Measurement and modeling of the bidirectional reflectance of snow

The angular distribution of reflected sunlight from snow is strongly anisotropic with most of the energy scattered in the forward direction. But, to obtain snow albedo from remote sensing data requires that we know the spectral bidirectional reflectance-distribution function (BRDF) as a function of snowpack physical properties such as snow depth, ice grain size, concentration of absorbing impurities and snowpack stratification. The authors establish the relationship between snow BRDF, snow physical properties, and solar/viewing geometries to improve estimates of surface albedo from remote sensing data. Using a discrete-ordinates radiative transfer model the authors have calculated the reflected spectral intensity of snow for a wide range of solar and viewing geometries. Corresponding snow reflectance measurements from Greenland and the Sierra Nevada, California, collected over a wide variety of snowpack conditions, show some agreement with the model results but the model appears to overestimate reflectance for solar zenith angles greater than 70 degrees. While snowpack physical properties are known to strongly affect the spectral reflectance of snow, the authors show how grain size, snowpack layering, and other physical properties also influence the angular reflectance from snow