Effects of snow cover on UV irradiance and surface albedo: A case study

A heavy snowfall, followed by several days of cloudless skies before significant snow melt had occurred, enabled a quantitative study of the effects of snow on downwelling UV spectral irradiances at the National Institute for Water and Atmospheric Research UV measurements site in Lauder, New Zealand. The largest UV enhancements (>70%) were seen during partly cloudy conditions immediately after the snowfall. A radiative transfer model was used to quantify the enhancements due to the snow cover and the spectral albedo of the snow under clear-sky conditions. The first cloudless day on which the radiative transfer model could be used with confidence occurred 7 days after the snowfall. By this time, the maximum enhancements due to snow at solar zenith angle (SZA) 70 degree were approximately 22% in the UV-A region. In the UV-B region, the enhancements were approximately 28% and tended to increase slightly at larger SZA. The corresponding surface albedo was 0.62 plus or minus 0.08, and comparison with supplementary measurements indicated that the albedo decayed with time. Any spectral or SZA dependencies in the enhancements were below the measurement uncertainties in the UV region. Comparisons with supplementary data indicated that the albedo immediately after the snow was greater than 0.8.