Effect of Axisymmetrical Spectral Response Function on Microwave Radiance Simulation of Quadruple-Sideband Channel

Actual spectral response function (SRF) of the quadruple-sideband channel of the microwave sounder is axisymmetrical, and its distortion degree affects the difference in observations between the actual microwave sounder and the ideal design situation. Effects of the actual quadruple-sideband channel’s SRF on observations has been evaluated by applying the actual SRF of the quadruple-sideband channel of FengYun-3D (FY-3D) microwave temperature sounder-2 (MWTS-2) and rapid radiative transfer model (RTM) of China Meteorological Administration-Global Forecast System (CMA-GFS). Compared with ideal SRF, actual SRF could improve microwave-radiance simulation, and the observation-minus-background could be decreased by about 0.7 K. The improvements in the mid-high latitudes of the south hemisphere are more evident than in other latitudes. Effects of distortion types of actual SRF on observations have been analyzed by comparing simulated radiance differences between reference SRF and originally proposed SRFs with eight typical distortion types. Inner sideband drifting type and outer sideband drifting type have the greatest influence of about 1 K on microwave radiance. Inner–outer sideband symmetrical difference type has the least impact of approximately 0.02 K. Effects of the axisymmetry of SRF on observations have also been evaluated. Compared microwave radiance simulation by reference SRF with axisymmetrically and nonaxisymmetrically distorted SRFs, respectively, simulation errors induced by axisymmetrically distorted SRFs are less than that by nonaxisymmetrically distorted SRFs of about 0.1–0.4 K.