Estimating the cloudy-sky albedo of sea ice and snow from space

While satellites provide the means to monitor the temporal and spatial vari ability of surface albedo, their use has been limited to clear-sky areas be cause clouds obscure the surface at wavelengths in the solar spectrum. Howe ver, the effect of clouds on the surface albedo, especially that of snow an d ice, is significant and should be considered in satellite retrievals. In this paper theoretical and observational evidence is given that shows the s now/ice albedo to be on the average 4-6% (absolute) higher under cloud cove r than for clear skies, with a range of slightly less than 0 to approximate ly 15%. A method for retrieving the clear-sky broadband albedo of snow/ice from the advanced very high resolution radiometer is presented, and an adju stment for cloud optical depth is proposed. The cloudy-sky adjustment is in dependent of sensor type and could also be used with nonsatellite data sets . An application of the algorithm to data from the Surface Heat Budget of t he Arctic Ocean experiment demonstrates that clear- and cloudy-sky snow sur face albedo can be obtained from space with an uncertainty of approximately 7% absolute. While it may be sufficient to adjust a monthly clear-sky surf ace albedo climatology for clouds by incorporating the mean cloud effect of approximately 5%, adjustments for cloud optical depth should be performed with instantaneous retrievals.