Seasonal variability of raindrop size distribution over Cuddalore, a coastal station in Tamilnadu of southern peninsular India

Abstract

Cuddalore (11.46° N / 79.46° E), a tropical coastal station in Tamilnadu of southern peninsular India receives precipitation from pre-monsoon (March – May), southwest monsoon (June – September) and northeast monsoon (October – December). While the precipitation during pre-monsoon (PM) and southwest monsoon (SWM) is mostly convective, that received during northeast monsoon (NEM) is mostly stratiform albeit a juxtaposition of both convective and stratiform is also feasible.  The seasonal variability of raindrop size distribution (DSD) has been studied using the data obtained from electro-mechanical disdrometer (Joss-Waldvogel type) at Cuddalore. The modal drop size is less than 2.0 mm diameter in stratiform precipitation whereas drops of  higher diameter (more than 3 mm) is quite probable in convective precipitation events. The mean concentration of rain drops of size more than 3 mm is highest during pre-monsoon followed by southwest monsoon in rain rates exceeding 10 mm h^-1 due to rapid collision and coalescence taking place in afternoon mixing and convective currents.  The concentration of smaller size drops (of size less than 2 mm dia) especially in rain rates exceeding 8 mm h^-1 is more during NEM than the SWM because the condensed particles could not grow effectively into larger drops due to the prevalence of either weak instability or nocturnal stability conditions during NEM. Convective type precipitation has higher rain rates than the stratiform type. Inverse relationship between drop concentration and rain rate is seen during convective situations, while the relationship is linear during stratiform conditions. Lognormal distribution fits the DSD of northeast monsoon (mostly stratiform precipitation) extremely well. However, this fitting has some deviation in the rain rate 10-50 mm h^-1 during pre-monsoon and southwest monsoon season (mostly convective precipitation) based on the limited data sample obtained during 2003.