Direct radiative effect of aerosols as determined from a combination of MODIS retrievals and GOCART simulations

The aerosol direct solar effect under clear sky is assessed by (1) combining multiple aerosol characterizations and (2) using the satellite‐retrieved land surface albedo. The aerosol characterization is made through an integration of the MODerate resolution Imaging Spectroradiometer (MODIS) retrievals and the Georgia Tech/Goddard Global Ozone Chemistry Aerosol Radiation and Transport (GOCART) model simulations. The spectral and bidirectional albedo of land surface is derived from MODIS. On a global average, the solar forcing at the top of atmosphere (TOA) DFTOA is −4.5 Wm−2, of which about 1/3 is contributed by a sum of natural and anthropogenic sulfate and carbonaceous aerosols. Though the optical depth is about 50% higher over land than over ocean, no significant land‐ocean contrast in this TOA forcing is observed. It is reduced by larger aerosol absorption and higher surface albedo over land. As a result of absorption by soot and dust, a much larger surface cooling and substantial atmospheric absorption coexist over land and adjacent oceans. Globally, the surface cooling DFSFC is about −9.9 Wm−2, and the atmospheric absorption DFAIR is about 5.4 Wm−2, suggesting that more than half of the surface cooling results from the atmospheric absorption. Sensitivity tests show that an inclusion of MODIS‐derived anisotropy of land surface reflection reduces the diurnal variation of TOA solar forcing, because of aerosol‐induced changes in the fraction of direct beam and hence in the effective reflection from the surface. Constraining the GOCART dust absorption with recent measurements reduces DFAIR and DFSFC by 1.3 Wm−2 and 0.9 Wm−2, respectively, and increases the TOA cooling by 0.4 Wm−2.