Analysis and correction of the difference between the ascending and descending orbits of the FY-3C microwave imager

The Microwave Radiometer Imager (MWRI) onboard FY-3C satellites was successfully launched on December 23, 2013. MWRI observes the Earth's atmosphere and surface at 10.65, 18.7, 23.8, 36.5, and 89.0 GHz with dual polarization and can provide an important initial field for Numerical Weather Prediction (NWP). However, the O-B (observation minus simulation) of MWRI shows a clear bias difference between ascending and descending orbits. The magnitude of this ascending-descending bias is approximately 2 K for all channels, thereby restricting its operational application in NWP data assimilation systems. This research analyzes the causes of the bias and makes appropriate corrections. The parameters of the calibration equation were analyzed, including physical temperature of the warm load, brightness temperature of the hot reflector's back-lobe, physical temperature of the hot reflector, physical temperature of the cold reflector, receiver channel instrument temperature, warm load radiometric counts, cold space radiometric counts, antenna brightness temperature calibration scale, antenna brightness temperature calibration offset, and a sensitivity analysis of each term of the calibration equation was conducted. Results indicated that high values of the hot load reflector are the main causes of the bias. The reflector was heated periodically by incident solar radiation and emitted a variable radiation with space and time, which caused the ascending-descending bias. Thus, the brightness temperature was simulated using the basic atmospheric parameters of ERA5 in conjunction with the radiative transfer model known as RTTOV. With the principle that the probability density difference between the O-B of ascending and descending orbits is minimum, the emissivity of the hot load reflector is estimated. Results show that before adjusting the emissivity of the hot reflector, the probability density plot of the O-B of ascending and descending orbits was separated. After correction, the bias difference between the ascending and descending orbits were clearly reduced, thereby identifying the main error source of the ascending-descending bias. Such identification can guide the development of future instruments and provide the condition for direct assimilation of MWRI radiance data. Although the accuracy of NWP fields, the radiative transfer model, calibration, and cloud detection are not the main error source of the ascending-descending bias, they may affect the estimation accuracy of the emissivity of the hot load reflector. Thus, strict quality control should be carried out in the future, and after the samples of greater uncertainty are eliminated, more accurate on-orbit emissivity of the hot load reflector can be estimated. © 2019, Science Press. All right reserved.