## 5 Accuracy and Uncertainty of COSMIC-2 Refractivity, Temperature, and Water Vapor Retrievals
### Accuracy and Uncertainty of COSMIC-2 Refractivity in the Troposphere  
Ref. [48] stated that although RO signals with better SNR may penetrate to the deeper troposphere, it does not guarantee an improved bending angle and refractivity profiles after the inversion. Studies ([1,49]) demonstrated that RO derived water vapor results are highly accurate. The water vapor retrieval results may be weakly dependent on the a priori [1,29]. To examine how exactly COSMIC-2 refractivity biases would be improved when better SNR data are used in the inversion package, we compared the COSMIC-2 retrievals under different atmospheric conditions (dry and moisture troposphere in different latitudes) in this section. Figure 12a-d depict the COSMIC-2-RS41 refractivity mean differences and standard deviation from June to October 2019 over global (45\({}^{\circ}\) N to 45\({}^{\circ}\) S), mid-latitude North Hemisphere (45\({}^{\circ}\) N to 20\({}^{\circ}\) N), tropical region (20\({}^{\circ}\) N-20\({}^{\circ}\) S), and mid-latitude South Hemisphere (20\({}^{\circ}\) S to 45\({}^{\circ}\) S). In general, the COSMIC-2-RS41 fractional refractivity differences vary in different latitudinal zones (see Table 2). The COSMIC-2-RS41 mean fractional refractivity difference from 45\({}^{\circ}\) N to 45\({}^{\circ}\) S is equal to \(-\)4% near the surface to about 0.3% at 5 km altitude. The std below 2 km is equal to about 4% to 5%. Note that the refractivity uncertainty also depends on moisture variation [1]. The mean difference values in Figure 12a-d are computed from the surface to 30 km altitude. The relatively larger mean difference at 3 km altitude occurred over 30\({}^{\circ}\) N-45\({}^{\circ}\) N, where the mean difference is equal to 1% at 3 km altitude then decreased to \(-\)4% at the surface. Over Tropics (30\({}^{\circ}\) N-30\({}^{\circ}\) S), the mean difference is equal to 0.1% at 3 km, then also reduces to close to \(-\)5% at the surface. As demonstrated in Figure 2, most all of the COSMIC-2-RS41 pairs occur over the 30\({}^{\circ}\) N-30\({}^{\circ}\) S latitudinal zone since more than 70% of COSMIC-2 occur over that region. The sample numbers shown in Figure 12 are the sample number at the 10 km altitude. Figure 11: Similar to Figure 10 but for the COSMIC-2–RS41 pairs over (a) day time, and (b) night time. The more substantial mean difference in the tropical regions below 2 km altitude may be owing to a considerable tracking uncertainty from COSMIC-2, mostly below the SR. The mean difference between COSMIC-2 and RS41 is smaller over higher latitudes (mid-latitudes for both North and South Hemispheres). The water vapor variation and amount over those regions are lower than those from Tropics. We also compared the COSMIC-RS41 refractivity data collected from June to October from 2010 to 2020 over the same latitudinal zones. The COSMIC-RS41 mean refractivity difference and uncertainty over 30\({}^{\circ}\) N-30\({}^{\circ}\) S near the surface is about \(-4\%\) with a std of 5% (not shown). The results show that the COSMIC-2 mean refractivity biases and uncertainty (in terms of std) are very compatible with those of COSMIC. These results are similar to those for COSMIC-1-ECMWF comparisons shown in [16] (Figure 2). Accuracy and Uncertainty of COSMIC-2 refractivity and Water Vapor Retrievals for Different SNR Groups  
We further compare the COSMIC-2-RS41 fractional refractivity difference from surface to 5 km altitude in each SNR group in Figure 13. Figure 13 depicts that the mean fractional refractivity