GLOBAL SCALE AEROSOL PROPERTIES: IMPLICATIONS FOR SURFACE SHORTWAVE RADIATION BUDGET

Abstract

Aerosols are known to affect the shortwave radiation budget of the Earth-atmosphere system. Using truncated Empirical Orthogonal Functions (EOF) fitting, we derive monthly mean aerosol optical depth (AOD) at 0.55 μm using information from: the Goddard Global Ozone Chemistry Aerosol Radiation and Transport (GOCART) model; the MODerate resolution Imaging Spectro-radiometer (MODIS); and the AErosol RObotic NETwork (AERONET). The single scattering albedo, the asymmetry parameter and the normalized extinction coefficient over the solar spectrum are estimated from GOCART data, MODIS Ångström exponent and AERONET almucantar retrievals.

The University of Maryland (UMD) Global Energy and Water Cycle Experiment (GEWEX) shortwave Surface Radiation Budget (SRB) model is updated to allow the treatment of complex aerosol properties. The modified model is implemented with the International Satellite Cloud Climatology Project (ISCCP) D1 for a one year period. From the evaluation of the improvements against ground measurements we find that the bias in retrieved AOD at 0.55 μm is reduced from 0.20 to 0.05. The overall bias in the estimated surface SW fluxes is reduced by about 7 Wm-2 for the total irradiance and 11 and 4 Wm-2 for the direct and diffuse parts, respectively. The new version of the UMD SRB model has now the capability to address the issue of aerosol direct radiative effects. Annually averaged global clear-sky direct radiative aerosol forcing is estimated to be -1.31 Wm-2 at the top of atmosphere and -2.71 Wm-2 at the surface. This indicates that the effect of aerosols on the SW energy absorption is comparable with their effect on the reflection at the TOA. At regional scales, aerosol effects can be much larger. In a case study preformed at a sub-Sahel site in Africa, the depletion of the daily surface irradiance can be as large as 120 Wm-2. Compared with other methods used to estimate aerosol direct effects, the advantage of our scheme is that it preserves closure with TOA satellite measurements. With anticipated progresses in aerosol research and satellite observations, the UMD SRB model has the potential to address aerosol radiative effects in a realistic and coherent way.