RETRIEVAL OF SURFACE RADIATION FLUXES FROM SATELLITE DATA

The retrieval of the downward solar (0.3-4-mu-m) and longwave (4-100-mu-m) irradiances and the direct retrieval of the solar net flux at the surface from satellite observations are discussed. A close linear coupling between the solar radiation field observed by the satellite and the surface radiation field enables surface insolation estimates with standard errors of about 5% for monthly mean values in comparison with surface measurements. It is pointed out that the net solar nux density at the surface can be calculated directly from the net at the top of the atmosphere with an adequate estimate of absorption in the atmosphere. In the longwave the satellite estimates are more difficult since the downwelling longwave irradiance at the Earth's surface is largely decoupled from the radiation field at the top of the atmosphere. The clear-sky longwave irradiance is basically determined by the near-surface temperature and humidity field. Clouds with low bases are very effective for increasing the downward longwave flux. The accuracy of the present longwave retrievals is not yet adequate for the derivation of a global climatology while shortwave retrievals provide useful results.