TOWARD THE INFLUENCE OF CLOUDS ON THE SHORTWAVE RADIATION BUDGET OF THE EARTH ATMOSPHERE SYSTEM ESTIMATED FROM SATELLITE DATA

The purpose of this paper is to investigate the influence of cloudines s on the shortwave radiation budget at the top of the atmosphere, at t he surface, and, as a residual, for the atmosphere itself. The data us ed for this study are derived exclusively from satellite measurements. Calculations for the top of the atmosphere are based entirely on meas urements of the Earth Radiation Budget Experiment (ERBE). For the sola r radiation budget at the surface, the incoming surface solar radiatio n is derived from Meteosat data and the surface albedo is calculated f rom ERBE clear-sky planetary albedo measurements by applying an atmosp heric correction scheme. As results, maps of absorbed solar radiation for the total earth-atmosphere system, the surface, and for the atmosp here are presented for the area of investigation, +/-60-degrees longit ude and latitude. To infer the contribution of clouds, the concept of cloud radiative forcing is applied to these different datasets. It is shown that the solar cloud forcing at the top of the atmosphere (CF(TO A)), and at the surface (CF(SUR)), are of the same order of magnitude and well correlated with cloud cover (R = 0.83). On the contrary, the solar cloud forcing of the atmosphere itself, CF(ATM), is about one or der of magnitude less and not very highly correlated with cloud cover (R = 0.37). The mean value of the annual averaged solar cloud forcing for the area of investigation is calculated for the top of the atmosph ere to be CF(TOA) = 50 +/- 4 W m-2, for the surface to be CF(SUR) = 55 +/- 6 W m-2 , and for the atmosphere to be CF(ATM) = -5 +/- 10 W m-2. Related to the annual mean solar insolation, the CF(ATM) corresponds to an additional contribution of the clouds to atmospheric solar absor ption of 1.4%. The uncertainty range for this additional absorption is calculated to be -1.4% to +4.2%.