## 4 Discussion  
According to [PERSON] et al. [4], the dry temperature derived from COSMIC has a bias of \(-0.03\) K and an RMSE of 1.98 K compared to radiosonde observations from 12 km to 25 km in China; and below 12 km, the dry temperature is significantly lower than the actual temperature because water vapor effects become important. In our results, the average temperature difference between COSMIC and radiosonde data from 0 to 40 km in China is \(-0.10\) K, with an RMSE of 4.84 K. These results may imply that the increase in water vapor in the troposphere increases the COSMIC cold temperature bias compared to radiosonde observations and the RMSE between them. Moreover, the absolute temperature bias between COSMIC and radiosonde data has a positive correlation with station altitude. Thus, the temperature difference between COSMIC and radiosonde data increases in the QTP, and the temperature RMSE between COSMIC and radiosonde data exhibits the same trend. Because a background atmospheric state is used in the retrieval of the temperature profile in COSMIC RO [39; 59] and the model temperature bias is large in the QTP area because of its complex topography [60], the limited ability of the background may contribute to the temperature difference between COSMIC and radiosonde data, especially in the QTP region. A few studies have compared the pressure and vapor pressure profiles of COSMIC and radiosonde data over China especially for the QTP region [51; 52; 53]. In this study, the COSMIC pressure has an average bias of 0.69 hPa in China, and the average RMSE is 10.66 hPa. Moreover, the absolute pressure bias between COSMIC and radiosonde data has a negative correlation with station latitude, yet this correlation is not significant for the pressure RMSE. Note that both the pressure bias and RMSE are generally larger in the QTP region than in other areas at the same latitude, and this finding may be related to the uncertainty of the model and the complex terrain in the QTP area. For the vapor pressure, the average bias and RMSE between COSMIC and radiosonde data in China are \(-0.01\) hPa and 0.64 hPa, respectively. As shown by [PERSON] et al. [21], the relationship between  
Figure 9: The refractivity RMSEs between COSMIC and radiosonde data in China. The number of radiosonde stations for each RMSE class is listed in parentheses. errors in temperature and water vapor pressure, under the assumption of no errors in N or P, is as follows:  
\[\Delta\mathrm{e}\approx\frac{(2\mathrm{TN}-77.6\mathrm{P})}{3.73\times 10^{5}} \Delta\mathrm{T}, \tag{2}\]  
where e is vapor pressure in hPa, T is temperature in K, N is refractivity, and P is pressure in hPa. \(\Delta\mathrm{e}\) and \(\Delta\mathrm{T}\) are the errors of vapor pressure and temperature, respectively. For typical values of N = 300, P = 1000 hPa, and T = 273 K, the following expression is valid:  
\[\Delta\mathrm{e}\approx 0.23\Delta\mathrm{T}\mathrm{\ or}\mathrm{\ }\Delta\mathrm{T}\approx 4.35\Delta \mathrm{e} \tag{3}\]  
Specifically, T would have to be known to within an accuracy of 1 K to obtain vapor pressure estimates accurate to within 0.23 hPa. Because the temperature bias is \(-\)0.10 K with an RMSE of 4.84 K in this study and there are errors in P and N, the vapor pressure bias of \(-\)0.01 hPa with an RMSE of 0.64 hPa is reasonable. Additionally, the absolute vapor pressure bias has a negative correlation with station latitude. However, the vapor pressure RMSE is negatively correlated with station altitude. The small vapor pressure RMSE in the QTP region may be due to the low vapor pressure in this area. The COSMIC refractivity profile from 0 to 40 km exhibits little difference compared to radiosonde observations in China, and the average bias between them is 0.17 N with an RMSE of 3.42 N. As with the vapor pressure, the absolute refractivity bias has a negative correlation with station latitude, while the refractivity RMSE is negatively correlated with the station altitude.