R.: 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 types of highly reflective surfaces are presented, with the emphasis on the dependence of these fluxes on total cloud coverage. Measurements were performed in summer during three field campaigns: one in Greenland and two in Antarctica. It was found that especially the strength of the dependence of the shortwave fluxes on cloud amount differed widely among the four locations; this is due to differences in surface albedo and cloud shortwave transmissivity. At two locations the net allwave radiation was found to increase with increasing cloud coverage (sometimes referred to as the ‘radiation paradox’). It is demonstrated that this is due mainly to the fact that the shortwave cloud transmissivity at these sites is relatively high, which, in turn, is thought to be caused mainly by a low cloud optical thickness and by multiple scattering between surface and cloud-base. Whether or not the net surface radiation increases with increasing cloud coverage is found to depend chiefly on the values of the surface albedo and effective shortwave cloud transmissivity. KEY WORDS: Antarctica; Greenland; radiation; clouds; snow and ic