The effect of clouds and surface albedo on UV irradiances at a high latitude site

At high latitudes the Earth's surface is covered by snow large parts of the year. In spring the snow cover significantly increases UV radiation for cl oudless as well as cloudy situations. In Tromso (69.65 degrees N, 18.95 deg rees E), Norway, independent measurements have been made of the effective r egional spectral surface albedo, the total ozone column, the effective clou d optical depth, and the surface UV irradiance. These measurements are used together with model simulations to study the effect of snow and clouds on surface erythemal radiation doses. Snow on the surface increases the monthl y erythemal doses by more than 20%. Relative to cloudless sky, the clouds r educe the monthly erythemal doses by 20-40%. During snow free conditions mo nthly doses derived from Earth Probe/Total Ozone Mapping Spectrometer (EP-T OMS) total ozone and cloud reflectivity agree with the measurements within the experimental uncertainties. In presence of snow, however, the EP-TOMS d erived data are too low by 30-40% due to snow covered surfaces being misint erpreted as clouds.