Examining Scattering Signatures from Underlying Snow/Ice Layers in the Microwave Sea Ice Emissivity

Emissivity retrieval for the sea ice from passive microwave (MW) measurements has been an important problem in climate/environmental research because of its link to various snow/ice parameters. But, so far it has been a difficult task because of the influences of surface roughness and snow/freeboard ice induced scattering. Here, we examined the influences of roughness and scattering-induced light depletion on the sea ice emissivity from 10.65, 18.7, 23.8, and 36.5 GHz AMSR (Advanced Microwave Scanning Radiometer)-E brightness temperatures over the Arctic Ocean. It was done so from the use of a two-dimensional roughness parameterization with the assumption that the surface emission follows Fresnel relationship, but modified with surface facet orientations that determine the surface roughness. Emitting surface temperature and refractive index retrieved at 6.9 GHz were used for other channels in interest. It was shown that obtained roughness index is in a linear relationship with measured geometric rms height, suggesting that the roughness index, that also appears in the MW radiative transfer in the sea ice/snow layers, reflects the surface roughness condition.

Retrieved roughness index, which is found to be nearly equivalent with the departures of effective incident angle from the AMSR-E viewing angle, showed that the surface roughness effect on the effective emissivity is rather minimal except for some fresh seasonal ice regions. Results further indicate that it is indeed scatterings giving significant influences on the emissivity retrieval, but even so scattering influence over the first-year sea ice appears to be minor. We suggested that the Fresnel-type emissivity can be retrieved once roughness index and depletion rate are combined (i.e., finding the correction factor) to remove their effect from the effective emissivity. Further suggestion is made for the possibility of obtaining the correction factor from measured brightness temperatures.