Spatial and seasonal variability of raindrop size distributions in southeast India

Long-term measurements of raindrop size distribution (DSD) made with the Joss-Waldvogel disdrometers at two sites (Gadanki, an inland station, and Cuddalore, a coastal station) in southeast India are utilized to study the seasonal and spatial variations of DSD. The stratified DSD data (based on rain rate R) show significant seasonal variation at both sites. Smaller-drop concentration is higher in the northeast monsoon (NEM) than in the southwest monsoon (SWM) for the same R. Paucity of smaller drops in SWM increases the mass weighted mean diameter (D-m) considerably. The seasonal differences are pronounced at Gadanki. The seasonal differences are found to be a regular feature at these locations as they are observed in all the years. The DSD has also shown clear diurnal variation with large Dm values in evening hours. The possible causative mechanisms for the observed spatial and seasonal DSD differences are investigated in detail using satellite and radiosonde observations. In particular, the research attempted to address the following question: Are the observed seasonal differences in DSD arising at the cloud formation level or related to the microphysical processes occurring in the evolution of DSD. The low-level wind pattern and cloud effective radius and Dm distributions in these seasons reveal that the cloud systems in SWM and NEM are continental and maritime, respectively, in nature. However, the microphysical and dynamical processes related to evaporation and convection also seem to play an important role in modifying the DSD. These processes are found predominantly in SWM and are, primarily, responsible for the changes in DSD during their evolution.