## 5 Accuracy and Uncertainty of COSMIC-2 Refractivity, Temperature, and Water Vapor Retrievals
### Accuracy and Uncertainty of COSMIC-2 Refractivity in the Troposphere
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Comparison Regions**} & \multirow{2}{*}{**45\({}^{\circ}\) N to 45\({}^{\circ}\) S**} & \multicolumn{2}{c}{**North Hemisphere**} & \multirow{2}{*}{**Tropical Region**} & \multirow{2}{*}{**Mid-Latitude South**} \\  & & **(45\({}^{\circ}\) N to 20\({}^{\circ}\) N)** & & **(20\({}^{\circ}\) N-20\({}^{\circ}\) S)** & \\ \hline Mean Difference (Stds) (\%) & \(-0.33(1.83)\) & \(-0.25\) (2.02) & \(-0.36(1.77)\) & \(-0.08\) (1.43) \\ \hline Sample number at 10 km altitude & 2833 & 662 & 1821 & 307 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The COSMIC-2–RS41 fractional refractivity differences and stds in different latitudinal zones. Figure 12: The COSMIC-2 fractional refractivity mean differences and standard deviation compared to those collocated RS41 profiles from June to October 2019 (**a**) over global (45\({}^{\circ}\) N to 45\({}^{\circ}\) S), (**b**) mid-latitude North Hemisphere (45\({}^{\circ}\) N to 20\({}^{\circ}\) N), (**c**) tropical region (20\({}^{\circ}\) N-20\({}^{\circ}\) S), and (**d**) mid-latitude South Hemisphere (20\({}^{\circ}\) S to 45\({}^{\circ}\) S). The mean biases and standard deviation relative to the mean are computed from surface to 30 km altitude. The sample numbers are in the blue line, the mean differences are in red line, and the standard deviation values are in green line. difference for COSMIC-2 and RS41 is slightly larger for the 0-500 \(v\)/\(v\) group compared to other groups. Except for the \(>\)2000 \(v\)/\(v\) group, the high SNR measurements from COSMIC-2 seem to improve the mean fractional refractivity difference for the higher SNR group slightly from the 2 km to 5 km altitude. The mean fractional refractivity difference from surface to 5 km altitude for these five groups are equal to \(-\)1.7%, \(-\)0.63%, \(-\)1.25%, \(-\)0.89%, and \(-\)1.69%, respectively (Table 3). Note that most of the COSMIC-2 occultations are within 500-2000 \(v\)/\(v\) (with a mean of 1250 \(v\)/\(v\), see Figure 13c). The mean difference and the std for the 500-1000 \(v\)/\(v\) group (similar to those for COSMIC and KOMPSAT-5. Figure 13b) are equal to \(-\)0.63% with a 3.82% std, which is slightly smaller than that of the 1000-1500 \(v\)/\(v\) group (Figure 13c). Figure 14 depicts the mean water vapor comparison results for different SNR groups. Results show that the mean water vapor difference in the lower troposphere is slighter larger for the 0-500 \(v\)/\(v\) group than those from other groups, except for the \(>\)2000 \(v\)/\(v\) group. The results show that the COSMIC-2 \begin{table}
\begin{tabular}{c c c} \hline \hline
**L1 SNR Range** & \
\begin{tabular}{c} **Mean Difference for Fractional Reactivity (std)** \\ **from the Surface to 5 km Altitude (in \%)** \\ \end{tabular} &
\begin{tabular}{c} **Sample Number at 5 km Altitude** \\ \end{tabular} \\ \hline
0–500 \(v\)/\(v\) & \(-\)1.7(3.65) & 199 \\ \hline
500–1000 \(v\)/\(v\) & \(-\)0.63 (3.82) & 738 \\ \hline
1000–1500 \(v\)/\(v\) & \(-\)1.25(3.88) & 865 \\ \hline
1500–2000 \(v\)/\(v\) & \(-\)0.89(3.51) & 787 \\ \hline \(>\)2000 \(v\)/\(v\) & \(-\)1.69(3.95) & 292 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: The mean fractional refractivity difference and standard deviation from the surface to 5 km altitude for five SNR groups. Figure 13: Similar to Figure 12 but for COSMIC-2 profiles for SNR of **(a)** 0–500 \(v\)/\(v\) group, **(b)** 500–1000 \(v\)/\(v\) group, **(c)** 1000–1500 \(v\)/\(v\) group, **(d)** 1500–2000 \(v\)/\(v\) group, and **(e)** \(>\) 2000 \(v\)/\(v\) group. The SEM is in the horizontal black lines superimposed on the mean. The sample numbers at 5 km altitude are listed in the figures. The sample numbers are in the blue line, the mean differences are in the red line, and the standard deviation values are in the green line. water vapor for the 1000-1500 \(\
u\)/\(\
u\) group is slightly drier than that from the 500-1000 \(\
u\)/\(\
u\) group from the surface to 4 km altitude.