Aerosol radiation cloud interactions over the tropical Indian Ocean prior to the onset of the summer monsoon

Abstract

Prior to the summer monsoon, from December to April every year, the trade wind blowing from the northeast brings polluted continent air over the Arabian Sea and the tropical Indian Ocean. A variety of fine suspended particles, majority of them manmade, get accumulated over the ocean region till the summer monsoon sets in and cleans the atmosphere. Aerosols have long been recognized for their role in cloud development and rain formation. They act as condensation nuclei for the formation of raindrops and help precipitation. But, if they are present in large quantities they produce a narrow spectrum of small cloud droplets and do not allow the droplets to grow. These clouds do not precipitate and the rain formation is inhibited. The aerosol-cloud interaction could be playing a crucial role on the onset of monsoon over different regions and the spatial distribution of rain over India. Recent observations over the Arabian Sea and the tropical Indian Ocean, conducted as part of the Indian Ocean Experiment, have revealed the presence of large amount of aerosols over these regions. The aerosols brought from the Indian subcontinent and other surrounding regions have relatively high concentration of soot particles, which substantially reduce the amount of solar radiation reaching the ocean surface. Results on this direct effect of aerosols on the radiation budget as well as their impact on cloud properties are presented and discussed.