EFFECTS OF SHOW COVER ON UV IRRADIANCE AND SURFACE ALBEDO - A CASE-STUDY

A heavy snowfall, followed by several days of cloudless skies before s ignificant snow melt had occurred, enabled a quantitative study of the effects of snow on downwelling UV spectral irradiances at the Nationa l Institute for Water and Atmospheric Research UV measurements site in Lauder, New Zealand. The largest UV enhancements (>70%) were seen dur ing partly cloudy conditions immediately after the snowfall. A radiati ve transfer model was used to quantify the enhancements due to the sno w 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 ti me, the maximum enhancements due to snow at solar zenith angle (SZA) 7 0 degrees were approximately 22% in the UV-A region. In the UV-B regio n, the enhancements were approximately 28% and tended to increase slig htly at larger SZA. The corresponding surface albedo was 0.62 +/- 0.08 , and comparison with supplementary measurements indicated that the al bedo decayed with time. Any spectral or SZA dependencies in the enhanc ements were below the measurement uncertainties in the UV region. Comp arisons with supplementary data indicated that the albedo immediately after the snow was greater than 0.8.