MILLIMETER-WAVE RADAR SCATTERING FROM SNOW .1. RADIATIVE-TRANSFER MODEL

Millimeter-wave (MMW) remote sensing of ground snow has attracted considerable interest in recent years. Because the size of the snow ice particle is comparable to the wavelength in the millimeter-wave region, we can no longer use a simple Rayleigh phase function or the small particle approximation usually used at microwave frequencies for calculating the extinction coefficient. In this paper we present a model for MMW scattering from snow using the vector radiative transfer theory and a Mie phase function. Assuming snow to consist of randomly distributed spherical particles embedded in a mixture of air and water, the vector radiative transfer theory is solved using the discrete ordinate method. The values of the extinction coefficient used in the calculations are based on a combination of experimental data and calculations using the quasi-crystalline approximation. The backscattering coefficient is calculated for different liquid water contents at 35, 95, and 140 GHz. We show that the backscattering coefficient is sensitive to liquid water content at all three frequencies, with 35 GHz being the most sensitive. Except for normal incidence, the effect of snow surface roughness is negligibly small for dry snow, and it is somewhat significant for wet snow at 35 GHz, but not at the higher frequencies.