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This study evaluates the performance of four impact-type disdrometers installed at the Indian Institute of Tropical Meteorology, Pune, located on the leeside of the Western Ghats (WG). Further, seasonal variation of rain microphysical properties is studied during premonsoon (March-May), monsoon (June-September), postmonsoon (October-November), and winter (December-February). The four disdrometers exhibit comparable raindrop size distribution (DSD) patterns, though negligible differences are found at smaller drop diameters. The DSD shows higher concentrations of smaller (large-size) drops during monsoon (pre-monsoon and postmonsoon). Principal component analysis revealed three distinct modes of DSD characteristics. Monsoon DSDs are associated with a group of numerous smaller drops allied with shallow storm heights (warm rain). The premonsoon and postmonsoon DSDs are clustered in a group where ice-based processes dominate, resulting in higher median drop diameters (D0) and smaller normalized intercept parameters (Nw). The fitted gamma DSD model indicates higher mass-weighted mean diameter (Dm) during premonsoon, and higher intercept parameter during monsoon, whereas postmonsoon and winter have intermediate values. DSD stratified with rain rate shows that the Dm values increase with an increase in rain rate during winter, monsoon and postmon-soon, whereas in premonsoon, Dm increases initially and then decreases. Higher Dm and lower Nw are observed during convective rain in all seasons. The fitted slope-shape parameter relationships show a considerable seasonal variation. The DSD on the WG's leeside is notably different from the windward slopes and other WG locations. Different microphysical and dynamical mechanisms lead to seasonal differences in DSD characteristics. In monsoon, a considerable volume of water vapour advected from the Arabian Sea promotes the formation of raindrops through collision-coalescence processes, which may result in a higher proportion of smaller and midsized raindrops. The deeper clouds during pre-monsoon and postmonsoon indicate mixed-phase processes, which lead to mid and large-sized raindrops. [ABSTRACT FROM AUTHOR]

Surface raindrop size distributions over a rain shadow region during the Indian summer monsoon are clustered using the k‐means algorithm. The rainfall for five dominant clusters has distinct vertical features in polarimetric radar and micro rain radar. The deep convection with low cloud bases and high liquid water is associated with sharply increasing radar reflectivity at low levels and the largest drops at the surface. The large drops, originated by ice processes, break while falling below the melting layer, causing a peak in raindrops smaller than 0.5 mm diameter near the cloud base, where the falling raindrops rapidly grow by collision‐coalescence without breaking. The number concentration in the stratiform rain is relatively moderate; hence a gradual raindrop growth, and uniform vertical reflectivity. However, few raindrops in stratiform rain grow to a larger diameter than in convective rain, for the same rain rate due to the absence of collisional breakup. Plain Language Summary: Different clouds produce raindrops of various sizes in different concentrations. Therefore, we classified ground‐based measurements of raindrop sizes to study the rainfall characteristics during the monsoon. Generally, raindrops grow inside the cloud as they collide and combine with smaller drops, and break when they collide with larger drops. These processes affect the rain intensity and the sizes of raindrops. We found the abundance of tiny raindrops in convective clouds at lower levels. The larger drops collect them and grow while falling from this region of the cloud. The concentration of the tiny drops is higher when the cloud is taller and the cloud base is lower with sufficient water. The growing raindrops are detected by the radar with an increasing reflectivity signal. On the other hand, stratiform rain has a lower concentration of raindrops and less water. Therefore, the raindrop growth is slow, and the radar reflectivity generally appears uniform from cloud to ground. Despite this, due to a low concentration of large drops in the stratiform cloud, some raindrops gradually grow without breaking. Therefore, larger raindrops are often found in stratiform rain than convective rain, for the same rain intensity. Key Points: Five drop size distribution clusters at the surface during the Indian monsoon have distinct rainfall features in polarimetric radar and micro rain radarA low‐level peak in tiny drops helps collisional growth of raindrops in convective rain, increasing the radar reflectivity downwardsRaindrop growth is strong in deep convection with low cloud base and high liquid water but weak in stratiform rain due to low drop density [ABSTRACT FROM AUTHOR]

