Classification of tropical coastal precipitating cloud systems using disdrometer observations over Thumba, India

Abstract

This paper presents the seasonal mean, rain integral parameters, and gamma parameters at the surface associated with the different precipitating systems over a coastal station Thumba (8.53°N, 76.87°E). The primary datasets consist of Joss-Waldvogel disdrometer (JWD) observations collected during 2007–2015 and 2018 over Thumba. The precipitating cloud systems observed over three seasons (pre-monsoon: March-May, Monsoon: June-September and post-monsoon: October and November) are classified into convective, transition and stratiform rain based on the ratio of mass-weighted mean diameter (Dm) and rain rate (R). The results show a distinct difference in raindrop size distribution (DSD) patterns in the convective, transition and stratiform rain types. In general, the mean DSD spectra for convective precipitation is found to be broad compared to that of the transition and stratiform rain types. The DSD spectra in the convective precipitation have relatively higher (smaller) number concentration of large (small) raindrops as compared to other precipitation types, resulting in larger Dm values than the other two rain regimes. These differences in DSD for the convective, transition and stratiform resulted in a systematic variation in Z-R relationships. A distinct inverse relationship is found between A and b for convective and stratiform rain types irrespective of seasons. The coefficient A (exponent b) is found to be larger (smaller) for stratiform (convective) rain regime. Out of about 1826 h of total precipitation, stratiform rain is observed for 63% of the time, while convective and transition rain is observed for about 8% and 29% respectively. However, the contribution of stratiform rain is 11% of total rainfall, while that of convective and transition rain are about 55% and 35% respectively, consistent with the earlier studies.