gan than we've seen up to now
(Figure 2). The
infrared image showed patchy high clouds to the
northwest of Gan (Figure
3). The visible image indicated that these clouds
were separated convective cells
(Figure 4). Zoomed
in, the visible image showed the cells forming at the
edges of empty zones, which were probably cold pools
(Figure 5). We
followed these images in time lapse sequence and saw
that they corresponded to empty zones on radar
(e.g. compare Figure 6
to Figure 5), and
that the convective echoes again today systematically
were forming on cold pool boundaries left behind by
previous convection. During the day clouds seen from the
S-PolKa site did not grow to extreme heights, possibly
because of the dryness aloft, but the appeared to be
growing quite vigorously
(Figures
7 and 8). Some formed
towers (probably undiluted) to slightly higher levels
(Figure 9). And some formed
small cumulonimbus clouds with heavy showers
(Figure 10). During
the night more vigorous convection developed to the
northwest of Addu Atoll, and formed mesoscale organized
squall lines. One squall line came directly over
S-PolKa. Figure 11 shows
a reflectivity PPI for 2131 UTC, just after the leading
line passed over
S-PolKa. Figure 12 shows
a set of cross sections along the yellow line
in Figure 1. These sections
display various Doppler and polarimetric variables in
the convective region of the squall line. The
reflectivity cross section shows dBZ values up to the
high 50's. The radial velocity cross section shows the
gust front convergence at a range of about 19 km. The
radial velocity pattern also indicates that rearward of
the gust front the updraft was extremely sloped,
consistent with the westerly component winds at low
levels and easterlies from midlevels upward
(Figure 1). The polarimetric
particle identification retrieval shows that the
particles in the main rain cell were a robust mixture of
large raindrops and graupel. The graupel (dark green)
extended in a narrow column up to about 6-7 km. The
positive ZDR values in the convective cells were primary
indicators of the large raindrops. Another set of cross
sections obtained at the same time, along the yellow
line in Figure 13 and
shown in Figure 14,
give a slightly different view of the convective
line. Two cells are seen in the reflectivity cross
section, at ranges of about 13 and 23 km. The leading
gust front is seen near the surface at about 26 km
range, and the sloping updraft signature is seen in the
grey and green/blue colors indicating flow toward the
radar. The yellows indicate flow away from the
radar. Note also the green patch indicating a
particularly strong divergent downdraft signature at a
range of about 13 km range associated with the stronger
cell. The particle identification algorithm was
triggered to indicate a hail signature (light green) in
the midst of the rain shaft of the primary
cell. Probably the particles were graupel of larger size
or concentration. The cell farther to the east between
20-25 km range appears to be a newer cell since within
it the ZDR signature (second row
of Figure 14) shows
near zero values in the easternmost cell, while the ZDR
is distinctly positive in the cell closer to the radar,
indicate the presence of large raindrops. The Kdp values
in this inner cell are highly positive indicating that
this cell was mature and producing heavy rain at a range
of about 13 km, consistent with the location of the
strong downdraft signature in the radial velocity cross
section. About an hour and a half after the squall line
convective region passed over, the trailing stratiform
region became
evident. Figure 15
shows the PPI pattern of reflectivity,
and Figure 16 shows
the polarimetric and Doppler variables associated with
this part of the squall line system. The easternmost
part of the echo seen at a range of about 39 km is in
the remaining convective region, which had become
somewhat weaker but still showed a strong gust front in
the radial velocity data (first row
of Figure 16). The
other cross section show several of the polarimetric
variables, and they all show the bright band in the
melting layer of the stratiform zone. The particle
identification algorithm (second row
of Figure 16) shows
a layer of melting aggregates below non-melting ice
particles. |
| Figure 1.
DOE Gan sounding for 0300 UTC 14 October 2011. |
| Figure 2.
METEOSAT 10 micron water vapor image for 0830 UTC 14
October 2011. |
| Figure 3. METEOSAT IR image for 0830 UTC 14 October 2011. |
| Figure 4. METEOSAT visible image for 0830 UTC 14 October 2011. |
| Figure 5. Zoomed in METEOSAT visible image for for 0830 UTC 14 October 2011. |
| Figure 6. METEOSAT visible image for 0830 UTC 14 October 2011. |
| Figure 7.Photos looking NW from S-PolKa site at 0607 UTC 14 October 2011. |
| Figure 8.
Photo from S-PolKa site at 0621 UTC 14 October 2011. |
| Figure 9.
Photos looking SSW from S-PolKa site at 1141 UTC
14 October 2011. |
| Figure
10. Photos looking E from S-PolKa site at 0713 UTC
14 October 2011. |
| Figure 11. S-PolKa S-band reflectivity PPI for 2131 UTC 14 October 2011. |
 
 
|
| Figure 12.
S-PolKa RHIs at 120 deg azimuth at 2131 UTC 14
October 2011. Left to right: 1st row: dBZ, radial
velocity; 2nd row: particle type, ZDR; 3rd row: LDR,
RhoHV. |
| Figure 13. S-PolKa S-band reflectivity RHI for 2131 UTC 14 October 2011. |
 
 
 |
| Figure 14. S-PolKa RHIs at 82 deg azimuth at 2131 UTC 14 October 2011. Left to right: 1st row: dBZ, radial velocity; 2nd row: particle type, ZDR; 3rd row: LDR, RhoH; 4th row: kdp |
| Figure 15. S-PolKa S-band reflectivity PPI for 2231 UTC 14 October 2011. |
 
 
|
| Figure 16. S-PolKa RHIs at 80 deg azimuth at 2242 UTC 14 October 2011. Left to right: 1st row: dBZ, radial velocity; 2nd row: particle type, ZDR; 3rd row: LDR, RhoHV. |
| The ECMWF large-scale
wind pattern (Figure 1) shows the
patterns changing over the last three days such that the
cyclonic gyres at low levels are becoming better defined
with stronger westerlies over the S-PolKa region. At upper
levels the anticyclonic gyres have shifted somewhat
eastward with corresponding decrease in easterlies
overhead. The DOE Gan soundings showed general dryness
above 500 mb during most of the day (Figure
2). The METEOSAT infrared image for 1330 UTC 15
October (Figure 3) shows a general
increase in high cloud tops in the region surrounding the
DYNAMO/AMIE array, but the S-PolKa was lying in a
relatively cloud free zone between major regions of deep
cloud events. In the image shown, a pixel of coldd cloud
(red) is seen lying to the southeast of S-PolKa. This
feature was part of a cloud line that appeared to form
near the boundary of the region influenced by yesterday's
squall line. During the earlier part of the day, the sky
was at first filled with debris from the mesoscale
systems, in the form of altostratus, altocumulus, and
cirrus (Figure 4). Also evident
far to the NE in Figure 4 is
cumulus in a line, which was likely forming at the
boundary of the region affected by the mesoscale system.
During midday the sky was very clear except for fragments
of cirrus, cirrostratus, and cirrocumulus left behind by
the system (Figure 5). As the
day wore on, a linear convective cloud feature formed far
to the southeast of S-PolKa (75-150 km in range).
Figure 6
shows views of this new system at three times over a
period of about two hours. Visible are two lines of
clouds. A line of cumulus congestus was closer to the
S-PolKa site, and a line of deeper cumulonimbus with anvil
clouds was somewhat farther away, behind the line of
cumulus. The anvils of the distant cumulonimbus exhibited
an excessively tilted structure. An S-PolKa radar PPI
display of reflectivity at 1146 UTC shows both lines (Figure 7).
The line of cumulus is evident as a
thinner line of weaker echoes just to the NW of the main
line of wider more intense echoes. This line of cumulus
was forming along the outflow boundary of the main
convective line. Vertical cross sections of reflectivity
and polarimetrically derived particle type in Figure 8 show that the cells in
the main line were reaching about 12 km height. Figure 9 shows that the main line
had spread out into a broader stratiform system by 1446
UTC, and a line of smaller cells had
continually tried to form along the edge of the spreading
cold pool. The cold pool is evident by a clearing out of
the boundary layer echo. (The hazy green azimuthally
elongated echoes appearing within the cold pools in this
particular image are artifacts know as "second trip"
echoes, which come from distant targets encountered after
the next pulse of radiation was sent out from the radar.)
Figure 10 shows the system at a
still later time (1647 UTC), with the antenna tilted at
2.5 deg elevation to observe the upper anvil part of the
convective system. RHI displays in Figure
11 show the anvil and a convective cell in vertical
section. The reflectivity RHI shows the main convective
cell reaching 16 km and the anvil reaching over 12 km. The
radial velocity RHI shows midlevel inflow under the anvil.
The distribution of ZDR is consistent with heavy rain in
the convective cell. The particle types are similar to
what we have seen in other convective cells: in the
convective cell, heavy rain at low levels with graupel
extending up into the upper portion of the cell surrounded
by larger (blue), and smaller ice (pink). The anvil region
particle signature is consistent with non-melting
aggregates above the 0 deg C level. |
|
|
| 12
October |
12
October |
|
 |
| 15
October 2011 |
15
October 2011 |
| Figure 1.
ECMWF analyses for 12 and 15 October. The blue shades
are proportional to humidity. |
| Figure 2.
DOE Gan soundings for 15 October 2011. |
| Figure 3. METEOSAT IR image for 1330 UTC 15 October 2011. |
| Figure 4. Photo looking SW of S-PolKa site at 0402 UTC 15 October 2011. |
 |
 |
| Figure 5. Photos looking NW (left) and SW (right) from S-PolKa site at 0517-0518 UTC 15 October 2011. |
 |
| Figure 6. Photos looking SE from S-PolKa site at (left-to-right) 1031, 1130, and 1211 UTC 15 October 2011. |
| Figure 7. S-PolKa S-band reflectivity PPI for 1146 UTC 15 October 2011. |
 |
| Figure 8.
S-PolKa RHIs at 120 deg azimuth at 1143 UTC 15
October 2011. Left to right: dBZ and polarimetrically
derived particle type. |
| Figure 9. S-PolKa S-band reflectivity RHI for 1446 UTC 15 October 2011. |
| Figure 10. S-PolKa S-band reflectivity RHI for 1647 UTC 15 October 2011. Data in this PPI are taken with the antenna at an elevation angle of 2.5 deg. |
 
 |
| Figure 11. S-PolKa RHIs at 138 deg azimuth at 1659 UTC 15 October 2011. Left to right: 1st row: dBZ, radial velocity; 2nd row: ZDR, particle type. |
| It rained all day starting at
about midnight local time. The entire 150 km radius radar
PPI display of S-PolKa was covered with rain most of the
day. The sandy streets in town were largely inundated
during the day (see Figure 1
taken at about noon local time). Surface winds had a
strong westerly component, turning from due west to
northwest during the day. The ECMWF maps show the
low-level northwesterly over Gan at 925 hPa, between two
weak cyclonic gyres south and north of the equator (Figure 2). At upper levels (e.g., 200
hPa), the wind was easterly between anticyclonic gyres.
The DOE Gan sounding for 0900 UTC shows nearly saturated
moist adiabatic conditions through most of the troposphere
(Figure 3). A sequence of METEOSAT
infrared images shows the development of the precipitating
cloud system over Gan and surroundings. Figure
4 is for the preceding afternoon (1030 UTC, or 1530
local, 15 October) when the region around the radar was
relatively suppressed (see yesterday's summary). Figure 5 shows that by 2230 UTC 15
October (0230 local), the region west of Gan had filled in
with an outbreak of cold-topped cloud systems of mesoscale
proportions. In the next hours, the cloud pattern
continued to evolve, and other cold-topped mesoscale cloud
systems developed. They began to extend over the Gan area
during the day. Figure 6 shows an
example of the high-cloud-top pattern at 0630 UTC, or
about 1330 local on 16 October. Note the cold tops near
Gan, well within radar range. Figure
7 shows the S-PolKa coverage over a 9 hour period.
The area was essentially covered with widespread rain
throughout the period (note that the beam is partially
blocked by trees to the west, so the western half of the
domain is more completely covered by rain than it appears
in the images). The first column of the figure shows the
S-PolKa reflectivity superimposed on the infrared
satellite image. The second column shows the rain rate
computed from all of the S-PolKa S-band radar data. The
method uses a Z-R relation from MISMO (the Japanese field
project conducted in Addu Atoll) and from all the
dual-polarimetric parameters measured by S-PolKa. The
third column shows the subdivision of the echo into
convective (yellow) and stratiform (red) rain. The
algorithm used for convective stratiform separation is a
version of the algorithm developed by Steiner et al. (JAM,
1995) and refined by Yuter and Houze (JAM, 1997). The
algorithm has been tuned to conditions that apply in this
region by subjectively testing the separation results
against vertical cross sections of the S-PolKa radar data.