The variability in the microphysical properties of raindrop size distribution (DSD) over the rain shadow region of Indian peninsula is investigated in this study. Simultaneous in-situ measurements of precipitating cloud systems by an impact disdrometer, Micro Rain Radar (24 GHz), and wind profiler (1.28 GHz) are used in this study. The normalized Gamma distribution parameters for five DSD clusters of precipitating systems, corresponding to the non-convective, deep convective, shallow convective, mixed-type, and stratiform rain types were obtained using the k-means clustering algorithm. Four DSDs are nearly bounded by shape parameters, −3 < μ < 25, except for stratiform DSDs (−3 < μ < 45) indicating a wide variation in DSD shape. Bimodal (mono-modal) distributions in terms of frequency of occurrence of N w are observed in the non-convective, deep convective and shallow convective (stratiform and mixed) clouds. The bimodality is attributed to the large raindrops formed by the collision and coalescence (CC) process, and the small raindrops due to breakup that survive evaporation and reach the ground. The mono-modality of stratiform rain is attributed to the in-efficient CC and CC-breakup process. These rain microphysical processes are further characterized using the radar reflectivity factor (Z)-Rainfall Intensity (R) relationships of the form Z = aR b . Finally, we found that the convective DSDs over the region appear to be a composite of both maritime and continental characteristics. • Microphysical properties of non-convective, shallow, convective, mixed, and stratiform rains are described. • Parameterization of normalized Gamma DSD is done for DSD clusters. DSDs are maritime-continental origin • Collision-coalescence process dominates in non-convective, shallow, deep convective, and mixed rain. [ABSTRACT FROM AUTHOR]

A study of rain Drop Size Distributions (DSDs) and associated microphysical processes is carried out during the premonsoon and monsoon seasons at Kohima (25.67° N, 94.08° E) in the Patkai hill range of the Northeast India. The study is carried out with the help of Micro Rain Radar (24.1 GHz). The rainfall is classified in three categories namely, Bright Band (BB), No Bright Band Low rain rate (R) i.e. 0.10 mm h−1< R ≤ 10 mm h−1 (NBBL) and No Bright Band High rain rate i.e. 10 mm h−1 < R ≤ 20 mm h−1 (NBBH). During the premonsoon and monsoon, for R ≤ 20 mm h−1, rainfall occurrence time constitute around 96% and 98% respectively, whereas rainfall accumulation constitute around 51%. and 72% respectively. The BB height is found to be lower during the premonsoon (∼2.2 km from surface level) compared to monsoon (∼3.6 km from surface level) season. The observational results suggest that, during the premonsoon as well monsoon seasons, BB rain is predominantly associated with collision–coalescence–breakup process with varying proportion, whereas NBBL and NBBH rain are predominantly associated with collision–coalescence process with varying proportion. In each season, collision-coalescence process is most dominant for NBBH type of rain. Seasonally, collision-coalescence process is found to be stronger during the premonsoon season. For the falling rain drops, there is a transition in the microphysical process from number controlled (collision-coalescence-breakup) to size-controlled (collision-coalescence) process with varying proportion. For each season, for R ≤ 10 mm h−1, larger rain drops are found for BB type of rain compared to NBBL rain. Overall, relatively larger raindrops are found to be associated with NBBH rain. Seasonally, larger drops are found during the premonsoon compared to the monsoon season. The rain DSDs and associated microphysical processes during the premonsoon are found to be associated with stronger convection compared to the monsoon season. • During the premonsoon / monsoon seasons, BB rain is predominantly associated with collision–coalescence–breakup process. • In both the seasons, NBBL and NBBH rain are predominantly associated with collision–coalescence process. • The collision-coalescence process is found to be stronger during the premonsoon compared to the monsoon season. • For the falling rain drops, transition from number-controlled to size-controlled microphysical process is observed. • Seasonally, larger rain drops are observed during the premonsoon compared to the monsoon season. [ABSTRACT FROM AUTHOR]