Millimeter Microwave Emissivities and Effective Temperatures of Snow-Covered Surfaces: Evidence for Lambertian Surface Scattering

The remote sounding of atmospheric temperature and humidity via satelliteborne microwave instruments requires knowledge of the surface emissivity in order to separate the atmospheric and surface components of emission. To date, microwave sounding of the lower troposphere is only carried out over open sea surfaces because of large uncertainties in the surface emissivity and effective emitting temperature of other surfaces. In addition to this, recent literature has brought into doubt the assumption of specular reflection which is commonly used in retrieving surface emissivity. This paper presents analysis of airborne data in the 89-183-GHz frequency range taken over snow-covered surfaces during the second phase of the Cold Land Processes Experiment (CLPX-II). Two methods for retrieving emissivity and effective temperature using the three 183-GHz channels are introduced. Both methods give independent estimates of the emissivity and effective temperature. These methods can be applied to estimate emissivity under both specular reflection and diffuse scattering. Evaluation of emissivities under diffuse scattering requires the summing of contributions over the upper half sphere. An analysis of flight data from CLPX-II shows that Lambertian scattering gives much more consistent solutions for emissivities and effective temperatures than the traditional approach assuming specular reflection.