Raindrop size distribution in stratiform precipitation: Insights from spectral bin simulations over the high-altitude cloud physics observatory, Western Ghats

An understanding of the Raindrop Size Distributions (RSD) derived from the bin microphysics scheme is vital for improving rainfall estimates and elucidating cloud microphysical processes over the complex terrain of the Western Ghats. This study investigates the structure and evolution of RSD using simulations from the Weather Research and Forecasting (WRF) model with the fast version of the spectral bin microphysics scheme (SBM-Fast), and co-located observations from the micro rain radar and ceilometer at the High-Altitude Cloud Physics Observatory (HACPO), Munnar. Three prolonged stratiform events (>2 h) are analysed to examine key microphysical signatures of melting layer structure, raindrop growth, and vertical variation in RSD. MRR and model results indicate a clear increase in RSD below the melting layer (∼4.8 km). The SBM shows an enhanced rainwater mixing ratio beneath the melting layer, in agreement with observed RSD growth. Above the melting layer, graupel and snow aloft contribute to the growth of raindrops. The model tends to overpredict mid-sized raindrops in the range 1–2 mm near the surface, leading to an overestimation in rainfall for the stratiform case. The enhanced particle growth and subsequent hydrometeor loading were supported by vertical wind with higher updrafts, as captured by the model simulation.