Revelle (TOGA) Radar Report
For October 2011
S. Rutledge, T. Lang, and A. Rowe
Photo courtesy of T. Lang
Operations summary:
This report covers the Revelle radar operation for DYNAMO Cruise 2. The TOGA
radar operated 24/7 with no problems. The Cruise 2 data has arrived back at CSU
and the dataset is fully intact (Cruise 1 as well). The radar has been very stable and
reliable. During Cruise 1 radar uptime was 95%, and this improved to 99% during
Cruise 2. The various calibrations done over the course of the month have been
repeatable. At this point in time we estimate the TOGA radar is about 1 dB high.
Science summary:
Fig. 1 shows the areal-averaged rainfall within a 150 km radius centered on the R/V
Revelle. Lesser rain amounts were observed between 1-15 October, with heavier
rain amounts in the second half of the month. There was a marked difference in the
type of convection observed between 1-15 October and 16-30 October. (Note: The
Revelle departed its station point on 29 October to make its port call in Phuket,
Thailand). In general, isolated convective cells dominated the radar observations
between 1-15 October. These cells generally drifted west to east. Most cells were
shallow and dominated by warm rain production. Occasionally deeper convection
was present, but remained isolated. In the latter half of the month, deeper
convection was present, organized on the mesoscale. Large areas of stratiform
precipitation were also observed.
Fig. 1. Areal-averaged rainfall observed by the Revelle radar between 1-30 October
2011. The DYNAMO Z-R was used. No attempt was made to discriminate between
convective and stratiform rain. Analysis by Dr. Timothy Lang, CSU.
Examples of these isolated cells are shown in the following figures. Figure 2 shows a
RHI from 1009 UTC on 11 October along 33 degrees azimuth. Echo tops were to 8
km, but these cells were largely warm rain producers given the low level echo
centroids.
Fig. 2. RHI at 33 degrees azimuth from 1009 UTC, 11 October 2011.
An example of the scattered nature of cells around this period is shown in Fig. 3.
During the first part of the month the sounding time series over the Revelle showed
reduced upper tropospheric moisture and moderate tropospheric shear. Westerly
winds persisted up to approximately 600 mb, explaining cell motion from west to
east. Why mesoscale organization was generally absent in the first part of the month
remains elusive, but the reduced upper level moisture and moderate shear are
possibilities. (See Fig. 4).
Fig. 3. Scattered cells on
10 October 2011 at 1000
UTC.
Fig. 4. Monthly sounding
time series at the R/V
Revelle location.
Dr. R. Johnson.
We now turn to a brief discussion about the later half of the month. The next set of
images is from the middle of the cruise when convective organization began to
increase in an environment of directional vertical wind shear. Despite the increase
in organization and overall radar echo depth, shallower, shorter-lived convection
persisted. Fig. 5 shows these shallow and deeper modes of convection, as well as the
sunrise. Fig. 6 shows the echo top product with impressive echo tops, marking onset
of deeper convection around this time period. The corresponding RHI at that time
(Fig. 7) highlights the deep embedded convection. Even in the shallower cells,
reflectivities exceeded 50-55 dBZ.
Fig. 5. Example of a shallow
and a deeper convective cell.
The echo near the top of the
panel is associated with
the sunrise.
Fig. 6. Radar echo tops at 1740 UTC on
15 October 2011. Approximate azimuth
of the RHI shown in Fig. 7 is indicated
on the figure.
Fig. 7. Example of a deep
convective cell on 15 October
2011 at 1749 UTC.
During this period of increasing mesoscale organization, outflow boundaries played
a key role in convective initiation, organization, and convective strengthening. One
example occurred on 20-21 October, where colliding outflow boundaries resulted in
the rapid intensification of convection near the radar (Fig. 8). On 24 October, an
outflow boundary propagated over 100 km toward the east-southeast, later
interacting with an ongoing MCS, leading to intensification of convection along a line
embedded within the stratiform area (Fig. 9). In this case, the convective line
continued to move toward the SE within the low-level westerly flow, while the
stratiform area slowly extended back toward the NW, driven by the easterly winds
above 600 mb on this day, although overall storm motion remained relatively slow.
This trend of opposing directions of movement between the convective and
stratiform components appeared to be a common characteristic of MCSs in this
region forming within an environment characterized by weak winds, but directional
vertical wind shear.
Fig. 8. A short,
intense
convective
element that
formed from
colliding
outflow
boundaries.
Fig. 9. A) Shallow
cells along a
well-defined
outflow boundary
east-southeast of
the Revelle radar.
B) Resulting line
of convection
triggered by the
boundary.
Another example is provided on 28 October, with enhancement of convection in an
area where an outflow boundary converged with the prevailing low-level
environmental flow (Fig. 10). On this day, wave after wave of squall type MCSs
overran the Revelle moving northeast to southwest. Total rain at the ship on 28
October was 191.6 millimeters, the largest accumulation during cruise 2. Convection
generally decayed after 29 October following the passage of the MJO and the onset
of drying conditions.
Fig. 10. Intense
convective
line near Revelle
on 28 October
2011. Strong rear
inflow is evident
in the Doppler
velocity pattern
(right panel).
Some examples of the extensive stratiform precipitation in the later portion of the
month when extensive mesoscale systems were frequent are also
of interest. Fig. 11 shows a well defined bright band that seems to be extremely
uniform. Note the sheared flow stratified by the melting level. Fig. 12 is another
example, but this time the bright band is more convective in nature owing to the fact
that this stratiform area formed via the decay of cells rather than the advection of
ice particles over sloping trajectories from convection.
Fig. 11. RHI
through
stratiform
precipitation
on 22 October
2011.
Fig. 12. RHI
through
stratiform
precipitation
on 28 October
2011.
Fig. 13 shows a lightning Hovmoller plot using Vaisala's new GLD360 product,
courtesy of Dr. R. Holle of Vaisala. The Hovmoller goes from the Equator to 10S to
cover the DYNAMO area. First, the CG lightning patterns show the general system
movements that have been previously documented. Convective elements moving
west to east in the first part of the month, and then east to west after say 15 October.
The first lightning event around 6-7 Oct was not really seen in any of the rain time
series from Gan or Revelle. We are now analyzing daily maps to get a better handle
on this (e.g., the lightning generating systems were south of the Gan/Revelle
latitude). The zonally moving lightning signal around 14 Oct coincided with that
peak rainfall on Gan Is. at that time. Revelle and Gan Is. both had a lot of rain during
the 19-29 October period but the only significant lightning events were at the
beginning and end of that period. Assuming that lightning indicates the presence of
not just deep convection, but vertically developed convection (that is, cells are not
only deep, 10 dBZ to > 16 km but the 30 dBZ contour extends well into the
supercooled region) this might suggest that the deep and intense convection
occurred near the start of the heavy rain period and was then again at the end.
Monsoon-like rain falls between, where lightning is known to be infrequent at best.
This is all very preliminary though. We are anxious to see if a similar pattern holds
for the next MJO bursts.
In summary, a marvelous dataset was collected by the Revelle radar during cruise 2.
We are anxious to combine the radar measurements with the ocean measurements
made on the Revelle to begin addressing the air-sea coupling important to MJO
initiation.
Fig. 13 Lightning Hovmoller. Courtesy Vailasa’s GLD360 dataset.