This tuning is recommended in both of the above mentioned
papers. The stratiform rain was very sharply defined with
a strong bright band at the melting level (4-5 km),
sometimes reaching 50 dBZ in intensity. Top panel in Figure 8 shows a PPI display of
the S-PolKa reflectivity at the 2.0 km level in the
sectors scanned by a sequence of RHIs to obtain the finest
possible vertical resolution in this sector. The blue
arrows illustrate the locations of cross sections shown in
the middle and lowest panels of the figure. The cross
section in the middle panel shows reflectivity along the
NW-SE line, with the right-hand side of the section
corresponding to the arrowhead in the top panel. The
reflectivity in the bright band approached 50 dBZ, which
is about as large as is ever seen in bright bands of
stratiform precipitation. The lowest panel shows the
particle types. derived from the S-PolKa polarimetric
variables in a cross section along the azimuthal arrow
shown in the top panel. The blue-grey color in the melting
layer is the category identified as "wet snow," which is
most likely melting aggregates. The greens at the bottom
of the blue gray layer are probably the large drops
resulting from the melting, while the dark greens at the
top of the layer are probably triggered by graupel
particles mixed in with the aggregates. The light blue
above the melting layer indicates "dry snow", which we
think are likely non-melting aggregates. The pink color at
higher levels is some kind of smaller ice particles. The
widespread echo made it possible to use the Doppler radial
velocities to diagnose the wind structure in the
disturbance passing over the radar. Figure 9 shows the radial
velocity patterns at 4 different times over an eleven-hour
period, during which the rain remained widespread over the
radar. In the images, blues and greens indicate inbound
radial velocity and yellows and reds represent outbound.
From these colors, note how the low-level wind (left
column) is strong and changes from westerly to
northwesterly over the time period of observations. The
northwesterly pattern at the end of the period (1330 UTC)
is consistent with the ECMWF analysis at 925 hPa in Figure 2. The upper-level winds are
easterly at all times, also consistent with the ECMWF
analysis. Vertical cross sections in the right column
of Figure
9 show that the westerly jet sloped downward early
in the day (top right panel). Later in the day, a slope of
the jet was not evident. Figure 10 shows an
example of the midlevel radial velocity pattern at 0606
UTC. The arrows indicate the direction of the radial flow
in several regions. This pattern is a complex
superposition of large-scale and mesoscale disturbed
winds. It is still under investigation but seems to be
consistent with a deformation wind pattern. |
| Figure 1.
Addu Atoll, 0414 UTC 16 October 2011 |
| Figure 2.
ECMWF analyses for 16 October. |
| Figure 3.
DOE Gan sounding for 0900 UTC 16 October 2011. |
| Figure 4. METEOSAT IR image for 1030 UTC 15 October 2011. |
| Figure 5. METEOSAT IR image for 2230 UTC 15 October 2011. |
| Figure 6. METEOSAT IR image for 0630 UTC 16 October 2011. |
 |
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|
 |
 |
 |
 |
 |
| Figure 7.
S-Polka low-level images of the rainfall at three times
on 16 October 2011. Left hand column shows the
reflectivity at 2.5 km. The middle column displays
the rain rate derived from reflectivity and
polarimetrically variables. The third column shows the
areas of precipitation diagnosed as convective (yellow)
and stratiform (red). |
| Figure 8.
The PPI display in the top panel shows the S-PolKa
reflectivity at 2.0 km in the sectors scanned by RHI in
order to obtain the finest possible vertical resolution.
The blue arrows show the locations of the cross sections
in the middle and lowest panels. The cross section in
the middle panel shows reflectivity along the NW-SE
line, with the right-hand side of the section
corresponding to the arrowhead in the top panel. Note
that the reflectivity in the bright band approaches 50
dBZ. The lowest panel shows particle type derived from
the S-PolKa polarimetric variables along in a cross
section along the azimuthal arrow shown in the top
panel. |
|
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 |
 |
| Figure 9.
Doppler radial velocity from the S-PolKa S-band radar
for 0146 (row 1), 0605 (row 2), 1020 (row 3), and 1330
UTC (row 4). The left-hand column shows radial velocity
at constant height levels of 0.5 km in row 1 and 1.0 km
in the other rows. The middle column shows radial
velocity at the 11 km level. The right-hand column shows
radial velocity in vertical cross sections along the
yellow lines. |
| Figure 10.
Doppler radial velocity from the S-PolKa
S-band radar at the 6 km level at 0605 UTC. The arrows
indicate the direction of the radial velocity in various
part of the pattern. |
| One of the goals of the radar
supersite on Addu Atoll is to be able observe
nonprecipitating as well as precipitating clouds. The DOE
soundings for Gan at 0000 and 0300 UTC 17 October 2011 (Figure 1) suggest three cloud layers,
at about 950, 650, and 425-450 hPa. The METEOSAT images
for 0400 UTC 17 October 2011 in Figure
2 are consistent with the presence of a layer of
middle cloud, indicated by the middle tone of grey in the
infrared image and the medium level of visible reflectance
surrounding the region. Figure
3 shows data from the DOE micropulse lidar image. It
shows the middle and upper cloud layers at 4 and 7 km,
respectively, early in the day, consistent with the
soundings. The upper panel of Figure 4 shows a
cross section of S-PolKa S-band data, in which all 3 cloud
layers can be seen at heights of about 0.5, 4 and 6 km.
The lower panel shows the corresponding Ka-band cross
section in which the two lower cloud layers are evident
but the highest layer does not appear, probably because of
attenuation. After daylight the layer of altocumulus
became evident visually, for example as seen at 0448 UTC
in the photo in Figure 5.
This cloud layer was clearly evident in both the S-band
and Ka-band cross sections taken at the same time as the
photo (Figure 6) at a level
around 6 km. Two hours later the altocumulus layer was
still visually evident, with some small cumulus at lower
levels (Figure 7), and both
the low and middle level clouds were evident in the
S-PolKa S-band (Figure 8),
evidently as a Bragg scattering signature, but not evident
at Ka-band (not shown). Later in the day a line of
building convection was seen as a southwest-northeas line
of small high clouds in the METEOSAT infrared and visible
images (Figure 9). Looking to
the west and northwest, we could see the building
convection along the leading edge of the squall as it was
moving in from the northwest (Figure 10). The S-PolKa
S-band showed the line of intense cells preceded by a gust
front line (Figure 11). |
| Figure 1.
DOE Gan soundings for 17 October 2011. |
| Figure 2. METEOSAT IR and visible images for 0417 UTC 17 October 2011. |
| Figure 3. DOE Gan miropulse lidar data for 17 October 2011. |
| Figure 4.
S-PolKa S-band and Ka-band reflectivity at 2246 16
October 2011 at azimuth 142 deg. |
| Figure 5.
Cloud photo looking SSE from S-PolKa at 0448 UTC 17
October 2011. |
| Figure 6.
Cloud photo looking SSE from S-PolKa at 0446 UTC 17
October 2011. |
| Figure 7.
Photo looking SW from S-PolKa at 0648 UTC 17 October
2011. |
| Figure 8.
Photo looking SW from S-PolKa at 0726 UTC 17 October
2011. |
| Figure 9. METEOSAT IR and visible images for 1217 UTC 17 October 2011. |
| Figure 10. Photo looking W from S-PolKa at 1125 17 October 2011. |
| Figure 11.
S-PolKa PPI display of reflectivity at 1101 UTC 17
October 2011. |
| This day had two major events in
the vicinity of S-PolKa. Both events were associated with
SW-NE lines of convection. Early in the day, a gust front
associated with one convective line rushed dramatically in
a southeastward direction over S-PolKa, while the upper
stratiform anvil portion was sheared off to the east. The
later system consisted of several lines of convection that
evolved into widespread stratiform precipitation, mostly
southeast of S-PolKa. The DOE Gan soundings (Figure 1) showed a fairly dry upper
troposphere at 0600 UTC. By 1800 UTC, with the upper
levels dominated by stratiform anvil clouds, the upper
troposphere had become nearly saturated. The METEOSAT
infrared image in Figure 2 shows
the early morning convective line just northwest of Gan. Figure 3 shows the progression of
the radar echo pattern on S-PolKa between 0300 and 0500
UTC 18 October. The upper portion of the convective line
is seen shearing off to the west, while a line of weak
echo along the gust front moves rapidly toward the
southeast, across the S-PolKa site. The radial velocity
pattern in the lower right panel of Figure 3 shows a relatively strong
northwesterly flow
behind the gust front line. The echoes along the gust
front seen in the lower left panel have a streaky
appearance, probably because these echoes are produced by
the non-precipitating clouds along the gust front, and
these cumuli were highly sheared
(Figure 4). At about 0730
UTC (1230 local), a new
southwest-northeast oriented line of convection was
forming to the southeast of S-PolKa. In the METEOSAT
infrared image in Figure 5, it
exhibited a few cloud tops with brightness temperatures
<208 K. The photographic panorama in Figure 6 shows the visual
appearance of the line, with building convection on the
southwest side and a thick expanding anvil on its
northwest end. The radar echo pattern seen by S-PolKa in Figure 7 showed echo tops up to
13-14 km. The infrared images in Figure
8 show how the line continued to grow and new lines
formed parallel to it for the next few hours. This growth
led to the S-PolKa radar echo pattern in Figure 9 around 1700 UTC, which
shows several mesoscale echo features, the most prominent
being the relatively new convective lines northwest and
southeast of the radar site at close range and large
stratiform echo patterns at farther ranges to the east and
south east. The RHIs in Figure
9 show a penetrative cell extending up to over 17 km
and completely surrounded by stratiform precipitation with
a bright band. The polarimetric particle identification
algorithm sees melting aggregates (blue/gray) at the
bright band level and some green spots likely indicating
graupel in the convective cell. The radial velocity
pattern shows a channel of outbound flow (yellows and
oranges) extending all the way to echo top, a likely
signature of the updraft. The top of this updraft exhibits
a divergence signature.
The convection close to S-PolKa
continued to intensify over the next hour
(Figure 10). A cross-section
through the most intense core (top RHI panel) shows very
high reflectivities (>55 dBZ) along the leading edge
with a stratiform region marked by a bright band with
fallstreaks on its northeast side. The particle
identification algorithm at this time triggers the light
green category, which is nominally "rain mixed with
hail" below the 0 deg C level (middle RHI
panel). However, this (and other categories of
the particle identification algorithm) are tuned for
midlatitude continental storms, and we think that in
this case it is more likely that the algorithm is
triggered by some larger graupel particles mixed with
heavy rain. There is
a small amount of darker green (graupel) at the top of
the core. The radial velocity signature (bottom panel)
shows a convective-scale downdraft signature in the
form of inbound velocities (green colors) near the
surface at about 12-15 km range. These inbound
velocities collide with the strong outbound velocity
(yellow/orange) near the surface. The outbound
velocities are carried upward, at first steeply and
then along a highly sloped path. Underneath this
sloping outbound updraft flow was an inbound subsiding
midlevel inflow. As the evening progressed,
the system became dominated by stratiform. An
example of the robust stratiform precipitation is shown in
Figure 11, where the RHI of
reflectivity shows values of 50 dBZ in the bright band.
The polarimetric hydrometeor algorithm shows al lot of
nonmelting aggregates in the melting layer (grey/blue),
with some indication of graupel or some other larger ice
particle (medium green) above the dry aggregates, and
large raindrops (darker green) on the bottom of the
melting layer. At 2000 UTC, Gan was still located under
and surrounded by high cloud tops of the various
convective systems in the area (Figure
12). The S-PolKa radar data along the yellow line in
the PPI of Figure 13 show and
example of the structure of the stratiform precipitation
at the edge of one of the widespread stratiform
precipitation regions. It shows a very thin but well
defined bright band all the way to the edge of the echo.
The particle identification algorithms shows snow and and
nonmelting aggregates but no evidence of graupel or bigger
ice particles. |
| Figure 1.
DOE Gan soundings for 18 October 2011. |
| Figure 2.
METEOSAT infrared image for 0530 UTC 18 October2011. |
|
|
|
 |
| Figure 3.
Clockwise from upper left, S-PolKa PPI displays of
reflectivity for 0301, 0401, and 0501 UTC and radial
velocity for 0501 UTC 18 October 2011. |
| Figure 4.
