Radiative Transfer Modeling of Microwave Emission and Dependence on Firn Properties

Abstract

The microwave emission from a model polar firn was calculated using a numerical solution of the radiative transfer equation that included angledependent Rayleigh scattering. The depth-dependent parameters in the equation were physical temperature and the coefficients of scattering and absorption. The coefficients were based on Rayleigh scattering from the snow grains. The bulk emissivity and the seasonal dependence of brightness temperature were calculated for seven locations at which grain sizes were measured as a function of depth. When the absorption and scattering coefficients are adjusted, the modeled emissivities agree with observed emissivities at these locations. The modeled seasonal dependence of brightness temperatures also compares well with values obtained at 1.55 cm wavelength by the Nimbus-5 satellite. Good agreement with data did not occur when the imaginary part of the index of refraction (and, hence, the absorption coefficient) had a significant temperature dependence between 210 and 250 K.