Snow-cover effects on emission from sea ice

A combined surface-volume emission model based on the radiative transfer formulation for an inhomogeneous layer above an inhomogeneous half space is developed using a dense medium phase function. This phase function gives the scattering properties of coherent scattering from a group of randomly positioned scatterers within a unit volume. Laboratory data collected at CRREL indicate a drastic difference between emission from a bare saline ice with and without a snow cover over the frequency range from 10 to 37 GHz. In general, with snow cover there is a substantial rise in emission for horizontal polarization in level and a much slower drop off in angular trend from nadir. For vertical polarization a similar but relatively less drastic change takes place. Two possible reasons may be cited for the change: (1) the emission from snow is significant because of the reduction in the effective number of scatterers due to group scattering as opposed to independent scattering by the snow grains and (2) the discontinuity at the air-snow interface is much smaller than the air-ice interface causing a significant increase in emission at large nadir angles. On balance both snow and ice are contributing to the total emission at these frequencies. The relative percent contribution by the ice medium, of course, is decreasing with frequency. At a snow volume fraction of 0.2 the snow layer actually enhances emission in the 10 to 37 GHz range due to a decrease in the effective number density of the snow grain. Beyond 37 GHz the presence of a snow cover turns into a darkening effect due to further increase in albedo and the optical depth of the snow layer. The latter effect causes a decrease in the contribution from the ice below. At 90 GHz emission is dominated completely by snow.