Cloud photo looking SW from S-PolKa at 0422 UTC. |
| Figure 5.
METEOSAT infrared image for 0730 UTC 18 October 2011. |
| Figure 6.
Cloud photo panorama looking (left-to-right)
from SE to S to SW from S-PolKa at 0726-0727 UTC 18
October 2011. |
 |
 |
| Figure 7.
S-PolKa reflectivity data for 0731 UTC 18 October 2011.
Vertical cross section in the lower panel is along the
yellow line in the PPI in the top panel. |
| Figure 8.
METEOSAT infrared images for 1330 and 1730 UTC 18
October 2011. |
|
|
|
| Figure 9.
S-PolKa reflectivity data for 1702 UTC 18 October 2011.
Vertical cross sections are taken along the yellow line
in the PPI in the top panel. Vertical sections in
descending order are for reflectivity, polarimetrically
derived hydrometeor type, and radial velocity. |
|
|
|
| Figure 10.
S-PolKa reflectivity data for 1802-1809 UTC 18
October 2011. Vertical cross sections are taken along
the yellow line in the PPI in the top panel. Vertical
sections in descending order are for reflectivity,
polarimetrically derived hydrometeor type, and radial
velocity. |
|
|
| Figure 11.
S-PolKa reflectivity PPI for 1917 UTC 18
October 2011 is shown in the top panel. Vertical cross
sections along the yellow line in the PPI are shown for
1947 UTC in the second and third panels. The cross
sections show reflectivity (middle panel) and
polarimetrically derived hydrometeor type (lower panel). |
| Figure 12.
METEOSAT infrared image for 2000 UTC 18
October 2011. |
 |
 |
| Figure 13.
S-PolKa reflectivity data for 2101 UTC 18
October 2011. Vertical cross sections are taken along
the yellow line in the PPI in the top panel. Vertical
sections in descending order are for reflectivity and
polarimetrically derived hydrometeor type. |
| This day was dominated by
nonprecipitating layer clouds over the region surrounding
S-PolKa. The day started with dying anvil clouds in the
region (0300 UTC panel in Figure 1). As
these dissipated, the region surrounding Gan was broadly
covered by medium brightness temperature in the METEOSAT
infrared images (0600, 0900, and 1200 UTC panels in Figure
1). The DOE Gan sounding at 0300 UTC in Figure 2 showed a classic "onion"
sounding, a name coined by Ed Zipser to indicate a
sounding through an anvil cloud, which typically has
subsidence warming and drying below the cloud layer. Figure 3 shows that an anvil
cloud indeed extended over the radar, which is near the
Gan sounding site. The anvil contained moderate stratiform
precipitation, with a bright band and fallstreaks. The
blue/gray and light blue at the bright band level in the
hydrometeor-type cross section is typical of the bright
bands we have described in previous summaries. This anvil
as viewed from the S-PolKa site is shown in the photo in Figure 4. The soundings for 0600,
0900, and 1200 UTC in Figure 2 show
evidence of multiple thin cloud layers, at low, middle and
high levels. These thin cloud layers are consistent with
the infrared satellite imagery at these same times in
Figure 1
and with sky conditions over the radar site during most of
the day (Figure
5) Figures
6, 7,
and 8 show examples of S-PolKa
data in which thin cloud layers can be seen at both S- and
Ka-band wavelengths. In all three figures, both S- and
Ka-band radars see cloud layers at 10-12 km. The S-band
generally sees the extent of the clouds more completely
than the Ka-band. At close range, however, the Ka-band
sees the internal structure more clearly. Later in the day
(1431 UTC), convective lines were forming to the southeast
of the radar site in a generally SW-NE orientation. One of
these lines showed an extraordinarily well defined
semicircular cold pool around its northeast end (Figure 9). The ZDR signature was
positive (red in the lower panel of the figure). |
 |
 |
 |
 |
| Figure 1.
METEOSAT infrared images for 19 October2011. |
|
|
|
|
| Figure 2.
DOE Gan soundings for 19 October 2011. |
|
|
| Figure 3.
S-PolKa reflectivity data for 0316 UTC 19 October 2011.
Vertical cross sections are taken along the yellow line
in the PPI in the top panel. Vertical sections in
descending order are for reflectivity and
polarimetrically derived hydrometeor type. |
| Figure 4.
Photo looking south from S-PolKa at 0334 UTC. |
| Figure 5.
Photo looking east from S-PolKa at 0620 UTC. |
|
|
| Figure 6.
S-PolKa reflectivity data for 0657 UTC 19 October 2011.
Vertical cross sections in descending order show
reflectivity S-band reflectivity at 116 deg azimuth, and
K-band signal to noise ratio at 116 deg azimuth. |
|
|
| Figure 7.
S-PolKa reflectivity data for 0800 UTC 19 October 2011.
Vertical cross sections in descending order show
reflectivity S-band reflectivity at 142 deg azimuth, and
K-band signal to noise ratio at 142 deg azimuth. |
|
|
| Figure 8.
S-PolKa reflectivity data for 0916 UTC 19 October 2011.
Vertical cross sections in descending order show
reflectivity S-band reflectivity at 142 deg azimuth and
K-band signal to noise ratio at 142 deg azimuth. |
| Figure 9.
S-PolKa radar data for 1431 UTC 19 October 2011. The
upper panel shows reflectivity. The lower panel shows
ZDR. |
| This day was marked by a
substantially higher level of convective activity than we
have seen so far. This increase is consistent with the
ECMWF (and other) forecasts of MJO-related convective
activity over the Indian Ocean region during the present
time period (Figure 1). This MJO has
been forecast by several global models to be unsustainable
over time. Figure 2 shows
Wheeler's forecast of this event, and it can be seen to
die out rather quickly as it moves to the east. The DOE
soundings in Figure 3 show
generally moist conditions through the depth of the
troposphere throughout the day, consistent with the
presence of the deep cloud systems. The sequence of
METEOSAT infrared images in Figure 4
provides an overview of the cloud system development on
this day. Gan was under mesoscale high cloud-topped
systems the whole day, but they were all generally smaller
and less intense that those to the distant south between
Gan and Diego Garcia and in the center of the four-corners
DYNAMO array. Nevertheless the cloud systems in the Gan
region intensified during the day, reaching maximum
intensity just south of Gan by 1800 UTC (last panels in
Figure 4).
Figure 5 shows three views of the
precipitation pattern seen on the S-PolKa S-band radar at
0100 UTC, the reflectivity PPI superimposed on the
satellite infrared image, the hybrid S-PolKa rain rate
product, and the subdivision of the pattern into
convective and stratiform areas. The hybrid product uses
all of the polarimetric variables and incorporates the
MISMO Z-R relation (Z=175R^1.44). These patterns show
several SW-NE oriented lines that initiated along outflow
boundaries. Such lines were present all day. At this time
they contained narrow tall cells of reflectivity. Figure 6 shows and example of one of
these reflectivity cells in an early stage of development.
The polarimetric hydrometeor identification showed it was
lofting dry snow all the way up to 12 km in a very narrow
tower (lower panel). These penetrative towers were present
generally in the area for several hours. An example of
tower penetrating up through an anvil is shown in a photo
in Figure 7 and in a radar
RHI in Figure 8. Again we
see dry snow lofted to high levels even though the turret
in the photo appears to be composed of liquid water in its
visible exterior portion. By 0800 UTC, the areas covered
by echo lines were broader and about evenly split between
convective and stratiform precipitation components (Figure 9). This summary cannot easily convey the behavior of these convective lines. Anyone interested in this case is referred to the time-lapse loop of S-PolKa images seen on the EOL DYNAMO Catalog. There one will see that the actively convective lines move toward the northeast. But as their components weaken and broaden into stratiform components they move rapidly westward. Looping of the convective stratiform maps shows the convective regions moving northeastward for the most part, and the stratiform regions moving westward. This behavior is generally consistent with the wind profile in the region. As seen in the soundings and has been generally the case here, the winds are easterly aloft and have a strong westerly component at lower levels. On this day, the convective bands were aligned generally along the shear vector with their most active growth tending to be on their southeast sides, suggestive that they were in zone of confluence and fed at low levels on their southeast sides. Apparently younger convective cells were advected by the low-level flow while the older stratiform regions were advected by the easterly flow aloft. When one of the SW-NE lines was just to the northwest of S-PolKa it spawned a gust front that moved over the S-PolKa site. The blue box in Figure 10 shows the location of the fine line at the gust front. Figure 11 shows RHIs of several parameters in a cross section across the gust front. The S-band reflectivity shows the gust front marked by a thin radar echo probably produced by Bragg scattering from a roll cloud preceding the line (inside blue box in upper left panel). Figure 12 contains a photo of the roll cloud as it approached the radar site. The radial velocity RHI clearly shows the thin layer of outflow toward the radar producing the roll cloud (lower right panel). The cold pool is overridden by outbound flow ascending into the upper part of the cloud. Both the S-band reflectivity (upper left) and Ka-band signal-to-noise ratio show non-precipitating clouds forming in this updraft current feeding into the cloud. The RHI of ZDR shows red (positive) pixels at the leading edge of the gust front (lower left panel). We have commented in previous summaries about how the gust front often appears as a sharp red line in ZDR PPI patterns, and we have suggested that birds might be producing this signal. Looking up from the radar site, we saw birds soaring in the gust front as it passed over the radar site (Figure 13). By 1200 UTC (1700 local time), the echo coverage had increased, and stratiform precipitation had taken an increasing proportion of the area covered by echo (Figure 14). One odd feature noted at about the time of Figure 14 was the convective cloud shown in Figure 15. It had an unusual circular appearance thin circular clouds seemed to be emanating from it like gravity waves. By 1800 UTC the stratiform coverage was still greater, although convective cells were still forming SW-NE lines and bands (Figure 16). By 0000 UTC on 21 October, clouds in the region were become more electrified than we have seen thus far (Figure 17). Some of the lightning flashes were near the Revelle(located at NE in Figure 17), which was seeing the echo shown in Figure 18 around this time. Observers on the ship reported lightning at this time. |
| Figure 1.
ECMWF Hovmoeller diagram of OLR between 5 N and 5 S. The
period after the horizontal line is forecast. |
| Figure 2.
M. Wheeler's Australian Bureau of Meteorology forecast
of MJO OLR. |
|
|
 |
| Figure 3.
DOE Gan soundings for 20 October 2011. |
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 |
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 |
 |
| Figure 4.
METEOSAT infrared images for 20 October 2011 with
S-PolKa reflectivity overlaid in the right-hand images. |
 |
 |
 |
| Figure
5. From left to right, S-PolKa S-band reflectivity,
polarimetrically derived rain rate, and convective
(yellow) and stratiform (red) rain areas for 0100 UTC. |
|
|
| Figure
6. S-PolKa RHIs of dBZ (upper) and hydrometeor type
(lower) at 136 deg azimuth for 0014 UTC 20 October 2011 |
| Figure 7.
Photo looking east from S-PolKa at 0429 UTC. |
|
|
| Figure 8. S-PolKa RHIs of dBZ (upper) and hydrometeor type (lower) at 82 deg azimuth for 0412 UTC 20 October 2011. |
 |
 |
 |
| Figure 9.
