Satellite based estimation of anthropogenic aerosol radiative forcing over tropical Indian Ocean

Aerosols influence the radiative balance of the Earth through scattering of radiation (direct effect) and by serving as a nuclei for cloud droplet and ice particle formation, thereby modifying cloud reflectance, absorption and precipitation formation (indirect effect). Estimates of the aerosol direct effect and its direct radiative forcing (defined as the anthropogenic fraction of the aerosol direct effect) are largely model based, since techniques for measuring global anthropogenic aerosol amount and distribution are scarce. Previous studies suggest that satellite data that provide a high temporal and spatial coverage and that distinguish fine from coarse aerosols can be used to study the radiative forcing due to anthropogenic aerosols.

In this study the aerosol radiative forcing due to anthropogenic aerosols had been estimated for a period of one year using CERES (Clouds and Earth's Radiant Energy System) derived shortwave flux and MODIS (Moderate Resolution Imaging Spectroradiometer) derived aerosol optical depth (AOD). The Indian Ocean basin (Bay of Bengal and Arabian Sea) has been chosen as the region of study because it provides a unique laboratory to study anthropogenic aerosols that owe their origin to a high degree of pollution from the Asian sub-continent. Comparison of satellite based AOD observations against extensive collocated measurements made over the Indian Ocean during several ship cruises in 2003 will be presented. Seasonal variation of aerosol optical thickness and short wave radiative forcing due to anthropogenic, dust and marine aerosol will be discussed. Validation results of satellite derived shortwave radiative forcing attributable to the anthropogenic aerosols against estimates obtained from a radiative transfer model will also be presented.