3.2.1 An overview of the synoptic conditions
Favourable synoptic and thermodynamic conditions help in the organization of convective storms to develop into severe ones (Mukhopadhyay et al., 2009). High moisture, atmospheric instability, vertical wind shear and a lifting mechanism are the different necessary conditions for the development of thunderstorms. Hence, an overview of the synoptic conditions before and during the events will give more insights into their development. Geopotential height anomaly and wind data from ECMWF Reanalysis v5 (ERA5) at 12 UTC and vertical profile of equivalent potential temperature (θe), mixing ratio, wind speed, wind direction from radiosonde measurements by IMD, Thiruvananthapuram at 00 UTC and 12 UTC were used to look into the environmental conditions for the events.
A low-pressure area formed in the south west Arabian Sea (Figure 7a) on 13th May, 2018 which was evident from the minimum geopotential height anomaly at 700 hPa levels between 55-65E and 4-10N, was far from the study region. Under this influence, the mean wind was from Bay of Bengal to the Arabian Sea in the easterly direction (figure 7a). The strong negative gradient of the θe profile up to 3 km height shows the instability in the lower atmosphere (Figure 7c). The mixing ratio profiles indicate the presence of moist layers between 2-6 km levels, also suggest the existence of favourable atmospheric conditions for the formation of thunderstorm. Wind direction changed abruptly along the vertical which is due to the turbulence associated with the unstable lower atmosphere. Heavy rainfall in isolated places were reported over Kerala and Tamil Nadu by IMD. These conditions lead to the formation of convective system over inland region on 13 May 2018 in the afternoon hours between 16:00-22:30 IST.
The convective event occurred over southern peninsula on 25th May 2018, between 13:00-19:00 IST. The lower geopotential height anomaly at 700 hPa level clearly demonstrates the convection is active and strong (Figure 7b) evident with scattered low and medium clouds favouring the intense to very intense convection. The sounding analysis over the study region indicates that, θe and mixing profiles in the morning and evening hours shows unstable and moist layers in the near surface levels. Also, the near surface wind was more than 10 ms-1 from the south westerly directions (near to monsoon onset). Further to the west, cyclonic vortex Meknu (T5.0) was formed over west central adjoining south west Arabian sea with lay centred over 15.2°N and 54.3°E.