From left to right, S-PolKa S-band reflectivity,
polarimetrically derived rain rate, and convective
(yellow) and stratiform (red) rain areas for 0800 UTC. |
| Figure
10. PPI of S-PolKa S-band reflectivity at
0731 UTC 20 October. Lower panel is zoomed in. The box
shows the location of a gust front fineline. |
 |
 |
 |
 |
| Figure
11. RHIs of S-PolKa radar data. Clockwise
from upper left are S-band reflectivity, Ka-band
signal-to-noise ratio, S-band radial velocity, and
S-band ZDR. The box in the upper left panel surrounds
the near-surface gust front. |
| Figure
12. Photo looking NE from S-PolKa at 0741 UTC 20
October 2011. |
| Figure
13. Photo taken at S-PolKa site while pointing the
camera directly overhead. Birds were soaring in the gust
front. |
 |
 |
 |
| Figure
14. From left to right, S-PolKa S-band
reflectivity, polarimetrically derived rain rate, and
convective (yellow) and stratiform (red) rain areas for
1200 UTC. |
| Figure
15. Photo looking southwest from S-PolKa at 1805
UTC 20 October 2011. |
 |
 |
 |
| Figure
16. From left to right, S-PolKa S-band
reflectivity, polarimetrically derived rain rate, and
convective (yellow) and stratiform (red) rain areas for
1800 UTC. |
| Figure
17. World Wide Lightning Location Network data
superimposed on METEOSAT infrared image for 0000 21
October 2011. Flashes are accumulated over one hour, and
the colors show the locations of flashes and the number
of stations reporting each flash. |
| Figure
18. Revelle C-band radar reflectivity at 2259 UTC
20 October 2011. |
| Today S-PolKa sampled convection
on the southern edge of a major outbreak over the northern
Indian Ocean. This event occurred in the context of phase
1-2 of an MJO, which has begun to weaken
(Figure 1). Global model
forecasts suggest that it will not
survive beyond phase 2. The general context of the
convective events on this day are in a deep moist layer
that contrasts with the dry midlevel conditions of two
weeks ago (Figure
2). The ECMWF analyses show weak cyclonic gyres at
low levels over the western Indian Ocean (e.g. 925 hPa
in Figure 3) and upper-level
anticyclonic gyres over the
eastern Indian Ocean (e.g. 200 hPa in Figure 3)--displaced
from the cyclonic gyres. The convection over the Indian
Ocean has shifted well north of the equator, and north of
Addu Atoll. Figure 4 shows the
locations of lightning at four times over a two-day
period. A marked increase in electrification of the clouds
over the northern Indian Ocean has occured over the last
day. Figure 5 shows the development of
the high cloudiness over the Indian Ocean over a 34 hour
period. At 1900 UTC 20 October, fairly intense cold cloud
topped convection was occurring in a region centered just
west of Gan (upper left panel). This convection expanded
as newer deep convection formed in a giant ring around
this region, and this ring of convection expanded leaving
its center relatively undisturbed. The ring is seen at
0430 UTC 21 October in the upper-right panel of Figure 5. Another ring emanated from a
region of convection centered at about 2 N and 76 E at
0800 UTC 21 October 2011 (lower left panel). In the lower
right panel, it can be seen that this giant ring had it
southmost portion over Gan at 0500 UTC 22
October. This summary describes what was seen by the
S-PolKa radar of this portion of the giant ring.
(Anecdotally, RAH remembers similar giant ring formation
by convection over the West Pacific during TOGA COARE.) Figure 6 shows SW-NE lines of convection over the S-PolKa area at 2000 UTC 21 October. Note the cold cloud tops reaching brightness temperatues as low as ~200 K in the southern portion of the giant convective ring, located just outside the S-PolKa 150 km range circle. A cross section of reflectivity near this feature showed echo top heights reaching 20 km. A cross section along one of the intense convective lines showed cells lofting larger non-melting ice particles to great heights (narrow light blue shafts at 50, 70, and 100 km range in the lower-right panel of Figure 7. The ability of these cells to loft ice to such great heights is consistent with the formation of lightning seen in Figure 4. However, these cells were not producing lightning, which leads to the inference that the convection far to the north was even more intense than that seen on the S-PolKa radar! The precipitation seen by the S-PolKa S-band at 2130 UTC showed that the precipitation in the SW-NE bands was predominantly convective, but some of the northernmost precipitation seen by the radar was turning stratiform (rightmost panel of Figure 8). Although the precipitation in the northern region of radar coverage was gradually turning more stratiform, it still contained embedded intense cells. Some of these were lofting larger ice particles to over 14 km at 2130 21 October (light blue column at 100 km in Figure 9). This stratiform precipitation was likely from collapsing previously intense convective cells. An hour later, portions of the strong bright band (e.g. 30-40 km range in Figure 9) showed evidence of graupel particles (green in the particle idenfication panel, lower right of Figure 9) both above and below the layer of melting aggregates (grey/blue layer in the lower right panel). This signature is highly suggestive that this stratiform precipitation was the collapsed phase of a previously intense convective cell. An hour and a half later, the stratiform precipitation was still more widespread. The particle identification panel in Figure 10 shows a cross section through the now huge region of stratiform precipitation at 0001 UTC 22 October. The polarimetric particle identification algorithm shows likely collapsing convection at 90-110 km range. Another feature that we are seeing regularly in the deep convection in this region is the fringe of purple around the upper edges of the ice echo. These are weakly reflective particles showing horizontal orientation. We think they are proably pristine columns at low temperatures. At 0115 UTC 22 October, the S-PolKa echo in the northern zone contained a large portion of embedded convective cells (yellow in the rightmost panel of Figure 11). The satellite infrared brightness temperatures were extremely cold (190-196 K) in the pink region of cloud tops NNE of S-PolKa, near where the embedded convective cells were located. Closer to the S-PolKa radar, the embedded convection was also still intense. Figure 12 shows three lines of convection near the southern edge of the broader echo region to the north at about the same time as Figure 11. At this location the three lines were separated by regions of weak or no echo. From the satellite image underlay in the upper panel of Figure 12, they appear to have been extensions of the lines of SW-NE oriented cloud lines to the southwest of S-PolKa. These cloud lines appeared to be "feeding" the larger cold-cloud-topped feature to the north. Two hours later, at 0300 UTC, the broad echo feature to the north was still very active convectively. Three regions of intense echo at low levels were embedded north of the radar (upper panel, Figure 13). A cross section through these convective regions shows that they were still convectively active (lower panels of Figure 13). These convective cells were not reaching quite as high as those seen earlier far to the north, but the more active cells were still lofting large amounts of larger ice particles (light blue, dry snow) up to 10-12 km. The active cells at 35 and 50 km range were also pushing wet snow and graupel (grey-blue and green) upwards, above the environmental 0 deg C level. At 100-110 km range, a cell was collapsing and becoming stratiform, and it shows melting graupel below the bright band. After sunrise, the area to the north of the S-PolKa radar appeared dark on the horizon with rain, and the sky was over cast with multiple cloud layers from the massive cloud system to the north (Figure 14). At 0300 UTC 22 October, the METEOSAT visible image (Figure 15) showed the massive cloud system north of Gan and the feeder lines to the south. At this time, the lines were oriented SSW-NNE. By 0515 UTC, the precipitation north of S-PolKa was indicating that the massive cloud system on the south side of the giant ring of convection was becoming mostly stratiform (rightmost panel of Figure 16). This region of echo continued to be stratiform and weakening for several more hours (e.g., see the pattern at 0700 in Figure 17). The visible image in Figure 18 shows that the feeder lines south of the giant ring of convection were becoming S-N oriented as the system moved generally westward. Radar data from the Revelle show echoes from these lines in Figure 19; however, the line orientations are only faintly visible since the radar-echo cell locations are primarily determined by cold pool boundaries and intersections as our previous summaries have pointed out. |
| Figure 1.
Australian Bureau of Meteorology phase space diagram. |
| Figure
2. Wind and humidity time-height series based on
the DOE Gan soundings since the end of September. |
| Figure
3. ECMWF wind and relative humidity analyses. |
|
|
|
 |
| Figure
4. World Wide Lightning Location Network lightning
flashes for 30 minutes preceding METEOSAT infrared
images. |
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| Figure 5. METEOSAT
infrared images. |
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 |
| Figure 6. S-PolKa
S-band reflectivity at 2 km at 2000 UTC 21 October 2011
superimposed on the METEOSAT infrared image (upper panel)
and cross section (lower panel) taken south to north along
the red line in the upper panel. |
|
 |
| Figure 7. S-PolKa S-band reflectivity at at 0.5 deg elevation at 2116 UTC 21 October 2011 (upper panel) and RHIs of reflectivity (lower-left panel) and polarimetric particle type (lower-right panel) along the yellow line in the upper panel. |
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 |
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| Figure 8. From left to right, S-PolKa S-band reflectivity, polarimetrically derived rain rate, and convective (yellow) and stratiform (red) rain areas for 2130 UTC 21 October 2011. |
 |
 |
| Figure 9. S-PolKa S-band reflectivity at at 0.5 deg elevation at 2231 UTC 21 October 2011 (upper panel) and RHIs of reflectivity (lower-left panel) and polarimetric particle type (lower-right panel) along the yellow line in the upper panel. |
 |
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| Figure 10. S-PolKa S-band reflectivity at at 0.5 deg elevation at 0001 UTC 22 October 2011 (upper panel) and RHIs of reflectivity (lower-left panel) and polarimetric particle type (lower-right panel) along the yellow line in the upper panel. |
 |
| Figure 11.From left to right, S-PolKa S-band reflectivity, polarimetrically derived rain rate, and convective (yellow) and stratiform (red) rain areas for 0115 UTC 22 October 2011. |
|
|
| Figure 12. S-PolKa
S-band reflectivity at 2 km at 0100 UTC 22 October 2011
superimposed on METEOSAT infrared image (upper panel) and
cross section (lower panel) taken NW to SE along the red
line in the upper panel. |
 |
 |
| Figure 13.S-PolKa S-band reflectivity at at 0.5 deg elevation at 0301 UTC 22 October 2011 (upper panel) and RHIs of reflectivity (lower-left panel) and polarimetric particle type (lower-right panel) along the yellow line in the upper panel. |
| Figure 14. Photo
looking northeast of the S-PolKa site at 0354 UTC 22
October 2011. |
|
| Figure 15. METEOSAT
visible image at 0300 UTC 22 October 2011. |
 |
| Figure 16.From left to right, S-PolKa S-band reflectivity, polarimetrically derived rain rate, and convective (yellow) and stratiform (red) rain areas for 0515 UTC 22 October 2011. |
 |
| Figure 17. From left to right, S-PolKa S-band reflectivity, polarimetrically derived rain rate, and convective (yellow) and stratiform (red) rain areas for 0700 UTC 22 October 2011. |
|
| Figure 18. METEOSAT visible image at 0730 UTC 22 October 2011. |
|
|
| Figure 19. C-band radar images from Revelle, 22 October 2011. |
| Gan remained moist through a
deep layer on 23 October. The relatively moist anomaly,
which extends to about 12 km, has persisted since the end
of the previous suppressed phase. (Figure 1). A consistently
moist region near the stable 0 deg C layer has also
persisted throughout the entire first three weeks of
DYNAMO. During much of the day, we observed westward or
southwestward moving scattered cirrus clouds extending
outward from deep convection forming to the northeast of
Gan (Figure 2). A more solid field of
cirrus cloud reached the site shortly after 13 UTC (Figure 3). Figures 3 and 4
show data from the DOE Gan Ka-band zenith-pointing cloud
radar (KAZR) reflectivities and doppler velocities from
the DOE site at Gan on 23 Octoberober 2011. Cirrus anvil
ranged between 9 and 15 km throughout much of the day.
Reflectivities between -30dBZ and 0dBZ are typical for
anvils, and reflectivities on the higher end of this scale
are more typically observed near convective regions.
Longer-lived anvil, such as that seen between 10 UTC and
15 UTC, is generally composed of smaller particles that
give a weaker return. Periodic precipitation and
non-precipitating shallow cumulus are observed during the
first half of the day. Cirrus anvil, as well as thin cloud
around the 0 deg level extending laterally from a
developing MCS to the northeast, are detected after 18
UTC. Typical fall velocities of the ice particles in the
long-lived thin anvil and hydrometeors in the mid-level
thin cloud are generally less than 1 m s^-1. However, near
the bottom of thicker anvil cloud (around 00 UTC 24
October 2011), larger ice particles, probably aggregates,
which have high reflectivity, fall out of the cloud at
rates greater than 2 m s^-1. Figures 5, 6, and 7 all depict the cloud population during the day. Figure 5 shows the large amount of cirrus cloud present during much of the day. Some shallow cumulus and cumulus congestus (right side of right panel) were also present. Figure 6 illustrates large ice particles falling out of a newly formed anvil cloud extending from a convective core at 1012 UTC. The upper, and more visible, portion of the anvil consists of much more numerous smaller ice particles that are transported throughout the upper troposphere. After a brief period of clearing, more scattered cirrus cloud and shallow cumulus became present by dusk (Figure 7). Several convective cells extended vertically to between 8 and 10 km and moved eastward, eventually merging into and pumping moisture into a growing stratiform precipitating region that reached the area shortly before 00 UTC. An example of KAZR becoming attenuated by heavy, convective precipitation is seen on the right of Figure 4. Convection deepens as it enters the moist stratiform environment and enhances the stratiform region by transporting moisture into its anvil. Figure 8 demonstrates the cloudiness observed over the DOE Gan site by the S-band radar (S-Pol). Each of the panels are RHIs taken at an azimuth angle of 140.9 deg, in the direction of the DOE site, which is located about 8 to 9 km away from S-PolKa. Thin cirrus, with reflectivity between -20 to -5 dBZ, is detected by S-Pol between 10 and 12 km above the surface, and a thin cloud layer with similar reflectivity is detected below 5 km. The shallow precipitation that passes over KAZR around 23 UTC is seen also on S-band between 10 and 20 km away from the radar at a height between 3 and 6 km (lower panels of Figure 8). Although KAZR is partially attenuated by precipitation after 2330 UTC, it can still detect the top of the stratiform anvil. Cloud top is observed between 12 km and 13 km by both KAZR and S-Pol. Figure 9 suggests that the particle ID product may not classify most of the cirrus cloud at all, and thin cirrus near the 0 deg level is also not classified. KAZR was heavily attenuated near 0130 UTC 24 October. An S-PolKa S-band scan at 2.5 deg at this time and a corresponding RHI over the DOE KAZR are depicted in Figure 10. Reflectivities of greater than 40 dBZ are observed in the precipitating core as high as 5 km. The 0130 UTC time was the beginning of widespread rain over the S-PolKa area, and that event is described in our 24 October summary. |
| Figure 1.
