## 5 Evaluation of STAR COSMIC-2 Wet Profile Retrievals through Comparison with CrIS and ATMS
### Comparison of STAR COSMIC-2 WetPrf with SNPP ATMS Measurements Using CRTM
#### 5.2.2 Comparison Results  
Figure 18 shows the mean BT bias and uncertainty between CRTM-simulated BT from multi-RO or ECMWF data and ATMS BT measurements over selected ATMS channels. Over the ATMS temperature-sounding channels (CH07-12), the BT biases and uncertainty derived from comparing multiple RO retrievals with SNPP measurements are comparable. The mean biases \(\mu\)(\(\Delta\)BT) are generally within 0.6 K, with the mean bias difference among multiple RO retrievals being less than 0.35 K. These channels have a relatively small standard deviation since they are less affected by the water vapor. For the temperature-sounding channel CH12 with peak sounding height at \(\sim\)26 km, the mean BT biases \(\mu\)(\(\Delta\)BT) derived from KOMPSAT-5 and TerraSAR-X data appear to be higher than those derived from STAR or UCAR COSMIC-2 data and UCAR COSMIC-1 data. This can be due to a much lower number of colocations between KOMPSAT-5 or TerraSAR-X and SNPP ATMS than COSMIC-1 and -2. Generally, the \(\mu\)(\(\Delta\)BT) bias derived from ECMWF background profiles is higher than those derived from various RO data for ATMS CH09-12 (Figure 18). This indicates the consistency among RO retrievals and improvements with RO data over background data, considering that these data can differ in the RO sensor type, data acquisition time, and 1D-Var retrieval algorithms. For the moisture-sounding channels (CH19-22 of ATMS), the standard deviations \(\sigma\)(\(\Delta\)BT) are more significant than those of CH07-12. For ATMS CH19-20, the BT biases from STAR or UCAR COSMIC-2 data, UCAR COSMIC-1 data, and ECMWF reanalysis data are consistently within 0.25 K.  
These additional comparisons further confirm the conclusion in [36] using the UCAR COSMIC-2 data alone that the RO data can be used to monitor and calibrate these two moisture-sounding channels (CH19-20) of microwave sounders such as ATMS. Due to fewer colocation samples, the biases are more significant between KOMPSAT-5 or TerraSAR-X and SNPP ATMS for CH19. For ATMS CH21-22, the uncertainty \(\sigma\)(\(\Delta\)BT) is large, and the \(\mu\)(\(\Delta\)BT) bias derived from ECMWF background profiles is higher (with a larger negative bias) than others, which suggests the uncertainty and difficulty in the characterization of these two moisture-sounding channels (Figure 18). Furthermore, this study also shows the consistency between UCAR and STAR COSMIC-2 wet profile data by using ATMS as the reference. In general, the BT biases of UCAR and STAR COSMIC-2 wet profiles relative to SNPP ATMS are consistent, with the bias differences being less than 0.1 K over all of the ATMS channels we studied. The STAR COSMIC-2 wet profiles  
Figure 18: Inter-comparison of mean BT bias and uncertainty between CRTM-simulated BT from multi-RO or ECMWF data and ATMS BT measurements over selected ATMS channels. The texts in the figure legend correspond to data used in the CRTM simulation, such as C2 STAR: STAR COSMIC-2 WetPrf; C2 WetPf 2: UCAR COSMIC-2 WetPrf 2; C1 WetPrf: UCAR COSMIC-1 WetPrf; KOMPSAT-5 WetPrf2; TXS WetPrf2: UCAR TerraSAR-X WetPrf2; and ECMWF: ECMWF reanalysis profile. have a smaller uncertainty for ATMS CH08-12 and CH21-22 than the UCAR COSMIC-2 data (Figure 18), which also reflect the proper selection of background covariance matrixes and error covariance matrix, and retrieval approaches described in Section 3. To evaluate the precision consistency among six RO sensors in the COSMIC-2 constellation, we further conducted a double-difference analysis using the collocated ATMS measurements as the reference. The STAR wet profile (temperature and humidity) retrievals of six COSMIC-2 RO sensors are fed to CRTM, and the simulated BT data for each RO sensor of COSMIC-2 are compared with the collocated ATMS BT data to calculate BT biases: \(\Delta\)BT\({}_{\text{STAR/C2(En)-ATMS}}\) for \(n=1\)-6 of COSMIC-2.