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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]

Pre-monsoon aerosols in the northern part of India may play an important role in the advancement of the monsoon. This study investigates the properties of aerosols and their spatial and vertical distribution near the foothills of the Himalayas using data from an instrumented aircraft during the Cloud Aerosol Interactions and Precipitation Enhancement Experiment (CAIPEEX), 2009. CAIPEEX was conducted over five days during May 2009 in the northern part of India near the foothills of the Himalayas. On all the flight days thick haze was observed, with elevated aerosol layers up to 4 km with varying concentrations. The sources are identified as being from local anthropogenic activities such as biomass burning, as inferred from MODIS fire maps and dust from local as well as from long-range transport, as suggested by trajectory analysis. The aerosol size distributions depict the increases in both fine and coarse mode aerosols in polluted layers. This indicates that aerosols over this region are well mixed and the vertical distribution is a mixture of both biomass burning and dust aerosols at different altitudes, as also observed by CALIPSO and inferred from Mie calculations. Clouds observed above the elevated aerosol layers showed higher droplet concentrations (200-1400 cm−3) with small effective radii (3.5 to < 6 µm). Ice phase observed above 6 km at temperatures lower than −14°C might be due to the presence of dust aerosol acting as potential ice nuclei. Copyright © 2012 Royal Meteorological Society [ABSTRACT FROM AUTHOR]