On the seasonal variability of raindrop size distribution and associated variations in reflectivity – Rainrate relations at Tirupati, a tropical station

• Understanding the seasonal characteristics of rain and DSD at a station located near the foothills of Nallamala hills.
• Segregation of different types of rain and quantifying their occurrence and rain fractions.
• Deciphering possible reasons for the occurrence of fewer small drops and more big drops during the SWM than in NEM.
• Retrieving season- and rain-specific relations appropriate for southeast India.

Three years of continuous OTT Parsivel disdrometer measurements made at Tirupati (13.6°N, 79.4°E), a tropical station near the foothills of Nallamala mountains, have been used to examine the climatological seasonal differences in bulk rainfall parameters, gamma parameters, raindrop size distributions (DSDs) and reflectivity – rainfall (Z–R) relationships. These relations are derived for both stratiform and convective rain during southwest and northeast monsoon (SWM and NEM) seasons, the two primary rainfall seasons for this region. The probability distribution functions for bulk rainfall and gamma parameters during the SWM and NEM suggest the dominance of evaporation and drop sorting during the SWM. The seasonal variations are also clearly apparent in DSD with fewer big drops and more small drops during the NEM than in SWM. These differences are seen more prominently at smaller R. As a result, the retrieved Z–R relations are found to be distinctly different during the monsoon seasons. The seasonal variations in Z–R relations are not only observed for the total data but also for the rain type-segregated data. The prefactor of the Z–R relation is found to be larger for SWM and also for stratiform rain, consistent with earlier reports from southeast India, indicating that these features are robust and representative of southeast India. The observed differences in Z–R relations are discussed in the light of microphysical differences between the seasons and rain types.