Direct observations of aerosol radiative forcing over the tropical Indian Ocean during the January‐February 1996 pre‐INDOEX cruise

Abstract

Simultaneous measurements of aerosol optical depth, size distribution, and incoming solar radiation flux were conducted with spectral and broadband radiometers over the coastal Indian region, Arabian Sea, and Indian Ocean in January‐February 1996. Columnar aerosol optical depth, δa, at visible wavelengths was found to be 0.2–0.5 over the Arabian Sea and <0.1 over the equatorial Indian Ocean. Aerosol mass concentration decreased from about 80 μg/m3 near the coast to just a few μg/m3 over the interior ocean. The sub‐micron‐size particles showed more than an order of magnitude increase in number concentration near the coast versus the interior ocean. This large gradient in particle concentration was consistent with a corresponding large increase in the Sun‐photometer‐derived Angstrom exponent, which increased from 0.2 over the Indian Ocean to about 1.4 near the coast. The change in surface‐reaching solar flux with δa was obtained for both the direct and the global solar flux in the visible spectral region. The solar‐zenith‐angle‐normalized global and diffuse fluxes vary almost linearly with normalized δa. The direct visible (<780 nm) solar flux decreases by about 42±4 W m−2 and the diffuse sky radiation increases by about 30±3 W m−2 with every 0.1 increase in δa, for solar zenith angles smaller than 60°. For the same extinction optical depth the radiative forcing of the coastal aerosols is larger than the open ocean aerosol forcing by a factor of 2 or larger.