THE INFLUENCE OF CLOUDS ON THE RADIATION BUDGET OF ICE AND SNOW SURFACES IN ANTARCTICA AND GREENLAND IN SUMMER

Shortwave and longwave radiative surface fluxes over four different ty pes of highly reflective surfaces are presented with the emphasis on t he dependence of these fluxes on total cloud coverage. Measurements we re performed in summer during three field campaigns: one in Greenland and two in Antarctica. It was found that especially the strength of th e dependence of the shortwave fluxes on cloud amount differed widely a mong the four locations; this is due to differences in surface albedo and cloud shortwave transmissivity. At two locations the net allwave r adiation was found to increase with increasing cloud coverage (sometim es referred to as the 'radiation paradox'). It is demonstrated that th is is due mainly to the fact that the shortwave cloud transmissivity a t these sites is relatively high, which, in turn, is thought to be cau sed mainly by a low cloud optical thickness and by multiple scattering between surface and cloud-base. Whether or not the net surface radiat ion increases with increasing cloud coverage is found to depend chiefl y on the values of the surface albedo and effective shortwave cloud tr ansmissivity.