Retrieving the refractive index, emissivity, and surface temperature of polar sea ice from 6.9 GHz microwave measurements: A theoretical development

A new method for retrieving the refractive index, horizontally and vertically polarized emissivities (ε H , ε V ) , and temperature of sea ice has been developed by using the “combined Fresnel equation,” which combines two Fresnel‐polarized reflectivity equations into one. By using low‐frequency 6.9 GHz brightness temperature measurements, the full microwave radiative transfer equation was simplified so that the atmospheric influence on the horizontally and vertically polarized brightness temperatures ( T H , T V ) can be ignored, and thus ε H / ε V  =  T H / T V . Since ε H can be expressed by ε V according to the combined Fresnel equation (or vice versa), ε H and ε V can directly be retrieved from T H / T V . The Advanced Microwave Scanning Radiometer–Earth Observing System (AMSR‐E) measurements based on the developed method indicate that retrieved ε V is close to 1, regardless of region and season, which is consistent with the theoretically expected value of ε V near the Brewster angle (close to AMSR‐E viewing angle of 55°). This finding strongly suggests that ε H and ε V hold the same degree of accuracy because ε H / ε V  =  T H / T V . Such expected accuracy can also be applied to associated sea ice temperature and refractive index retrievals. Likewise, the close agreement of retrieved sea ice temperature with in situ measurement at the ice surface suggests that emissivity and refractive index retrievals are also sound. Some caveats of this approach for the surface temperature over the area including open water are discussed.