Percentage deviations from the time-mean humidity during
the period 1-24 October 2011 based on DOE soundings at
Gan. |
| Figure 2. METEOSAT
infrared imagery over the Indian Ocean at 0830 UTC 23
October 2011. |
| Figure 3. Composite of the DOE Gan Ka-band Zenith Cloud Radar (KAZR) reflectivity for 23 October 2011. |
| Figure 4. Composite of
KAZR vertical velocity betwen 13 UTC, 23 October 2011 and
02 UTC, 24 October 2011 at Gan Island. |
 
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| Figure 5. Left: Photo taken 0330 UTC on 23 October 2011. Right: Photo taken at 1018 UTC on 23 October 2011. |
| Figure 6. Photo of
anvil cloud extending from an isolated deep convective
cell at 1012 UTC on 23 October 2011. |
 
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| Figure
7. Left: Scattered cirrus clouds becoming more
numerous at 1304 UTC 23 October 2011 (also seen in top of
right panel). Right: An isolated deep convective cell and
anvil develop to the south of S-PolKa around 13
UTC. Shallow convection is visible in the distant boundary
layer. |
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| Figure 8. Series of
S-band reflectivity in RHIs at an azimuth angle of 140.9
deg, or toward the DOE Gan site at 2030 UTC, 2100 UTC,
2130 UTC, 2200 UTC, 2245 UTC, and 2330 UTC. Cloud
layers are visible near the 0 deg C level and 10 km.
Gan is located between 8 and 9 km from S-PolKa. |
| Figure 9. Particle ID
product along an RHI at 140.9 deg at 2045 UTC 23 October
2011. |
 
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| Figure 10. S-PolKa
S-band reflectivity at 2.5 deg elevation at 0132 UTC 24
October 2011 (left), and an RHI at 140.9 deg at 0130 UTC
(right). |
| Yesterday was a major rain event
at Gan and over the whole area scanned by the S-PolKa
radar. The total rain area and rain amount over the radar
area are shown in Figure 1 along
with curves showing the breakdown into convective and
stratiform components. Before 0400 UTC, the convective
rain amount (upper panel) was greater than the stratiform.
After that the rain was predominantly stratiform over the
radar area. The rain area (lower panel) was mostly
stratiform for the entire period. This rain event was
associated with a giant ring of convection of the type we
discussed in our 22 October summary. However, in that
event S-PolKa observed convection on the edge of the ring
at a late stage. In this event, S-PolKa observed
convection in the outbreak that was the source of the ring
that propagated outward. The METEOSAT imagery in Figure 2 provides an overview of the
sequence. The first image (1400 UTC) was before the event
started. At this time, significant convection was well
east of Gan and over the Revelle (location NE). The Revelle radar was
observing a mesoscale echo system with SW-NE and WSW-ENE
oriented lines of convection and associated stratiform
precipitation areas (left hand panel of Figure 3). The line west of the Revelle at this time
was propagating toward the ship. However, this mesoscale
convection was not ultimately associated with what
occurred subsequently over the S-PolKa area. The 1830 UTC
image in Figure
2 shows several small cells of high cloud tops
appearing between the Revelle
and Gan. These cells are the beginning of the outbreak
observed by S-PolKa. In the next two panels, these cells
were growing larger and moving into the S-PolKa area. By
0440 UTC (fifth panel of Figure
2 ) it was raining steadily at Gan. The radar
sequence during this time will be discussed in some detail
below. By about 0840 UTC, the ring of convection could be
identified from the satellite infrared image. The cold
cloud tops had moved SW of Gan, there was some clearing
between Gan and the Revelle,
and the Revelle
radar was seeing mesoscale rain areas with embedded
convective cells (right panel of Figure
3). The giant ring of convection continued to expand
through 0030 UTC 25 October 2011 (last four panels of
Figure 2
). Figure 4 shows the echo
pattern seen on the S-PolKa S-band radar at 0100 UTC. The
area to the east of S-PolKa was convered by radar echo and
WSW-ENE oriented lines of convection were located to the
west. One line was extending nearly directly over Gan at
this time and intersecting the stratiform region. Although
a great deal of stratiform rain was to the east of the
radar, the rightmost panel of Figure
4 shows that convective cells were embedded in the
stratiform region. As will be shown by cross sections
below, these cells were very deep and intense. Time-lapse
viewing of the images shows that the cells in the lines to
the west were moving eastward into the stratiform region,
while the stratiform region was moving westward. Figure
5 shows the time sequence of the radar echo
pattern on the S-PolKa S-band radar for a little over five
hours. Where the lines of convection to the west were
intersecting the stratiform zone, very intense convective
cells were seen, for example ENE of the radar at 0146 UTC
and in the SE quadrant at 0246 and 0316 UTC. After that
time (0446 and 0701 UTC in Figure
5), intense convection was still present but just
out of radar range, as seen in the satellite image in the
last panel of Figure 5. At 0146 UTC, the S-PolKa S-band PPI shows one of the WSW-ENE bands west of Gan intersecting the radar site and Gan Island. The DOE Gan KAZR vertically pointing Ka-band radar obtained an interesting time section of this event (Figure 6). It shows downdraft at about 0117 UTc over ridden by updraft. The upper extension of the updraft slopes upward from ~0130-0145 UTC. The updraft speeds exceed 5 m/s. The S-PolKa rain maps at 0200 (Figure 7) illustrate the intersection of the intense convective cells in the lines to the west with the large stratiform region to the east. The embedded cells in the stratiform zone appear to be extensions of the lines into the stratiform region, and much of the stratiform region contains embedded cells, some of which were very intense. Figure 8 shows the structure of an intense cell to the east. This cell was reaching over 17 km in height (lower left panel). The radial velocity data showed a gust front at about 105 km and a divergent signature near echo top. A convective-scale patch of enhanced outbound radial velocity is seen at the cell location in the PPI (upper left panel), indicative of downward transport of westerly momentum. The polarimetric particle identification pattern (upper and lower left panels) shows heavy rain at low levels with the cell lofting larger ice particles (light blue) to about 15 km height. Graupel signatures are seen both above and below the melting layer, both in the active cell and in the heavier stratiform zones surrounding the cells. The latter signatures are consistent with the stratiform precipitation being formed as convective cells collapse. The rain pattern over the S-PolKa region at 0300 UTC (Figure 9) look much the same as at 0200 UTC. Figure 10 shows another example of an intense cell embedded in the stratiform echo reaching about 17 km. This cell is so intense that it seems to have produced some upper-level clearing on either side of the cell, as if by upper level downdrafts responding to the buoyancy in the cell. The particle identification signatures (lower right) show a strong graupel signature right above the 0 deg C level and also some possible graupel mixed with the rain below. As in the other example, larger (more reflective) ice particles were being carried up to about 15 km. Figure 11 shows an example similar to the previous two examples in all respects. The radial velocity footprint of momentum transport is particularly strong in this case (upper right panel). Figure 12 shows an example of one of the embedded cells in the process of collapsing and turning into stratiform echo. Apparently, cells fed into the zone of widerspread echo from the west, grew into very intense cells, and collapsed into dense stratiform echo, which in turn was advected back westward. Figure 13 shows that the S-PolKa rain pattern at 0400 UTC still contained substantial convection embedded in the broader stratiform echo pattern. At this time the S-PolKa site was under this echo region. Figure 14 shows the overcast and rain at 0419 UTC. It is interesting that the cloud base was low. The stratiform cloud was not midlevel-based as might have been expected. At 0600, the precipitation on radar was nearly all stratiform (Figure 15). The Gan 0600 UTC sounding had a Zipser "onion" profile, of the type associated with stratiform anvil clouds (Figure 16); the warming and drying below 600 hPa gives the sounding an onion-like shape on the Skew-T diagram. The 0800 UTC rain pattern on S-PolKa was nearly all stratiform (Figure 17). However, the METEOSAT infrared satellite underlay in Figure 17, shows extremely cold cloud tops (~200 K) to the southwest, just beyond the maximum range of S-PolKa. The deep convection was evidently still active in this region, just out of radar range. The World Wide Lightning Location Network data show a lightning strike in the region (Figure 18), which is consistent with the large ice particles that we have seen being lifted to high levels (Figures 8, 10, and 11). After the major event just described, a line of covection orinted SW-NE moved across the northwest quadrant (Figure19). It had a well defined gust front with (lower panel). The relationship of this line of convection to the other events of the day remains unclear. |
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| Figure 1. Rain area
coverage and accumulation over the area covered by
S-PolKa. Computed from polarimetric variables and the
MISMO Z-R relation. |
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| Figure 2. METEOSAT
Infrared images for 23-24 October 2011. |
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| Figure 3. Revelle C-band radar images from the 23 and 24 October 2011. |
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| Figure 4. From left to right, S-PolKa S-band reflectivity, polarimetrically derived rain rate, and convective (yellow) and stratiform (red) rain areas for 0100 UTC 24 October 2011. |
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| Figure 5. S-PolKa S-band reflectivity field for 24 October 2011, except last panel, which shows a METEOSAT infrared image with S-PolKa 150 km range ring highlighted. |
| Figure 6. DOE KAZR
vertically pointing Ka-band radar image for 0100-0200 UTC
24 October 2011. |
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| Figure 7. From left to right, S-PolKa S-band reflectivity, polarimetrically derived rain rate, and convective (yellow) and stratiform (red) rain areas for 0200 UTC 24 October 2011. |
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| Figure 8. S-PolKa S-band radial velocity and particle identification fields at 0.5 deg elevation at 0146 UTC 24 October 2011 (upper panels) and RHIs of reflectivity, radial velocity, and particle type along the yellow line in the (lower panels). |
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| Figure 9. From left to right, S-PolKa S-band reflectivity, polarimetrically derived rain rate, and convective (yellow) and stratiform (red) rain areas for 0300 UTC 24 October 2011. |
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| Figure 10. S-PolKa S-band reflectivity at at 0.5 deg elevation at 0252-0258 UTC 24 October 2011 (upper panel) and RHIs of reflectivity (lower-left panel) and polarimetric particle type (lower-right panel) along the yellow line in the upper panel. |
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| Figure 11. S-PolKa S-band reflectivity and radial velocity fields at 0.5 deg elevation at 0316-0328 UTC 24 October 2011 (upper panels) and RHIs of reflectivity (lower-left panel) and polarimetric particle type (lower-right panel) along the yellow line in the upper panels. |
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| Figure 12. S-PolKa S-band reflectivity and particle identification fields at 0.5 deg elevation at 0346-0357 UTC 24 October 2011 (upper panels) and RHIs of reflectivity (lower-left panel) and polarimetric particle type (lower-right panel) along the yellow line in the upper panels. |
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| Figure 13. From left to right, S-PolKa S-band reflectivity, polarimetrically derived rain rate, and convective (yellow) and stratiform (red) rain areas for 0400 UTC 24 October 2011. |
| Figure 14. Photo looking east from S-PolKa at 0419 UTC 24 October 2011. |
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| Figure 15. From left to right, S-PolKa S-band reflectivity, polarimetrically derived rain rate, and convective (yellow) and stratiform (red) rain areas for 0600 UTC 24 October 2011. |
| Figure 16. DOE Gan
sounding for 0600 UTC 24 October 2011. |
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| Figure 17. From left to right, S-PolKa S-band reflectivity, polarimetrically derived rain rate, and convective (yellow) and stratiform (red) rain areas for 0800 UTC 24 October 2011. |
| Figure 18. World Wide Lightning Location Network flash freqency for the 30 min preceding 1000 UTC 24 October 2011 superimposed on the METEOSAT infrared image. |
| Figure 19. S-Polka
radar reflectivity (upper panel) and radial velocity
(lower panel) fields at 0.5 deg elevation for 1216 and
1301 UTC 24 October 2011. |
| The upper-level winds were
northeasterly today (Figure 1). At low
levels, over us and to our northwest, was a region of
strong southwesterly wind, which was part of the flow
around a low-level cyclone gyre centered in the northwest
Indian Ocean (Figure 2). The DOE Gan
soundings show that the winds were southwesterly at low
levels and northeasterly at upper levels (Figure 3). The shear vector was
generally SW-NE, and the shear played a significant role
in the organization of the convection on this day.
