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.