Angular-Dependent Polarized Characteristics of Surface Emissivity at 150 GHz Over Saharan and Taklimakan Deserts

Abstract

This study characterizes the marginally-known angular-dependent polarization difference of the land surface emissivity (LSE), using the microwave humidity sounder (MWHS) onboard China’s sun-synchronous satellite FengYun-3B (FY-3B). The instrument has two quasi-polarization channels at 150 GHz in addition to the three water-vapor absorption channels in the vicinity of 183 GHz, allowing observations at quasi vertical and horizontal polarizations with its cross-track scanning geometry. The Saharan and Taklimakan deserts are taken as examples due to the relatively homogeneous surface conditions. In the two 150 GHz channels of MWHS, the polarization bias is corrected according to the measurements at the Earth incidence angle (EIA) of 0°. The monthly-mean quasi-polarization difference (QPD) at 150 GHz is found to be varying with the EIA and exhibits seasonal variations over deserts. The QPD reaches a maximum value at the EIA of ~35°, while the zero value appears near the EIA of ~53° (equal to the satellite viewing angle of 45°) as expected. In contrast to the relatively stable and low QPD over Sahara, the QPD, quasi polarized emissivity difference and pure polarized emissivity difference are generally larger in the Taklimakan Desert, where the maximum monthly QPD is ~4 K and the polarized emissivity difference increases monotonously to ~0.2 at the EIA of 50°. The complex surface parameters and temporal variabilities impinge on the uncertainties of the land polarization. The increasing soil moisture due to precipitation damps the dichroic ability of the land surface, while the frozen surface due to snowfalls in cold seasons enhances the polarization.