Midlevel winds varied from southeasterly to northeasterly,
and anvils moved in all this different directions, but
always with a westward component. At 1200 UTC on 25
October, cloud lines aligned along the SW-NE shear vector
could be seen in the METEOSAT visible imagery (Figure 4). Infrared images show that
after that time the stronger convection formed and
produced anvils along the NE portions of these lines, and
the lines moved closer to Gan
(Figure 5). By 1900 UTC 25
October echoes, from these lines were appearing on the
S-PolKa and SMART-R radars. Figure 6 shows examples of the overlap of radar and satellite data at Gan for 0000 and 0400 UTC. The radar data for this time period will be discussed in more detail below. The infrared images in Figure 5 show that after 0400 UTC the most active convective lines were shifting eastward, and by 2030 UTC (last panel of Figure 5), an area of newer intense convection had broken out along a SW-NE oriented region in the region enclosed by the DYNAMO four-corners island/ship array. Unfortunately, the Mirai (located at the SE position) had moved off station by this time, so the array consisted only of Gan, Diego Garcia, and the Revelle, located at the NE position. This outbreak will be discussed in tomorrow's summary. Now we will focus on the radar data collected during the period of 20 UTC 25 October through 0600 UTC 26 October. Figure 7 shows the PPI sequence of reflectivity seen by the S-PolKa S-band radar. Time-lapse viewing of these images shows that the convective cells seen in the first three panels were moving SW-NE in accordance with the low-level winds. Figures 8 through 10 illustrate a stratiform region and anvil to the southwest of a developing convective cell. We were fortunate in that the stratiform region was moving toward the northwest (toward the S-PolKa site) while simultaneously moving over the DOE Ka-band cloud radar (KAZR). This unique setup allows us to examine the radial and vertical components of velocity concurrently for one slice of the system. Figure 8 shows KAZR reflectiviy for the period 2130 UTC to 2330 UTC on 25 October. (See the second panel of Figure 7 for a view of reflectivity near 2200 UTC for a 0.5 deg elevation angle.) We notice that the precipitation falls over the DOE site for more than one hour, and particularly high reflectivities in excess of 30 dBZ are noted around 2200 UTC. A thin strip of about 10 dBZ echoes is detected near the base of trailing anvil that passes over KAZR between 2230 UTC and 2300 UTC. The higher reflectivity probably suggests that larger particles, which will yield higher returns, were falling out of the anvil more quickly. The strip of enhanced reflectivity extends toward a precipitating stratiform region that had passed over KAZR shortly before 2230 UTC. A look at the KAZR Doppler velocities in Figure 9 further indicates that particles are falling, with the additional caveat that an area of enhanced fall velocity exists at the location where the anvil meets the precipitating stratiform cloud. Figure 10 shows an RHI cross-section over the DOE KAZR at 2215 UTC. The stratiform and anvil structure shown by S-PolKa S-band are consistent with that seen by KAZR. In the right panel, light blue indicates inbound radial velocity at the base of the anvil around 6 km. A look at the sounding for 2100 UTC (first panel of Figure 3) reveals that winds around the 0 deg level were easterly to southeasterly; however, near the top portion of the anvil at 8 km, winds were northeasterly. As large ice particles fell through the anvil, they entered a layer of easterlies, thus changing direction such that they fed into and contributed to stratiform precipitation. Figure 11 shows the overcast conditions that prevailed around the radar site during the period 0500-0800 UTC. Thick stratiform clouds covered the sky, and some lower level lines of cumulus clouds appeared from time to time, as in the right-hand photo. Figure 12 summarizes the rainfall in the area covered by S-PolKa over the 13-hour period during which most of the rain fell at the site. During the earlier period, the echoes were more isolated and cellular. As the hours went by, the echoes became larger with many of the convective cells embedded in stratiform echo. In the earlier time period, the more isolated echoes reached heights of 10-12 km, and they produced local heavy rainfall. Figure 13 contains some example cross sections through these echoes. The cells were generally less than 10 km in horizontal dimension. However, they were transporting highly reflective ice particles to farily high levels and showing some signs of graupel production. The wet snow signature in the leftmost cell in the particle type cross section suggests the cells was starting to collapse into stratiform structure. About 2200 UTC 25 October was an interesting time. Large cold pools bounded by gust fronts were emanating from the convection of the SW-NE lines. An example of this behavior is seen in Figure 14, where the gust fronts are marked by positive ZDR, a behavior we have noted in previous summaries and which is most pronounced at night. Also at about 2200 UTC 25 October, the convective cells were becoming somewhat taller and wider (Figure 15). An interesting behavior in the particle identification cross section (right-hand panel of Figure 15). A very strong and spatially continuous concentration of indicators of small horizontally oriented ice crystals was seen at echo top. It is thought that these particles are pristine needles, plates, dendrites or small aggregates. We see this type of particle frequently indicated at echo top but seldom so continuously. By 0330 UTC 26 October, S-PolKa was seeing the convective cells moving in from the SW interacting with stratiform echoes advecting in from the northeast (Figure 16). The second row of panels shows a very intense convective cell to the distant NE and several other cells in the region between it and the radar. All these cells were reaching 14-16 km, higher than the cells outside the stratiform region, and the intense cell is stronger than any seen as isolated cells moving in from the SW. The intervening region cells were collapsing into the stratiform echo and displaying a melting band. All the cells were lofting highly reflective ice particles to 10-12 km heights. The intense cell was producing heavy rain and graupel (second row, right panel). The echo to the distant SE showed a similar structure in dBZ and particle type cross sections. The radial velocity panel (bottom row) showed a strong downward transport of momentum behind a gust front and clear divergence signature at cell top. Figure 17 shows a cross section through a portion of a stratiform region to the northeast. Its reflectivity cross section (bottom left) shows a bright band reaching 50 dBZ at a range of about 60 km. The particle type cross section (bottom middle) shows a robust melting layer in the region of the strongest bright band with graupel at the top of the wet snow layer and melting graupel at the bottom of the wet snow layer. The particle type section also shows interesting structure in the category of oriented small ice particles (purple). It is thought that the layer of purple just above the light blue is likely indicating plates, dendrites, or small aggregates, while the purple indicators at echo top are more likely relatively pristine columns near echo top. The radial velocity cross section (bottom right) shows the strong southwesterly shear governing the environment on this day; strong outbound velocities (toward the NE) were occurring at low levels and strong inbound velocities (from the NE) were occurring aloft. Figure 18 shows that the stratiform precipitation at 0600 UTC still had a similar structure, although it was not reaching as high at this late time. Finally, we note that the convection on this day and previous days in DYNAMO/AMIE has behaved generally as was seen in MISMO by Yamada et al. (J. Atmos. Sci., 2010). Figure 19 shows their conceptual model. Convective cells are advected by lower level westerly-component winds and when the intersect stratiform precipitation regions being advected by upper level easterly-component winds, they form deeper more intense convective cells, which then collapse and join the stratiform precipitation region. |
| Figure
1. Indian Meteorological Department 200 mb model
analysis for 0000 UTC 26 October. |
| Figure 2. Indian Meteorological Department 925 mb model analysis for 0000 UTC 26 October. |
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| Figure 3. Selected
DOE Gan soundings for 25-26 October 2011 |
| Figure 4. METEOSAT
visible image for 1200 UTC 25 October 2011. |
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| Figure 5. Selected
METEOSAT Infrared images for 25-27 October 2011. |
| Figure 6. METEOSAT infrared image overlaid with SMART-R reflectivity for 0400 UTC 26 October 2011. |
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| Figure 7. S-PolKa
S-band reflectivity field for 25-26 October 2011. |
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| Figure 8. Reflectivity
from DOE ARM KAZR short mode between 2130 UTC and 2330 UTC
25 October. |
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| Figure 9. Doppler
velocity from DOE ARM KAZR short mode between 2130 UTC and
2330 UTC 25 October. |
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| Figure 10.
Reflectivity and radial velocity returns at 2215 UTC 25
October from the S-PolKa S-band for an RHI cross-section
of the precipitation and anvil seen in Figures 8 and 9
while located over the DOE gan site. |
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| Figure 11. Photos
taken on 26 October 2011 from S-PolKa at 0541UTC looking
south (left) and 0731 UTC looking east (right). |
| Figure 12. Rain seen by the S-PolKa radar on 25-26 October 2011. From left to right, S-PolKa S-band reflectivity, polarimetrically derived rain rate, and convective (yellow) and stratiform (red) rain areas. |
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| Figure 13. ; S-PolKa S-band radar data for 2129 UTC 25 October 2011. Reflectivity PPI is in the upper panel. Lower panels show reflecitivity (left) and particle type (right) along the yellow line in the PPI. |
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| Figure 14. S-PolKa S-band radar relfectivity (left) and differential reflectivity ZDR (right) for 2201 UTC 25 October 2011. Note how gust fronts shows up as positive ZDR (red). |
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| Figure 15. S-PolKa S-band radar data for 2200-2230 UTC 25 October 2011. Reflectivity PPI in left panel. Particle type RHI in right panel. |
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| Figure 16. METEOSAT infrared image (upper left) and corresponding S-PolKa S-band radar data (other panels) for 0300 UTC 26 October 2011. The second row shows RHIs of reflectivity and particle type at 52 deg azimuth. The third and fourth row show RHIs of reflectivity, particle type, and radial velocity at 116 deg azimuth. Azimuths are shown as yellow arrows in the top row panels. |
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| Figure 17. METEOSAT infrared image (upper left) and corresponding S-PolKa S-band radar data (other panels) for 0400 UTC 26 October 2011. The second row shows RHIs of reflectivity and particle type along the yellow lines in the top row panels. |
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| Figure 18. S-PolKa S-band radar data for 0613 UTC 26 October 2011. Reflectivity PPI is in the upper panel. Lower panels show reflecitivity (left) and particle type (right) along the yellow line in the PPI. |
| Figure 19.
Schematic model of equatorial Indian Ocean mesoscale
convection in southwesterly shear. From Yamada et al. (Journal of the Atmospheric
Sciences, 2010) |
| The convection on this day
differed from that on previous days. We have been seeing
convection in highly sheared environments with low-level
westerlies and upper-level easterlies. This day convection
in the southern DYNAMO array was intense but occurred in
relatively weak shear and deep easterlies. The model
0-hour analyses in Figures 1 and 2 show 200 and 850 hPa wind patterns,
with the location of the southern array highlighted by a
red box. The S-PolKa site is at the upper left corner of
the box, and Diego Garcia is at the lower left. The 200
hPa winds throughout the box were mostly northeasterly.
The 850 hPa winds were also northeasterly throughout the
box, so that the shear was minimal throughout the array.
Note that at the northwest corner of the box Gan lay in a
weak wind zone separating southwesterly low-level winds to
the northwest from northeasterly low-level winds to the
southeast. Very different convective behaviors occured at
the Gan and Diego Garcia corners of the box in Figures Figures 1 and 2.
The METEOSAT infrared imagery in Figure 3
indicate the difference in the phenomena at each of these
corners of the box. AT 0030 UTC a long line of convection
can be traced from Sri Lanka all the way southwestward to
about 10 deg S at the western edge of the image on the
left side of Figure 3. Figure 4 shows the line as it appeared
in a MODIS image. Gan lay under a portion of this line of
convection, and S-PolKa's observations were mainly of this
feature. At Diego Garcia much stronger convection was
occurring at this time, and this convective outbreak
extended on a SW-NE diagonal through the southern array
toward but not reaching the Revelle. This pattern of convection
continued for the next few hours, as can be seen in the
0300 UTC image in the right-hand panel of Figure 3. Figure
5 shows sounding
data for Gan and Diego Garcia. The Gan soundings show
northeasterlies at the highest levels and easterlies
through most of the troposphere. Some westerly winds were
present in the lowest layers. The Diego Garcia soundings
show northeasterlies and/or easterlies at all levels all
day, constituting a weak shear environment for the intense
convective outbreak. Figure
6 shows a time-height series for the northern and
southern DYNAMO arrays. The top panels show easterlies
descending to lower level over the last few weeks.
