3.2.3 Convective event on 25th May, 2018
An organized mesoscale convective system over southern peninsular India
prior to the onset of the southwest monsoon occurred on
25th May, 2018 during 13:00-19:00 IST. The CAPE value
of 148 J kg-1 was observed from the nearest radiosonde
measurements in the morning hour (05:30 IST) and increased to 1092 J
kg-1 in the afternoon hours (17:30 IST) suggesting the
unstable layers favouring the formation of convective event. The
development of the convective system is presented in the PPI diagrams of
radar reflectivity field from 12:58 IST to 16:10 IST consecutive times
during the event (Figure 14). Unlike the convective system on
13th May, this system started forming over the oceanic
region (westward from radar) as well as over the land (north-east ward
from radar) around 13:00 IST and gradually covered more and more region.
Compared to the previous system, this system had a much larger spatial
extent with a lower value of the peak reflectivity during the
development stage, suggesting lesser convective activity compared to the
13th May event. Rainfall and the embedded rain DSD
during the event were recorded by the disdrometer at NCESS (Figure 9b).
The peak rain rate (~10 mm h-1) was
significantly lower compared to that for the previous system (100 mm
h-1). The drops of diameter greater than 3 mm are
absent on 25th May, 2018 suggesting lesser updraft
speed and hence lesser time for the growth of raindrops. The
spatial-temporal evolution of the brightness temperature (INSAT-3DR
data) during the event (Figure 10f-10j & 11) reveals a slow development
of the cloud system. The development is markedly different from the one
on 13th May event, which was much more rapid. Around
15:30 IST the lowest brightness temperature of ~220 K
was noted, which was significantly higher than the convective event on
13th May 2018 (<185 K), indicating less deep
cloud systems. The ceilometer observation shows the presence of clouds
having the base height below 5 km level (Figure 12b) during the
initiation of the convective system. The dissipation phase of the event
was registered with high level clouds (5 < CBH < 10
km) over the region.
The vertical structure inside the system is shown through a vertical
cross along a convective region (Figure 15). The spatial distribution of
the reflectivity averaged between 2.5 and 3.5 km height during
developing stage is shown in Figure 15a. A vertical cross section along
the line AB through the convective region is taken and hydrometeor
identification has also been done. The overall features of the different
variables are very much similar to those in Figure 13. The reflectivity
core at a distance between 5 and 20 km from point A was observed and it
reached up to a height of 7 km (Figure 15b), which was 10 km on
13th May event. The Zdr values (Figure
15c) along the convection line are much lower due to smaller drop size
as observed in the DSD from disdrometer. Values of ρhv(Figure 15d) are high (>0.9) indicating presence of rain
with smaller drops at lower levels. The structure of Kdp(Figure 15e) follows the structure of Zh but values are
less (<4° km-1) compared to that in
13th May case. This is mainly because of lower rain
rate and relatively smaller drops with less eccentricity, resulting in
smaller difference between the reflectivity at horizontal and vertical
polarization. Identified hydrometeor types shown in Figure 15f are quite
similar to the ones in 13th May case (Figure 13f).
Rain (RN), aggregates (AG) and graupels (HDG) are the main hydrometeor
types identified. The graupels are present mainly in the highest
reflectivity column. In this case the abundance of graupels is much less
compared to the 13th May case. This is because of
lower updraft as inferred from the brightness temperature data. The
occurrence of drizzle (DZ) types was also identified along the vertical
cross section.