Spectral aerosol radiative forcing at the surface during the Indian Ocean Experiment (INDOEX)

We describe a radiation experiment performed at the Kaashidhoo Climate Observatory (KCO), Republic of Maldives, during the Indian Ocean Experiment (INDOEX) field campaign in February through March 1999. Both the total solar broadband (0.3 to 3.81 μm) and visible spectral narrowband (seven spectral channels in the region from 0.4 to 0.7 μm) quantities were measured for the total (hemispherical), direct solar, and diffuse (including forward scattering) components of the radiation field. The aerosol optical depth at 500 nm, obtained from one of the narrrowband spectral channels, ranged from approximately 0.2 to 0.7 during the period encompassing the intensive field phase (IFP) of INDOEX. The diurnally averaged atmospheric forcing efficiencies determined for the total solar broadband and visible spectrum are −72.2 ± 5.5 W m−2 and −38.5 ± 4.0 W m−2, respectively. Model simulations driven by in situ measurements and realistic aerosol optical properties infer a single scattering albedo of 0.874 ± 0.028 at 500 nm.