Preliminary examination suggests that stations to the east
as far as Manus exhibit a similar descent of easterlies. S-PolKa observations in the long SW-NE line of convection seen in Figure 3 and lying in the shear zone seen in Figure 2 showed an interesting behavior between 1800 and 2000 UTC. Figure 7 shows PPI displays of reflectivity and ZDR during this period. They show wind discontinuities prominent as red lines in the ZDR data. These thin-line wind discontinuities do not show the usual close connection with specific convective cells that are characteristic of convective-scale gust fronts. They could be gust fronts, but they are far ahead of the long cloud line and may reflect a discontinuity connected with the larger-scale shear line, which was seen as a thin cloud line in Figure 4. Such long thin rope-like cloud lines have been observed before in the equatorial tropics. Figures 8 and 9 show some examples of convective cells embedded in the area of stratiform echo located north of the radar. This echo area was within the long cloud line. The convective cells have particle type structure generally characteristic of the convection we have described in previous summaries with highly reflective non-melting ice (light blue) being lofted upward and some graupel being produced (green). The convective cells were of moderate depth, some reaching 14 km, but most only to l0-12 km. The cells showed downward transport of outbound momentum (circled in the radial velocity cross sections). The pattern of reflectivity seen in Figure 8 and Figure 9 continued for several hours. Figure 10 shows the pattern at a little after 0200 UTC 27 October superimposed on an infrared satellite image. It can be seen that the echoes to the north were still associated with the long cloud line. Time-lapse viewing of these images showed a behavior much different from any we have seen up to now in this project. The cells to the southeast were moving rapidly westward, and they were moving in tandem with the stratiform echo to the north. Heretofore, we have seen the stratiform echoes moving oppositely to the convective cells. The deep layer of shear had apparently eliminated the shearing effect that disallows the lower and upper portions of mesoscale convective systems to remain together. Figure 11 is for later in the day (0800 UTC), and it shows the infrared image of convection over the southern array with lightning overlaid. A large number of lightning flashes were being recorded, and they indicate the intense nature of the convection in the array. We have noted ice lofted to great heights in nearly all the convective cells seen by S-PolKa during the past month, but we have not experienced lightning. Although we have no S-PolKa data in this outbreak, we can only conclude that this convection was even more productive at producing large ice particles and graupel than the convection we have seen here. Figure 12 shows a stratiform echo with one embedded active cell at 0846 UTC to the southeast of S-PolKa. It was on the edge of the complex of active convection in the southern array. It has particle type structure rather similar to the stratiform echoes we have seen previously. The radial velocity RHI shows inbound (easterly component) velocities aloft, consistent with the environment wind profiles seen in Figures 1, 2, and 5. The outbound velocities along the 140 deg azimuth at low levels would be consistent with southwesterly or northwesterly winds at low levels. The Gan soundings showed varying wind direction at low levels, and at 0900 UTC the Gan sounding showed northwesterlies in the lowest levels. However, based on echo motion, the southeasterlies seem most likely. The reflectivity RHI in Figure 12 shows multiple layers of nonprecipitating cloud close to the radar (see especially the last panel in the figure, which zooms in on the region close to the radar). The cloud photos in Figure 13 show the numerous cloud layers present at the time of the radar observations in Figure 11. The dark horizon in the left panel is the region of the large echo to the southeast. |
| Figure 1. Indian
Meteorological Department 0-hour model analysis for 200
hPa 27 October 2011. The red box is the approximate
location of the southern DYNAMO array. |
| Figure 2. Indian Meteorological Department 0-hour model analysis for 850 hPa 27 October 2011. |
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| Figure 3. METEOSAT
Infrared images for 27 October 2011. |
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| Figure 4. MODIS image
over the geograpical area shown by the red box. The MODIS
image shows the cloud line stretching southwestward from
Sri Lanka. Arrows point to locations along the cloud line. |
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| Figure 5. Selected
soundings taken at Gan (upper tier) and Diego Garcia
(lower tier) for 27 October 2011. |
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| Figure 6. Sounding
time series for the northern DYNAMO northern (left) and
southern (right) arrays for perturbation zonal wind,
meridional wind, temperature and relative humidity.
Courtesy R. H. Johnson and P. Ciesieselski. |
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| Figure 7. S-PolKa
S-band radar PPIs at 0.5 elevation for reflectivity (left) and differential reflectivity (right) for 2131 UTC 27 October 2011. |
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| Figure 8. S-PolKa
S-band radar data at 2231 UTC 26 October 2011. PPIs at
0.5 deg elevation (upper tier) and RHIs (lower tier) of
reflectivity, particle type, and radial velocity. Circle
annotation shows the locations of strong outbound
downdraft outlfow at low levels. |
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| Figure 9. S-PolKa S-band radar data at 2301 UTC 26 October 2011. PPIs at 0.5 deg elevation (upper tier) and RHIs (lower tier) of reflectivity, particle type, and radial velocity. Circle annotation shows the locations of strong outbound downdraft outlfow at low levels. |
| Figure 10. Combined
satellite infrared image and 2-km level S-PolKa S-band
reflectivity data. |
| Figure 11. Lightning
flashes from the World Wide Lightning Location Network
superimposed on METEOSATE infrared image for 0800 UTC 27
October 2011. Flashes are those accumulated over the 30
minutes prior to the time of the satellite data. |
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| Figure 12. S-PolKa
S-band radar data at 0846 UTC 26 October 2011. PPIs at 0.5
deg elevation (upper tier) and RHIs (middle tier) of
reflectivity, particle type, and radial velocity. Image on
the lower tier is a zoomed in portion of the reflectivity
image directly above. It shows the numerous layer clouds
present in the vicinity of the radar site. |
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| Figure 13. Photos
looking south (left) and west (right) from the S-PolKa
site at 0822 UTC 27 October 2011. |
| The global models are analyzing
the MJO as weakening (Figure 1).
However, the convection over the Indian Ocean remains
active, with a major mesoscale convective system occuring
directly over the Revelle
today. Figure 2 shows that between
Gan and the Revelle,
the 200 hPa winds were easterly, while the 850 hPa winds
were generally north-northwesterly. However, the Revelle was located
just west of a confluence zone of northerlies and
northeasterlies. The Mirai
was not on station, so soundings were available for only
three corners of the array (Figure 3).
Diego Garcia (top row) had easterlies through the
troposphere. Gan (middle row) had easterlies from ~600-770
hPa upward but somewhat variable wind direction at low
levels. Revelle
(bottom row) also had easterlies from ~600-770 hPa upward
with generally northerly winds at low levels, consistent
with Figure 2. The left panel of Figure 4
shows a satellite infrared image of the major mesoscale
system over the Revelle
when it was at peak development at about 2200 UTC 27
October 2011. The right panel shows that by 0500 UTC on
the 28th, the system was weakening and smaller systems
were propagating outward from it, some of them moving into
the region surveyed by the S-PolKa. Figure
5 shows the locations of radar observations by the Revelle C-band radar
and the S-PolKa S-band radar in relation to the satellite
images during the time that the mesoscale system over the
Revelle was going
through its lifecycle. Figure 6
shows the sequence of infrared images during the event
with lightning flashes superimposed. The mesoscale
convective system over the Revelle had very little lightning (see
especially the panels for 0000 and 0800 UTC 28 October).
The lack of lightning contrasts sharply with the highly
electrified convection seen near Diego Garcia on 27
October (first panel of Figure
6; see also yesterdays summary). Most of the
precipitation from the mesoscale system in the northeast
corner of the southern array was in view of the Revelle radar during
the entire lifetime of the mesoscale system. Informal
reports from the ship indicate that the ship raingauge
recorded over 190 mm during the event. Figure 7 shows the rain maps for
27 and 28 October computed from the Revelle radar. Areas
over 100 mm accumulation are seen scattered over the
region surveyed by the radar. Figure
8 shows a reflectivity RHI through one of the
convective cells embedded in the stratiform precipitation.
The echo top reached nearly 15 km. The remainder of this summary will discuss the events observed by the S-PolKa radar during the time that the major mesoscale system was affecting the Revelle. At 2200 UTC on the 27th, no major feature was in the infrared imagery over Gan (Figure 4, left panel). Convective cells were scattered around the area and several cold pool boundaries were present in the boundary layer, all moving with westerly components (Figure 9). By 0300 UTC on the 28th, smaller mesoscale systems emanating from the large disturbance over the Revelle were partially within view of the S-PolKa S-band radar (Figure 10). The convective cell at far range was reaching 14 km and lofting highly reflective nonmelting ice (blue) to nearly that height. It was producing an anvil at about 10-12 km height. At 0800 UTC a mesoscale echo system was north of S-PolKa and moving toward the radar (Figure 11). Its most intense cell was reaching 17 km and lofting highly reflective nonmelting ice to ~15 km. It had heavy rain at low levels and had a distinct divergence signature at cell top and an outbound radial velocity signature at about 50-70 km range that is probably associated with viewing the cold pool from an odd angle. Figure 12 shows the gust front of this system distinctly, especially in the ZDR and polarimetric data (top row). The bottom panels show that the convection along the line was producing graupel even though the cells were not too deep, reaching ~12-14 km. Figure 13 shows photos of a roll cloud associated with the gust front approaching (upper photo) and receding (lower photo). The RHIs in Figure 14 look through the stratiform region of this squall line system toward convective cells far to the north. The straiform region has a well defined melting layer, and the cell to the north, though not tall is producing some graupel signatures. The S-PolKa precipitation product at 0942 UTC (Figure 15) depicts the stratiform and convective components of the squall-line system. The most highly convective end of the line is the portion farthest to the east. The eastern end of the line was overlapping the mesocale cloud system seen in the infrared imagery to emanate from the large mesoscale system over the Revelle. Figure 16 shows the location of the eastern end of the line seen on S-PolKa with respect to the satellite imagery (left panel). A cross section along the black line in the left panel is shown in the right panel. The echo at the east end of the line is over 50 dBZ over a horizontal distance of ~20 km. Such a massive intense echo did not occur outside the portion of the line overlapping with the system coming from the east. It is as if the potentially explosive convective development cannot occur outside the environment of the stratiform echo of a system of the type that came from the east. The interaction of the two echo systems continued through 1115 UTC 28 October (Figure 17), with the cell to the east remaining more intense than any other echoes in the field of view. RHIs through a cell in the part of the convective line outside the region of interaction with the system to the east shows and intense but not especially deep or wide cell (lower panels of Figure 17). By 1415 UTC, the convective line had reformed an moved farther south (Figure 18). RHIs along a southeast azimuth (lower panels) show the stratiform echo that is a combination of the stratiform region trailing the line and the system to the east, and they also show again that the cells in the portion of the line outside of the interaction zone are intense (e.g. a substantial amount of graupel is seen in the particle type cross section) but not especially deep or wide. The S-PolKa rain analysis in Figure 19 depict the convective nature of the cells in the line to the south. Much of the precipitation to the east and south of the radar is stratiform. However, in the distant northeast sector of S-PolKa's field of view the echoes are so intense as to be classified as convective by the algorithm. These intense echoes cover a very large area, testifying to the strengh of the mesoscale systems located east of S-PolKa. |
| Figure 1. Bureau of
Meteorology MJO analysis. |
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| Figure 2. Indian
Meteorological Department 0-hour analyses for 0000 UTC 28
October 2011. Red box locates the DYNAMO southern array. |
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| Figure 3. Selected
soundings at Diego Garcia (top), Gan (middle) and Revelle (bottom). |
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| Figure 4.
Selected METEOSAT infrared images for 27-28 October 2011. |
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| Figure 5.
