Dependence of rain drop size distribution parameters on atmospheric instability over a tropical location near the land-sea boundary

The role of atmospheric instability, expressed by Convective Available Potential Energy (CAPE), in determining raindrop size distribution (DSD) parameters have been investigated over an urban tropical location, Kolkata (22.57°N and 88.37°E), India, near the land-sea boundary. DSD measurements obtained from a ground-based disdrometer during both pre-monsoon (March-May) and monsoon periods (June-September) have been used in conjunction with CAPE from ERA-5 data from 2014 to 2018. Based on collocated observations from a K-band Doppler micro rain radar, rain events are classified as convective or stratiform. The study has shown that enhanced CAPE values significantly influence DSD parameters by increasing mass-weighted mean diameter (D
m
) during convective rain, but it does not impact directly during stratiform rain. For stratiform rain events, the melting layer becomes a significant factor in determining D
m
, which decreases with increasing melting layer height. Convective rain exhibits a bimodal DSD pattern for higher CAPE values, while stratiform rain shows a unimodal DSD feature. The present study location provides a unique opportunity to study the combined influences of air mass flows from the hot land region in Chotanagpur plateau and from the sea region of the Bay of Bengal on precipitation features. The investigation reveals the dominance of continental activities during convective precipitation and of maritime airflows on stratiform rain events, which is more visible during the pre-monsoon season compared to the monsoon period. For the first time, the dual control of CAPE and melting layer on drop size distributions has been demonstrated for convective and stratiform rain events.