## 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  
#### 5.2.1 Validation Approaches  
Here we compare the STAR COSMIC-2 retrieval forward simulated ATMS BTs with SNPP ATMS measurements. The STAR COSMIC-2 1D-Var temperature and humidity profiles are fed to the CRTM to simulate BT data for SNPP ATMS channels [36; 48].  
Besides the temperature and humidity profiles collected from RO retrievals, surface temperature, wind properties, and gas profiles obtained from the ERA-5 are also used as  
Figure 16: Evaluation of precision of COSMIC-2 through deriving spectral BT biases between COSMIC-2 E1 and five other COSMIC-2 sensors by using CrIS as the reference sensor in the double-difference method.  
inputs for the CRTM to simulate the ATMS BTs. Details of the CRTM simulation setup can be found in [36].  
Researchers [38] focused on analyzing the stability and consistency between UCAR COSMIC-2 WetPrf or WetPr2 data, i.e., two types of wet profile data from the same RO sensor processed by the same center, and SNPP ATMS measurements using CRTM simulation. The focus of this section is to evaluate the inter-RO sensor (among four different RO sensors) and inter-RO retrieval algorithms (between UCAR and STAR 1D-Var for COSMIC-2) consistencies and observation-minus-background (O-B) bias. This paper compared CRTM-simulated BT data from five RO wet profile retrievals or ECMWF reanalysis data (background) and SNPP ATMS-observed BT. The consistency between 1D-Var retrievals or ECMWF and the SNPP ATMS measurements are evaluated by quantifying the mean difference \(\upmu(\Delta\)BT) and associated uncertainty \(\upsigma(\Delta\)BT).  
We compare STAR COSMIC-2 WetPrf and UCAR COSMIC-2 WetPrf2 data with SNPP ATMS BT measurements through CRTM simulations. Our analysis focuses on the ATMS temperature-sounding channels (CH07 to CH12) with peak sounding heights from 8 to 26 km and the ATMS moisture-sounding channels (CH19 to CH22) with peak-sounding heights ranging from 3.2 to 6.7 km (see Figure 17). In addition, multiple RO data processed by UCAR, such as COSMIC, KOMPSAT-5, TerraSAR-X, and ECWMF reanalysis profile data, were also fed to CRTM to derive the mean BT biases and uncertainties over the ATMS channels of interest.  
The RO and ERA-5 data used in the CRTM simulation are from 16 October 2019 to 31 December 2019. The matchup condition for RO vs. ATMS comparison is set as restricting the colocation between RO and MW data within a 150 km distance and a 2 h time difference. The matchup between ERA-5 and ATMS is carried out through spatial and temporal interpolation of global ECMWF data onto locations and time instants of ATMS observations. To compare RO/background with MW observations, a nadir view of ATMS is enforced to minimize the viewing geometry effect. The RO/ECMWF background data over the whole day, i.e., both day and nighttime data, are included in the comparison. The MW surface emissivity and surface skin temperature over land change significantly with surface types. Owing to the lack of accurate MW surface emissivity and surface skin temperature information over lands, we only conduct our comparisons over oceans.  
Figure 17: The weighting function profile and peak sounding heights (listed) for SNPP ATMS channels CH07–CH12 and CH19–22.