The role of snow on microwave emission and scattering over first-year sea ice

The primary objective of this paper is to investigate the geophysical and thermodynamic effects of snow on sea ice in defining the electromagnetic (EM) interaction within the microwave portion of the spectrum. We combine observational evidence of both the physical and thermodynamic characteristics of snow with direct measurements of scattering and emission at a variety of frequencies. We explain our observational results using various "state-of-the-art" forward scattering and emission models. Results show that geophysical characteristics of snow effect emission above about 37 GHz and above 5 GHz for active microwave scattering. We understand these effects to be driven by grain size and its contribution to volume scattering in both passive and active interactions within the volume. With snow cover, the Brewster angle effect is not significant and there is a gradual rise in emission from 10 to 37 GHz, We find emissivity to be dominated by direct emission from saline ice through the snow layer. Hence, the influence of grain size is small but the trend is clearly a drop in total emission as the grain size increases. We find that the role of the volume fraction of snow on emission and scattering is a complex relationship between the number density of scatterers relative to the coherence of this scattering ensemble. At low volume fractions, we find that independent scattering dominates, resulting in an increase in albedo and the extinction coefficient of the snow with frequency. The thermodynamic effects of snow on microwave scattering and emission are driven by the role that thermal diffusivity and conductivity play in the definition of brine volumes at the ice surface and within the snow volume, Prior to the presence of water in liquid phase within the snow volume, we find that the indirect effects are dominated by an impedance matching process across the snow-ice interface. We find that the complex permittivity at the snow-ice interface is considerably higher than over the bare ice surface. Our results showed that only a small change occurs between the cold and warm cases at lower frequencies, but as expected, the change in emissivity is larger at higher frequencies. Once water in liquid phase appears within the snow cover, we find that both emission and scattering are directly affected by the high complex permittivity of this volume fraction within the snow layer.