atmospheric science

In India, regions adjoining the Western Ghats are known to have relatively high (~118) lightning incidences (GHCC, NASA 2003). The nature of spatial and temporal distribution of past incidents, type of thunderclouds which cause lightning, the topography, proximity to mountain range and sea point to the possibility of the Western Ghats mountain weather aiding in Cumulonimbus (Cb) cloud formation. Our studies have shown that the lightning incidences are more in the midlands, while less in mountain and coastal region. Also, incidences are less in the region west of Palakkad gap, which are devoid of mountains. In order to understand convective thundercloud formation and the associated lightning phenomena, the CESS has set up a mountain station on the western slope of the Western Ghats at Braemore, adjoining the Ponmudi Hills near Trivandrum. Our measurements have shown that Western Ghat mountain slope leads to thundercloud formation. The monitoring stations with automatic weather stations are already operational, but needs to be strengthened with sophisticated equipments. The results indicate that the existence of strong updrafts followed by the formation of thunderclouds provide an ideal situation to study the Cb formation and electrical characteristics of lightning discharges. However, more studies are required to understand the phenomenon of convective thundercloud formation and the lightning phenomena associated with it.

CESS was also engaged in a programme with support from the Space Applications Centre (SAC, ISRO) to characterize rainfall and cloud formation using data derived from the Megha-Tropiques satellite. As a continuation of this study microphysical parameters of rain are to be measured along with rain drop size distribution (DSD) at few locations. It is also observed that heavy rainfall leads to soil erosion and DSD information could help in understanding soil erosion especially in the high lands. Our understanding of the rainfall and cloud phenomena has shown that adequate coverage of rain events in the Western Ghat region can be done efficiently from 8 locations along the Western Ghats from Augumbe (Karnataka) to Kanyakumari (Tamil Nadu) separated by 200 km. In addition, as the weather in Western Ghats region is conducive for the formation of clouds, it should also be possible to study the precipitation microphysics, cloud formation and its propagation over southern India.

Field stations will be set up at suitable places in the Western Ghats for collection of primary data. Since 62% of the total lightning incidents happen in the months of April, May, October and November, condensation particle number density and the electric field data would be collected during these months to study and understand the characteristics of Cb formation over the location. Simultaneously, cloud base height measurements would be undertaken using Laser Ceilometers. Lightning detectors would be used to gather spatial and temporal occurrences of lightning. Cloud Top Temperature (CTT) data from India Meteorological Department (IMD) will also be utilized along with the data collected using the above instruments to identify the location of Cb and its growth during traverse of Cb. Rain parameters such as rain drop size (including its vertical profile), fall speed spectrum, and electrical nature, would be measured from 0.2 to 6 km. An empirical model utilizing the above parameters would be derived and validated to understand precipitation microphysics.