## 4 Validation of the STAR COSMIC-2 Temperature and Water Vapor Profiles
### Using STAR COSMIC-2 1D-Var Results for Climate Monitoring  
As mentioned above, the quality of COSMIC-2 data products may vary with the receivers, sudden changes in processing configuration, and a priori profiles used in the 1D-Var inversion algorithm. The sudden quality change in RO data time series (i.e., bending angle, refractivity, temperature, and water vapor profiles) are usually owing to (i) malfunction of receivers, (ii) changes in satellite maneuvers, and (iii) changes in RO data processing, including quality control criteria, and calibration/validation approaches. As shown in Figure 1, we developed the STAR RO ICVS to routinely monitor the quality of multiple RO missions and identify the sudden quality change in the time series of RO data products (see [[https://ncc.nesdis.noaa.gov/GNSSRO/ICVS/index.php](https://ncc.nesdis.noaa.gov/GNSSRO/ICVS/index.php)]([https://ncc.nesdis.noaa.gov/GNSSRO/ICVS/index.php](https://ncc.nesdis.noaa.gov/GNSSRO/ICVS/index.php)) (accessed on 26 September 2022)).  
Figure 11: The same as Figure 10 but for fractional residual between refractivity simulated from STAR retrievals and refractivity simulated from RAOB; COSMIC-2 retrievals for October 2019. Imposed “Mean” and “STD” values are computed by averaging from 0 km to 20 km.  
The references used to quantify the quality of the RO data in the RO ICVS include NWP forecast (GFS 6 h), analysis (global data assimilation system (GDAS)/GFS analysis), and reanalysis data (i.e., national centers for environmental prediction (NCEP) reanalysis-2, modern-ERA retrospective analysis for research and applications version -2 (MERRA-2), and ERA-5) datasets.  
Researchers [15] quantified COSMIC-2 data quality and accuracy uncertainty using reference datasets. Using ICVS, we can also quantify the quality of UCAR's refractivity data products compared to the collocated ERA-5 simulated refractivity (see Figure 13). The ERA-5 temperature and water vapor are first interpolated to COSMIC-2 location and times and then put into Equation (1) to simulate the refractivity profile.  
The sudden jump in the O-B (observation minus background reference) time series (see Figure 13) after 24 March 2020, is because COSMIC-2 data were assimilated into ERA-5 on that date. Another noticeable jump in the O-B time series in October 2020 is due to UCAR's 1D-Var algorithm update after 25 September, which is also reflected when comparing the O-B time series of water vapor from UCAR WetPrf2 and STAR WetPrf at 700 mb (Figure 14). Using a similar approach, [36] used COSMIC-2-retrieved atmospheric profiles from WetPrf2  
Figure 12: Monthly mean difference (red lines) +/ – standard deviation (green lines) for (STAR-UCAR) profiles of (**a**) specific humidity and (**b**) temperature. Light-blue lines show a sample number of averaged soundings.  
Figure 13: The time series of the fractional difference between COSMIC-2 refractivity and ERA-5-simulated refractivity at 700 mb for STAR WetPrf (in red) and UCAR WetPrf2 (in blue).  
and WetPrf to simulate the SNPP ATMS BTs from October 2019 to the end of 2020. The times series of O-B shown in Figures 13 and 14 are consistent with the O-B ATMS time series shown in [37].