Raindrop size distribution of stratiform precipitation over Southwest India – The Gateway of Indian Summer Monsoon

The study focuses on exploring the bright band (BB) characteristics, using Micro Rain Radar and ceilometer during the monsoon season of 2019 over three stations - coastal (20 m above mean sea level (MSL)), mid (400 m above MSL), and high (1820 m above MSL) altitude cloud physics observatories, setup by the National Centre for Earth Science Studies. The raindrop spectral characteristics are analyzed by classifying the BB events into three time periods, (i) prior to the occurrence of the bright band (pre-BB), (ii) during bright band (BB), and (iii) the subsequent hours after the dissipation of bright band (post-BB). Stronger BB events are noted in the high-altitude station with reflectivity of 33 dBZ at the melting layer while the occurrence of high duration (~2 h) events are noted at the mid-altitude station. Pre-BB hours are contributing the highest surface rainfall from all stages of BB evolution at the coastal and mid-altitude stations. The vertical profile of high rain rates indicates the presence of shallow and mixed-phase precipitating systems during pre-BB hours. At the coastal station, light to moderate rainfall with narrow raindrop spectra having dominance of mid-size drops are noticed. The efficiency of collision-coalescence processes is relatively more at the mid- and high-altitude stations resulting in small to mid-size drops and larger drops respectively. The raindrops generated from the melting layer fall into the shallow feeder clouds formed at the saturated surface in the high-altitude station that creates a seeder feeder mechanism of orographic rain enhancement. The distribution of mass-weighted mean diameter (Dm) varies from 0.5 to 1 mm with wider generalized intercept parameter (log10Nw) values (3–5 m−3 mm−1) at the mid- and high-altitude stations during pre-BB hours. In BB hours, the distribution of Dm values at the high-altitude station extends to 1.5 mm with high log10Nw values and the shape-slope (μ-Λ) relation also shows the extension of raindrop spectra to larger drops. The radar reflectivity-rain rate relations coherently explain the results concerning the evolution of drop spectra. The analysis provides a comprehensive understanding of the precipitation microphysical processes in the elevated terrain of the southwest coast of India, which acts as the gateway of the Indian summer monsoon.