## 6 Conclusions  
RO is becoming a core observation at NOAA. This study describes COSMIC-2 neutral atmospheric profile inversion procedures developed by NESDIS STAR. Because RO measurements are not affected by clouds and precipitation, the RO-derived water vapor profiles in the neutral atmosphere are complemented by those from satellite IR and MW sounders, providing extra atmospheric information within and below clouds. We developed a standalone 1D-Var approach to convert refractivity to temperature and moisture profiles. We used a GFS 6 h forecast temperature and moisture profiles interpolated to the COSMIC-2 occultation locations and times as the a priori profiles. We also provided the initial validation results for the COSMIC-2 mission. We reach the following conclusions:  
1. COSMIC-2 (and other RO) refractivity is sensitive to temperature and water vapor. Like all RO data, COSMIC-2 refractivity is sensitive to temperature variation in the upper and lower stratosphere, where water vapor amounts are negligible. In the troposphere, RO refractivity is very sensitive to water vapor variation, which is reflected in the magnitude of COSMIC-2 water vapor averaging kernels. COSMIC-2 water vapor averaging kernels are more significant in the lower troposphere in the summer than in winter during drier seasons. In the tropical region within 20\({}^{\circ}\)N to 20\({}^{\circ}\)S, the magnitude of COSMIC-2 water vapor averaging kernels is very close to 1.0 from the surface to 8 km altitude. As a result, in the 1D-Var package, most of the COSMIC-2 refractivity in the upper and lower stratosphere is used for temperature retrievals. In the moisture troposphere, most of the COSMIC-2 refractivity information is used for moisture retrievals. 2. Validation Results. In this study, we used RS41 and ERA5, and UCAR 1D-Var products (wetPrf2) to validate the accuracy and uncertainty of the STAR 1D-Var thermal profiles. Researchers [15] have verified the precision and accuracy of UCAR temperature and water vapor data products. Compared to the collocated RS41, the STAR temperature differences are less than a few tenths of 1 K from 8 km to 30 km altitude with a std of 1.5-2 K. We have compared the STAR 1D-Var-derived water vapor profiles with those collocated RAOB profiles. The magnitude and altitude dependence of RO-RAOB water vapor difference for COSMIC-2 is close to what has been obtained from the COSMIC mission compared to other radiosondes [3]. Further comparisons of STAR RO thermal profiles with those derived from the UCAR 1D-Var algorithm and Vaisala radiosondes are detailed in [37]. 3. Using STAR COSMIC-2 1D-Var results for climate monitoring. Using reanalysis as a reference, we identified the sudden change in the time series in the current UCAR near-real-time COSMIC-2 data. When comparing with the reanalysis and STAR COSMIC-2 1D-Var results, we further recognized that the possible causes of the sudden jump of the UCAR COSMIC-2, which might be owing to their (i) inversion implementations and (ii) the a priori used. 4. Compared to CrIS and ATMS brightness temperatures. In general, the STAR WetPrf-derived BT differences between five COSMIC-2 (E2-E6) sensors and COSMIC-2 E1 sensor are within \(\pm\)0.1 K for ATMS temperature-sounding channels CH07 to CH011 (peak sounding height from 8.06 to 20.89 km) and water vapor sounding channel CH19-20 (peak sounding height from 3.18 to 4.27 km). The RO-derived water vapor profiles in the neutral atmosphere are complemented by those from satellite infrared (IR) and microwave (MW) sounders [1; 2; 3; 4] and provide total water vapor column and vertical water vapor variation under all-sky conditions and within and below clouds [7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27].