Performances between the FY-4A/GIIRS and FY-4B/GIIRS Long-Wave InfraRed (LWIR) channels under clear-sky and all-sky conditions

The observation-minus-background (O - B) bias characteristics are compared for the Long-Wave InfraRed (LWIR) channels of the Geostationary Interferometric Infrared Sounder (GIIRS) on board Fengyun-4A (FY-4A) and Fengyun-4B (FY-4B) satellites under clear-sky and all-sky conditions. Under the clear-sky condition, the data quality of the FY-4B/GIIRS is better than that of the FY-4A/GIIRS. Specifically, the standard deviations (<1 K) of the O - B values for FY-4B/GIIRS LWIR channels are smaller than those (>1 K) of the FY-4A/GIIRS, especially in carbon dioxide (CO2) and ozone (O3) absorption bands. The diurnal variations of the mean O - B biases for the O3 band of the FY-4B/GIIRS are less than those of FY-4A/GIIRS. To our surprise, the diurnal variations of O - B biases for the FY-4B/GIIRS and FY-4A/GIIRS channels show an obvious antiphase characteristic in the CO2 band, which could be caused by diurnal environmental field variations from the different geostationary (GEO) platforms. In addition, the FY-4B/GIIRS channel 7 (703.75 cm(-1)) has smaller pixel-dependent biases than those of FY-4A/GIIRS. Under the all-sky condition, the observation and cloudy simulation (Bcloudy) for channels 7 (703.75 cm(-1)), 36 (721.875 cm(-1)), and 320 (900 cm(-1)) of the FY-4B/GIIRS are more consistent than those of FY-4A/GIIRS with smaller root-mean-square errors (RMSEs), which also indicates a better observation quality in the cloudy area.