Sensitivity of Estimation of Snow Surface Direct Beam Spectral Albedo by Reciprocity Approach to Various Sky and Surface Conditions

Although snow surface albedo can vary substantially with cloud conditions, snow surface albedo under any sky conditions can be derived through spectral irradiance-weighted integration of direct beam spectral albedo over the hemisphere. Here, a large database of direct beam spectral albedo as a function of incidence angle for various types of snow cover is critical. However, there are some main difficulties that hamper the establishment of such a database: the clear sky conditions are not easily available during field campaigns, and the range of solar incident angles at high latitudes is limited. Based on the reciprocity between the directional-hemispherical and the hemispherical-directional spectral reflectance values of any types of surface, snow surface spectral reflectance measurements made under overcast conditions can be used to derive surface direct beam spectral albedo for a wide range of solar incidence angles. The only assumption made for the reciprocity is that the solar radiation from the sky should be perfectly isotropic. Because the sky diffuse light is seldom isotropic, whether the method would actually work in practice is a question that must be answered. We conduct a sensitivity analysis to investigate the impact of departure from perfect isotropic sky radiation on the accuracy of estimated direct beam spectral albedo. The study suggests that for various overcast conditions with either limb darkening or limb brightening (where the intensity of diffuse sky light either decrease or increase by a factor of 2 from zenith to horizon), the direct beam spectral albedo derived from the reciprocity method are accurate within 2 percent for large solar zenith angles and strongly anisotropic surface. The accuracy is even higher for small to moderate incidence angles or less anisotropic surface. This means that under a wide range of overcast conditions we can accurately derive direct beam spectral albedo, especially for small to moderate incidence angles. The direct beam spectral albedo for small to moderate incidence angles are missing in the current database for polar ice cover. Using the enhanced dataset we can derive albedo under all sky conditions given the distribution of sky diffuse light.