Atmospheric aerosols can interact with clouds and influence the hydrological cycle by acting as cloud condensation nuclei. The current study reviews the results obtained on aerosol-precipitation interactions over India and the surrounding oceanic regions. An analysis of aerosol and cloud characteristics over the Arabian Sea, India, and the Bay of Bengal during summer monsoon in the last decade reveals large regional, intraseasonal, and interannual variations. Aerosol optical depth (AOD) and aerosol absorbing index (AAI) in 2002 (a drought year) are higher over India when compared to normal monsoon years. Cloud effective radius (CER) and cloud optical thickness exhibit a negative correlation with AOD over India, which agrees well with the indirect radiative effects of aerosols. Over Bay of Bengal CER is positively correlated with AOD suggesting an inverse aerosol indirect effect. In future, observatories to measure aerosol characteristics (amount, size, type, chemical composition, mixing, vertical and horizontal distributions), and cloud properties (number and size) over several locations in India, and intense observational campaigns involving aircraft and ships are crucial to unravel the quantitative impact that aerosols have on Indian monsoon. Satellite remote sensing of aerosol distribution, their chemical composition, microphysical properties of clouds, solar irradiance, and terrestrial longwave radiation is important. 1. Introduction Aerosols affect the earth-atmosphere radiation budget directly by scattering and absorbing the incoming solar radiation and indirectly by influencing the processes of formation of clouds and precipitation. The properties of cloud get influenced by aerosols through their role as cloud condensation nuclei and/or ice nuclei. Aerosols alter the intensity of solar radiation scattered back to space, absorbed in the atmosphere, and reaching the surface of the earth which is known as direct radiative effect. Aerosols indirectly can modify the cloud characteristics and influence precipitation in different ways. Aerosols (a) can increase the lifetime of clouds and reflectivity (albedo) and decrease the precipitation and radiation reaching the surface of the earth (cloud lifetime effect), (b) can cause an increase in cloud droplet concentration and a decrease in the cloud effective radii (cloud albedo effect), and (c) can absorb the solar radiation, reemit as thermal radiation, and heat the air mass and may cause evaporation of cloud droplets (semidirect effect) [1]. The level of scientific understanding of the above processes
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