Aerosol radiative forcing over Bay of Bengal and Chennai: Comparison with maritime, continental, and urban aerosol models

[1] Aerosol radiative forcings are estimated using the measured aerosol optical depths and a radiative transfer model over the Bay of Bengal and Chennai located on the east coast of India during February - March 2001. The monthly mean aerosol radiative forcings obtained during February are found to be the largest over Bay of Bengal when compared to those determined over coastal India, the Arabian Sea, and tropical Indian Ocean. The top of the atmosphere (TOA) forcing over Bay of Bengal is found to be - 9 Wm(-2) compared to - 6 Wm(-2) over the Arabian Sea. Atmospheric absorptions are about + 22, + 13, + 6, and + 0.3 Wm(-2) over Bay of Bengal, coastal India, Arabian Sea, and tropical Indian Ocean, respectively. The high-atmospheric absorption over Bay of Bengal is attributed to the presence of higher soot content. The radiative forcings estimated over Bay of Bengal and Chennai are contrasted with those of maritime, continental, and urban aerosol models. TOA forcing for urban aerosols is found to change sign from negative to positive when land surface reflectance is used. The forcing sign change is a consequence of higher albedo over land when compared to the ocean surface and presence of larger amount of soot in urban aerosol than maritime aerosol. Thus a higher soot content over land can exert more significant influence in terms of climate change than over the oceans. The forcings over Bay of Bengal and Chennai are an order of magnitude higher compared to the pristine southern Indian Ocean indicative of high anthropogenic influence.