Impact of ice surface and volume scatterings on the microwave sea ice apparent emissivity

Abstract

Emissivity retrieval for sea ice from passive microwave measurements has been an important problem in climate/environmental research because of its link to various snow/ice variables. However, so far, it has been a difficult task because of the influences of surface and snow/ice induced volume scatterings. Here we examine the influences of scatterings on the ice emissivity from 10.65, 18.7, 23.8, and 36.5GHz Advanced Microwave Scanning Radiometer (AMSR)-E brightness temperatures over the Arctic Ocean. In doing so, we use a two-dimensional roughness parameterization, modified with surface facet orientations with an assumption that the facet emission follows the Fresnel relationship. Emitting layer temperature and refractive index retrieved from AMSR-E 6.9GHz brightness temperature measurements were used in this study and applied to other channels of interest. We demonstrated that the obtained roughness index has a strong linear relationship with the root-mean-square height measured by Atmospheric Terrain Mapper on the National Aeronautics and Space Administration (NASA) P-3 aircraft. The obtained roughness index showed that surface scattering on the emissivity is generally insignificant except for some first-year ice regions in particular at higher frequencies. This fact implies that Fresnel relations can be applicable for most of sea ice at the low-frequency microwave spectrum. By contrast, volume scattering is found to be significant in emissivity retrieval in case of multiyear ice. Nonetheless, volume scattering influence over first-year ice appears to be minor. We suggest that Fresnel-type emissivity can be estimated once a correction factor is used for removing surface scattering and volume scattering contributions from the apparent emissivity.