## 5 Evaluation of STAR COSMIC-2 Wet Profile Retrievals through Comparison with CrIS and ATMS
### Comparisons with CrIS Measurements Using CRTM and Double-Difference Method
#### 5.1.2 Comparison Results  
The CRTM-simulated BT (BT\({}_{\text{C2\_STAR}}\))-derived from STAR COSMIC-2 WetPrf data is compared with the observed BT (BT\({}_{\text{CrIS}}\)) data collected from the NOAA-20 CrIS sounding sensor. Here the inter-sensor BT difference (\(\Delta\)BT\({}_{\text{C2\_CrIS}}\)) evaluation is focused on the CrIS SWIR bands since these bands have a very particular spectral range which has substantially clean temperature sounding coverage with an almost monotonic increase in peak sounding height varying from \(\sim\)1000 to 10 hPa (see Figure 15b). For other spectral bands, such as MWIR, the weighting function peak height varies more frequently with wavelengths than the SWIR bands which makes the comparison with a CRTM simulation more challenging, and they are not further addressed in this study. Figure 14: Similar to 13 but for COSMIC-2 and ERA-5 water vapor specific humidity mean difference time series at 700 mb for STAR WetPrf (in red) and UCAR WetPrf2 (in blue). In the analysis, COSMIC-2 and NOAA-20 CrIS data are collected within a latitude of 45\({}^{\circ}\). To remove the effect of surface emissivity variation on the simulation, we use CrIS ocean pixels only. The collocation criteria are set with a time difference within two hours and less than 150 km spatial distance. The spatial difference is defined as the difference between the perigee point location of the COSMIC-2 RO profile at 5 km altitude and the locations of NOAA-20 CrIS nadir pixels to minimize the viewing geometry effect of CrIS. To screen out cloud impact in CrIS data, we remove collocated pairs with BT difference in surface channels larger than 4 K.  
Figure 15a shows the \(\Delta\)BT\({}_{\text{C2-CrIS}}\) differences from 1 October 2019, to 31 December 2019. The \(\Delta\)BT\({}_{\text{C2-CrIS}}\) difference analyses are divided into three groups, e.g., over the whole day, daytime, and nighttime. As the peak-sounding heights of CrIS SWIR channels vary from near-surface to 10 hPa, the \(\Delta\)BT\({}_{\text{C2-CrIS}}\) are mostly within \(\pm\)1 K. The day and nighttime differences matched well, except in the spectral regions with wavenumber ranging from 2275 cm\({}^{-1}\) to 2375 cm\({}^{-1}\) and 2400 cm\({}^{-1}\) to 2450 cm\({}^{-1}\). The spectral region 2275 cm\({}^{-1}\)-2375 cm\({}^{-1}\) is within a strong CO\({}_{2}\) absorption band with peak-sounding pressure varying from 100 hPa to 10 hPa. The difference in the daytime vs. nighttime \(\Delta\)BT\({}_{\text{C2-CrIS}}\) differences may be due to these channels' nonlocal thermal equilibrium (NLTE) effect during the day [35]. For the surface channels, 2400 cm\({}^{-1}\) to 2450 cm\({}^{-1}\), the daytime vs. nighttime \(\Delta\)BT\({}_{\text{C2-CrIS}}\) differences are mainly because IR SWIR surface channel radiances are strongly affected by the reflected solar radiation from the ocean during the day. Both NLTE effects and reflected solar radiation in SWIR bands were considered in CRTM simulation, but the simulation accuracy is worse for daytime samples. We also note that the primarily positive BT differences may be partly due to the cloud-screen method that may include some partially cloudy CrIS pixels, e.g., covered by a thin cirrus cloud, as the transparent sky pixels in the simulation; however, detecting these partly cloudy pixels is still challenging for existing cloud-screening methods. In addition, we would like to point out other sources that can also contribute to the differences, such as the biases and uncertainties in the surface emissivity model and surface temperature, the temperature and water vapor profiles from the RO retrieval fed to the CRTM simulation, and the inherent statistical nature of the CRTM simulation which derives simulation coefficients from the gaseous absorption model and training data. CrIS is a well-calibrated and validated stable hyperspectral sensor. The stability of CrIS radiance data has been validated spectrally and radiometrically.