Radiative forcing by aerosols over the Bay of Bengal region derived from shipborne, island-based, and satellite (Moderate-Resolution Imaging Spectroradiometer) observations

Measurements of spectral aerosol optical depths (AODs) were made over the Bay of Bengal region (adjacent to the Indian landmass) on board the oceanographic research vessel Sagar Kanya during February 2003. Simultaneous measurements of spectral AODs and mass concentrations of the composite aerosols and aerosol black carbon (BC) were made at an island location, Port Blair (11.63degreesN, 92.71degreesE), also in the Bay of Bengal. At the cruise locations the AODs were in the range of similar to0.3-0.6 at 500 nm (with a mean value of 0.41 +/- 0.14) and Angstrom wavelength exponent of similar to1.1 +/- 0.1; while at Port Blair the AODs were in the range of 0.11-0.48 at 500 nm and Angstrom wavelength exponent of 0.98 +/- 0.07. Aerosol BC constituted 5.8 +/- 0.6% of the composite aerosol mass concentration with a single-scattering albedo of similar to0.88, indicating the presence of a significant amount of submicron absorbing aerosols. Comparisons of AODs measured at Port Blair during cruise 188 and an earlier cruise (cruise 161B) during March 2001 (over the Bay of Bengal, Arabian Sea, and Indian Ocean) with those derived from Moderate-Resolution Imaging Spectroradiometer (MODIS) (on board the TERRA platform) showed excellent agreement with a mean difference of similar to0.01 and a root-mean-square difference of similar to0.03. Regionally averaged aerosol (net) forcing over the Bay of Bengal was in the range -15 to -24 W m(-2) at the surface and -2 to -4 W m(-2) at the top of the atmosphere in February 2003; these values were smaller in magnitude than those observed over this region during March 2001 and larger than that observed over the Arabian Sea and the Indian Ocean. The resulting atmospheric heating due to aerosol absorption was similar to0.5degreesK/d.