Theoretical development of retrieving refractive index, emissivity, and surface temperature of polar sea ice from 6.9 GHz microwave measurements

A new method retrieving refractive index, horizontally and vertically polarized emissivities (eH, eV), and temperature of sea ice has been developed using so-called 'combined Fresnel equation' which combines two Fresnel polarized reflectivity equations into one. By introducing low frequency 6.9 GHz, the full microwave radiative transfer equation was simplified so that atmospheric influence on the horizontally and vertically polarized brightness temperatures (TH, TV) can be ignored, and thus eH/eV = TH/TV. Since eH can be expressed by eV according to the 'combined Fresnel equation' (or vice versa), eH and eV can directly be retrieved from TH/TV. Results from applying the method to AMSR-E measurements indicate that retrieved eV is close to 1, regardless of region and season, which is consistent with theoretically expected eV near the Brewster angle (close to AMSR-E viewing angle of 55°). This finding strongly suggests that the same degree of consistency is expected from &epsilonH, because eH/eV= TH/TV. Such expected accuracy can also be applied to associated sea ice temperature and refractive index retrievals. Some caveats of this approach for the surface temperature over the area including open water were discussed.