S-PolKa S-band and Revelle C-band radar images
superimposed on METEOSAT infrared images on 27-28 October
2011. |
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| Figure 6. World Wide Lightning Location Network flashes accumulated over 30 min superimposed on infrared images at the end of the 30 min period. |
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| Figure 7. Rainfall
measured by the Revelle C-band radar on 27 and 28 October
2011. |
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| Figure 8. A
reflectivity RHI take by the Revelle C-band radar. |
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| Figure 9. S-PolKa
S-band PPIs of reflectivity (left) and ZDR (right) at 0.5
deg elevation for 2116 UTC 27 October 2011. |
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| Figure 10. S-PolKa S-band radar data for 0316-0329 UTC 28 October 2011. PPI of dBZ at 0.5 deg elevation (top), RHIs showing left-to-right dBZ, radial velocity, and particle type. |
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| Figure 11.
S-PolKa S-band radar data for 0801-0807 UTC 28 October
2011. PPI of dBZ at 0.5 deg elevation (top), RHIs showing
left-to-right dBZ, radial velocity, and particle type. |
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| Figure 12. S-PolKa S-band radar data for 1002-1012 UTC 28 October 2011. Left to right in upper row: PPIs of dBZ, ZDR, and particle type at 0.5 deg elevation (top). Left to right in bottom row: RHIs showing dBZ and particle type. |
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| Figure 13. Photos
taken from the S-PolKa site on 28 October 2011. Top:
looking NE at 0919 UTC. Bottom: looking SW at 0947 UTC. |
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| Figure 14. S-PolKa S-band radar data for 1051-1101 UTC 28 October 2011. PPI of dBZ at 0.5 deg elevation (top), RHIs showing left-to-right dBZ and particle type. |
| Figure 15. S-PolKa
S-band radar data for 0943 UTC 28 October 2011. Left to
right: reflectivity superimposed on infrared satellite
image, polarimetrically tuned rain rate, and convective
(yellow) and stratiform (red) precipitation areas. |
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| Figure 16. S-PolKa
S-band radar PPI for 1114 UTC 28 October 2011 superimposed
on the infrared image (left) and vertical cross section
(right) of reflectivity along the black line in the PPI. |
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| Figure 17. S-PolKa
S-band radar data for 1115 UTC 28 October 2011. PPI of dBZ
at 0.5 deg elevation (top), RHIs showing left-to-right
dBZ, particle type, and ZDR. |
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| Figure 18. S-PolKa
S-band radar data for 1400-1415 UTC 28 October 2011. PPI
of dBZ at 0.5 deg elevation (top), RHIs showing
left-to-right dBZ, particle type, and ZDR. |
| Figure 19. S-PolKa
S-band radar data for 0943 UTC 28 October 2011. Left to
right: reflectivity superimposed on infrared satellite
image, polarimetrically tuned rain rate, and convective
(yellow) and stratiform (red) precipitation areas. |
| From the METEOSAT infrared images
in Figure 1 it can be seen that today
Gan lay under old anvil cloud (altostratus and
cirrostratus) from convective outbreaks throughout the
central Indian Ocean region (Figure 1). Photographs
in Figures 2
and 3 taken at the S-PolKa site
during the day show the cirrostratus overcast. Cumulus
clouds were developing at low levels, indicating an active
moist boundary layer during parts of the day (Figure 2). The anvil cloud was very
thick and producing visible fallstreacks from the middle
and upper-level layer clouds (Figure 3).
The DOE Gan soundings in Figure 4
show that the anvil cloud was being advected into the
region by upper level easterlies. The apparentl lack of
saturation in the anvil layer is probably because
soundings are not computed for saturation with respect to
ice. Onion soundings were not prominent at the times
shown. The regime at lower levels seems independent of the
upper-level anvil clouds. The Gan DOE KAZR vertically
pointing Ka-band radar shows the anvil cloud in the range
of 8-15 km through most of the day (Figure
5). |
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| Figure 1. Selected METEOSAT infrared images for 29-30 October 2011. |
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| Figure 2. Photos looking NE and SE from S-PolKa site at 0542 UTC 29 October 2011. |
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| Figure 3. Photos looking NE and E from S-PolKa site at 0948 UTC 29 October 2011. |
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| Figure 4. DOE Gan soundings for 29 October 2011. |
| Figure 5. DOE Gan KAZR vertically pointing Ka-band radar data for 29 October 2011. |
| The DOE
sounding (Figure 1) wind regime at
Gan has changed back to westerlies at low levels and
easterlies at high levels, with particularly strong
easterlies aloft. A dry layer in northwesterlies is
prominent at about the 700 mb level. A map of the winds at
700 hPa (Figure 2) suggests that
the dry northwesterlies were coming from the cyclonic
disturbance in the Arabian Sea. University of Wisconsin Morphed
Integrated Microwave Imagery for this time period
suggests that the dry air coming into the Gan region
was of African origin (click
here to see the loop). The cyclonic circulation
extended over the Arabian desert and
winds proceeded southeastward. These winds would be
expected to be dry and possibly dust laden. Figure 3 shows that the major convective
feature seen by satellite was a large mesoscale convective
system NE of the Revelle
(located at point NE). Gan lay to the SW of this major
development under the influence of smaller convective
elements on the periphery of the large system during the
time period of the first three panels of Figure3. For the remainder of the day,
Gan was under theumbrella of the cirrus shield of the
major mesoscale system (last panel of Figure 3). Both the major mesoscale
system to the NE of Gan
and the convection on the periphery affecting Gan were
electrified, with lightning being observed throughout the
time period illustrated with data from the World Wide
Lightning Location Network in Figure
4, which contains images from about 2100 UTC 29
October to 0900 UTC 30 October. Lightning was being
detected in the vicinity of Gan throughout this period as
well as in the large mesoscale system to the northeast.
Lightning was visually observed and thunder heard at Gan
at from 0057-0107 UTC 30 October. Examples of the cells
that produced the lightning are illustrated by Figures 5 and 6. The cells were extremely
vigorous but only about 12 km deep. A question is why
these cells, which were very intense lacked vertical
development. Could the lack of vertical growth have been
due to the dry layer at 700 hPa or the strong easterlies
aloft? Also, could the lightning have been favored by the
presence of dust? Was the African air more potentially
unstable that that normally occurring over the equatorial
ocean? The particle type identification cross section
(lower right of Figures 5 and 6)
shows graupel and even small hail signatures all the way
up to 10 km. These signatures are consistent with the
presence of electrification. Figure
7 shows that the discrete electrified convective
cells seen 0053 UTC 30 October (first panel) were
gradually absorbed into the cloud mass of the giant
convective system to the NE of Gan (lower two panels). Figure 8 shows that after these
cells were absorbed, the portion of the cloud mass that
could be seen by S-PolKa had a stratiform echo and a
descending midlevel inflow (apparent as the long sloping
red feature showing enhanced outbound radial velocity).
During the rest of the day the nonprecipitating anvil of
the giant mesoscale system to the NE remained overhead (Figure 9). This anvil was well
observed by the DOE AMF2 instruments. The DOE Gan
vertically pointing Ka-band KAZR and MPL lidar
observations show the thin anvil with fallstreaks from
from about 0600-1500 UTC 30 October (Figure 10). After that time the KAZR
and MPL show patchy
thin layer clouds at both middle and upper levels. |
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| Figure 1. DOE Gan soundings for Gan (left) and Revelle (right) for 30 October 2011. |
| Figure 2. Indian
Meteorological Department 0-hour 700 hPa analysis for 0000
UTC 30 October 2011. |
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| Figure 3. Selected METEOSAT infrared images for 29-30 October 2011. |
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| Figure 4. World Wide Lightning Location Network flashes accumulated over 30 min superimposed on infrared images at the end of the 30 min period for 29-30 October 2011. |
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| Figure 5. S-PolKa radar data for 2215-2230 UTC 29 October 2011. Top row: PPI of reflectivity at 0.5 deg elevation. Bottom row: RHIs of reflectivity (left) and particle type (right). |
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| Figure 6. S-PolKa radar data for 0130-0136 UTC 30 October 2011. Top row: PPI of reflectivity at 0.5 deg elevation. Bottom row: RHIs of reflectivity (left) and particle type (right). |
| Figure 7. S-PolKa radar reflectivity data at 2 km superimposed on METEOSAT infrared image for 0053-0253 UTC 30 October 2011. |
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| Figure 8. S-PolKa
radar data for 0345 UTC 30 October 2011. Top: PPI of
reflectivity at 0.5 deg elevation. Bottom: RHI of radial
velocity. |
| Figure 9. Photo
looking east from S-PolKa radar data at 1057 UTC 30
October 2011. |
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| Figure 10. DOE
vertically pointing Ka-band radar KAZR (upper) and MPL
lidar (lower) observations for 30 October 2011. |
| A strong squall line passed over
the S-PolKa site today. It was the strongest gust front we
have seen here, and it was another situation in which
lightning occurred. The DOE Gan sounding at 0300 UTC (left
panel of Figure 1), taken before
squall line passage still had the dry layer and
northwesterlies at 700 hPa that we have seen the last two
days. The upper level easterlies were very strong.
At lower levels, the 700 and 850 hPa wind patterns both
showed the cyclonic circulation in the Arabian Sea with
winds streaming over the Arabian desert then eastward and
southeastward toward the DYNAMO array (Figure
2). The 0300 UTC sounding had a very unstable
temperature profile from 850 hPa downward, the lapse rate
was tending toward dry adiabatic. The occurrence of strong
vertical motion in the lower troposphere is understandable
in view of this low-level stratification. Very large
buoyancy could occur with the lifting of that layer. The
sounding at 0900 UTC, taken after the passage of the
squall line and its stratiform region showed dry air and
northwesterlies extending below 850 hPa. The question
arises as to what triggered the squall line convection.
The yellow arrows in Figure 3 show the
end points of a zone of convection that was initiating at
about 0130 UTC (first panel). This zone of convection
moved eastward in the form of a line that was slightly
convex in its direction of propagation. The size and
general behavior of this larger line is suggestive of a
larger-than-convective-scale wave controlling the
convection. The third image in the sequence was when the
squall line was being observed on the S-PolKa radar. It is
the bright red cold cloud shield near Gan. At the time of
the third infrared image, the entire pattern of convection
between 70 and 90 E is suggestive of a wave pattern. Figure 4 provides an overview of
the radar data obtained by the S-PolKa as the squall line
came through the radar area. The curved leading edge of
intense radar echo is the nose of the system being moved
along by the lower level westerly wind components, and the
red cloud top image is the anvil streaming off to the west
in the easterlies aloft. Figure 5
shows that lightning was associated with this squall line
and with other convective elements in the large complex of
which it was a part. A clear message from the satellite
images in Figure
4 and 5
is that a continuous anvil covering a broad area did not
occur despite the instability evident at low levels in the
0300 UTC sounding (Figure
1). Either the dry 700-850 hPa layer or the strong
shear with very strong easterlies aloft may have prevented
widespread anvil development. Figure
6 shows a very typical structure of a squall line
with trailing stratiform precipitation. The leading line
convective cells are intense. The lower row of panels show
a cell with high reflectivity, graupel mixed with heavy
rain, graupel lofted up to about 9 km, a strong gust front
with 15 m/s outbound velocity near the surface, and a
sloping midlevel rear inflow jet. The most interesting
feature of this squall line is how weak the trailing
stratiform precipitation was. Figure
7 was selected to show the most continuous and
broadest segment of stratiform precipitation, and the echo
does show a bright band in reflectivity that is
characterized by a layer of wet melting snow. But the
identifiable ice particles extend only up to about 7-10
km. The environment present on this day was evidently
unfavorable to the deveolpment of broad stratiform
precipitation. |
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| Figure 1. DOE Gan soundings for Gan for 31 October 2011. |
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| Figure 2. Indian
Meteorological Department 0-hour 700 hPa analysis for 0000
UTC 30 October 2011. |
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| Figure 3. Selected METEOSAT infrared images for 31 October 2011. |
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| Figure 4. METEOSAT
infrared image with S-PolKa reflectivity at the 2 km level
superimposed. |
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| Figure 5. World Wide Lightning Location Network flashes accumulated over 30 min superimposed on infrared images at the end of the 30 min period for 31 October 2011. |
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| Figure 6. S-PolKa
S-band radar data for 0546-0556 UTC 31 October 2011. Upper
row, left to right: PPIs at 0.5 deg elevation for
reflectivity, particle type, and radial velocity. Lower
row, left to right: RHIs of the same parameters along 64
deg azimuth. |
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| Figure 7. S-PolKa
S-band radar data for 0716-0727 UTC 31 October 2011. Upper
row, left to right: PPIs at 0.5 deg elevation for
reflectivity, particle type, and radial velocity. Lower
row, left to right: RHIs of the same parameters along 82
deg azimuth. |
