A New 3D Surface Snow Model and Effect of Sastrugi on Bidirectional Reflectance Distribution Function of Surface Snow

Due to snow surfaces having the same magnitude in visible reflectance as clouds, estimating cloud properties over polar regions using measured reflectance from satellite imagers is still a challenge. The properties of surface snow vary with snow particle sizes, micro/macro surface roughness, aerosol inclusions, and etc. The variations in properties of surface snow are then influence the reflectances at the top of atmosphere. Comparing with measurements from Multi-angle Imaging SpectroRadiometer (MISR), a 2D surface snow model is developed by assuming surface snow consisting of an ensemble of hexagonal plates and columns, bullet rosettes, and aggregates with specific habit fractions at different particle size bins. Variations of surface roughness and black carbon inclusion of snow particles, which are found to affect single-scattering properties of snow particles, are also considered in the surface snow model. A database of the Bidirectional Reflectance Distribution Functions (BRDF) of surface snows has been developed at visible and near infrared bands for 7 snow grain sizes ranging from 25 to 1000 μm, 6 snow particle roughnesses ranging from 0.01 to 1.0, and 7 black carbon inclusion amounts ranging from 0.001 to 100.0 ppmv under different geometrics. Furthermore, a new 3D surface model is developed to consider effects of sastrugi (parallel wavelike ridges caused by winds on surface snow) on BRDFs. The same microphysical and optical properties that are used for the 2D surface snow model are applied to the new 3D surface snow model with different macrophysical properties of sastrugi, including groove and ridge widths, ridge heights, and slopes. BRDFs of the 3D surface snow are compared with MISR and surface measurements, and applied to improve predicting clear-sky reflectance over snow/ice in polar regions for CERES-MODIS cloud retrievals.