## 4 Validation of the STAR COSMIC-2 Temperature and Water Vapor Profiles
### Water Vapor and Refractivity Comparisons with Different SNR Groups  
Researchers [15] have demonstrated that although RO signals with higher SNR may penetrate the deeper troposphere, they may not guarantee improved refractivity and moisture retrievals in the lower moisture troposphere. Figure 10 compared the retrieved water vapor from our 1D-Var to the matched RS41 RAOB for different SNR groups. Figure 8: The mean difference (red lines) +/ – standard deviation (green lines) for the COSMIC-2-RAOB pairs with matchup criteria within 3 h and 1 degree of latitude/longitude in October 2019 are collected for (**a**) RS92 during the daytime, (**b**) RS92 during the nighttime, (**c**) RS41 during the daytime, (**d**) RS41 during the nighttime. The sample numbers are in the blue lines. Above 4 km altitudes, the STAR water vapor retrievals are very close to those from RS41. The mean SNR for all the STAR-RS41 pairs is about 1350 (\(Volt/Volt\), \(V/V\) hereafter). Figure 10: Monthly averaged bias (red lines) +/ \(-\) standard deviation (green lines) for (STAR-RAOB) profiles of specific humidity, and the sample numbers are in the blue lines. COSMIC-2 retrievals for October 2019 and RAOB type RS41 are used for comparison. Panels present averaging in five SNR bins (**a**) 300–500 \(V/V\), (**b**) 500–1000 \(V/V\), (**c**) 1000–1500 \(V/V\), (**d**) 1500–2000 \(V/V\). Imposed “Mean” and “STD” values are computed by averaging from 0 km to 12 km. Figure 9: The mean difference (red lines) +/ \(-\) standard deviation (green lines) for (STAR-RAOB) profiles of specific humidity for October 2019 for **(a)** the 45\({}^{\circ}\)N–45\({}^{\circ}\)S, (**b**) 45\({}^{\circ}\)N–30\({}^{\circ}\)N, (**c**) 30\({}^{\circ}\)N to the equator, (**d**) equator to 30\({}^{\circ}\)S, and (**e**) 30\({}^{\circ}\)S to 45\({}^{\circ}\)S. Five panels are present, averaging in five latitude bins. RAOB type RS41 is used for comparison. The sample numbers are in the blue lines. Although not shown, the mean difference and the standard of all the STAR-RS41 pairs are \(-0.35\) g/kg and 1.2 g/kg, respectively; however, STAR moisture is dryer than RAOB in the lower troposphere below 3 km altitudes, mainly due to the super refraction (SR) effect, which is consistent with those in [15]. The negative specific humidity bias below 3 km altitudes is about 1 g/kg near the surface in all SNR groups, except for the 300-500 \(V/V\) group. The standard deviation is 2 g/kg near the surface, linearly reducing to zero at the height of 10 km. The magnitude and altitude dependence of STAR-RAOB water vapor difference for COSMIC-2 are close to what has been obtained from the COSMIC-1 mission compared to other radiosondes [4; 15]. The mean differences and standard deviations of the STAR-RS41 specific humidity for the five SNR groups over 0-12 km are summarized in Table 1. Table 1 depicts that COSMIC-2 humidity profiles with low SNR (300-500 \(V/V\)) have a more considerable bias than those with higher SNR, especially below 4 km altitudes; however, the STAR water vapor retrievals for the 300-500 \(V/V\) group from 2 km to 4 km are close to RS41 compared with the 1500-2000 \(V/V\) group and \(>\)2000 \(V/V\) group. The RO humidity profiles with a larger SNR (1500-2000 \(V/V\)) have more significant negative biases than those from the 500-1000 \(V/V\) group and 1000-1500 \(V/V\) group. While the emphasis of this paper is on the SNR-dependence of STAR COSMIC-2 humidity data in comparison with RAOB, we note that the day and nighttime dependence of STAR RO thermal profiles in comparison with those derived from UCAR 1D-Var and Vaisala radiosondes are studied in [38] which complements the study in this paper. Figure 11 evaluates fractional N-residual between refractivity simulated from STAR temperature and water vapor retrievals (i.e., \(N_{STAR}\)) and refractivity simulated from RAOB \(N_{RAOB}\) (defined as 100% \(\times\) (\(N_{STAR}-N_{RAOB}\))/\(N_{RAOB}\)).