Snow Albedo's Dependence on Solar Zenith Angle from In Situ and MODIS Data
Since snow cover is one of the fastest modifications to the land surface albedo, the treatment of snow-covered surface albedo is important for the simulation of land processes in weather and climate models. A simple formulation is developed here to represent the solar zenith angle (SZA) dependence of albedo under maximum snow cover condition on the basis of Moderate Resolution Imaging Spectroradiometer (MODIS) Bidirectional Reflectance Distribution Function (BRDF) algorithm. The SZA dependence of black-sky (or direct) albedo is weaker under snow condition than that under snow-free condition, and it does not differ much among different vegetation types. The blue-sky albedo (or combined albedo from direct and diffuse radiations) based on the above formulation and in situ diffuse ratio of solar radiation is consistent with in situ data from two Canadian sites (grassland and evergreen needleleaf forest) and one U.S. grassland site. In particular, the SZA dependence of bluesky snow albedo is almost always weak because of high diffuse ratios for high SZA in winter. With the snow albedo formulation from this study and snow-free albedo formulations from the authors' previous studies, albedos with partial snow cover can be obtained as the snow frac- tion-weighted average of snow and snow-free albedos.