Contrasting features of rainfall microphysics as observed over the coastal and orographic region of western ghat in the inter-seasonal time-scale

Abstract

The paper aims to highlight the contrasting inter-seasonal variability of raindrop size distribution (DSD) as observed over Mumbai (representing a coastal city) and Mahabaleshwar (representing an orographic station in the Western Ghat mountain range) of Indian peninsula for a continuous period of four years (2018–2022). Upon examining the microphysical features of precipitation patterns, it is observed that raindrops with diameter of 3 mm and above dominate the rainfall in Mahabaleshwar during the pre-monsoon period, while the same with diameter of 1.5 mm and above dominate Mumbai's rainfall during the monsoon months. Additionally, the study finds a strong diurnal variation in rainfall occurrences during the pre-monsoon period for both the stations, while such variation is absent during the monsoon period. The analysis suggests that the Convective Available Potential Energy (CAPE) plays a significant role in these divergent diurnal patterns. Furthermore, the paper explores the relationship between mass-weighted mean diameter (D_m) - rain rate (R) values and shape (μ) versus slope (Λ) parameters as obtained from the two stations during the inter-seasonal phases of the monsoon. The results indicate that, during the pre-monsoon period, higher rain intensities (>16 mm/h) correspond to a dominant D_m value in Mahabaleshwar's rainfall. Conversely, during the monsoon months, higher D_m values are noticeable in Mumbai's rainfall. Thus by connecting with all the analysis done through this paper, it can be said that convective rainfall dominates the pre-monsoon period over orographic station, with convectivity becoming evident at rain rates greater than 16 mm/h. On the other hand, during the monsoon period, local thermodynamic conditions and higher moisture availability trigger the formation of deep convective clouds over coastal areas, thereby resulting for more convective rainfall in the coastal city compared to the orographic station.