Validation of FengYun-3D Microwave Temperature Sounder using GPS Radio Occultation Observations

The fourth FengYun satellite (FY-3D) was successfully launched into orbit on November 15, 2017. The cross-tracking scanning instrument microwave temperature sounder (MWTS) onboard FY-3D provides 13 channels, at frequencies ranging from 50 to 60 GHz. It probes the earth's atmosphere under all weather conditions and thus could not only greatly improve the weather forecasting but also enhance our understanding of climate trend when the data is connected with other microwave sounding data such as advanced microwave sounding unit (AMSU) and advanced technology microwave sounder (ATMS). It is important to assess the on-orbit accuracy of FY-3D/MWTS for the above applications. Since atmospheric profiles from GPS radio occultation (RO) are stable and accurate, they are very valuable for assessing the microwave sounder performance in orbit as demonstrated by many previous studies. In this study, the community radiative transfer model (CRTM) is used to simulate MWTS with GPS RO data as its input for characterizing the MWTS data quality. The GPS RO profiles are made from FY-3C Global Navigation Satellite System Occultation Sounder (GNOS). The collocation criteria are set by a time difference of no more than 3h and a horizontal spatial separation of less than 50 km. A cloud detection algorithm based on the channels of 50.3 GHz and 51.76 GHz is applied to separate the data in clear-sky conditions over oceans. The collocations of MWTS and GNOS are constrained with 60°S and 60°N over oceans. From the data during June 27 to July 16, 2018, 1081 MWTS measurements are collocated with GNOS. Our initial analysis shows that for upper air sounding channels the mean biases of MWTS observations to GPS RO simulations were negative for channels 5-9 with absolute values smaller than 0.9K and positive for channels 3, 4, 10 with values smaller than 0.8 K. The bias distribution is slightly asymmetric across the scan line. The GNOS-derived simulations are also compared with the COSMIC-derived to show the performance of different RO data in establishing the on-orbit truth for microwave sounder calibration.