## 4 Quantify the Spire Retrieval Accuracy and Uncertainty
### Assessment of the Spire Bending Angle Accuracy by Comparing with ERA5
(**c**) Differences (dash lines) and uncertainties (dot lines) of Spire-specific humidity profiles retrieved by UCAR wetPf2 compared to RS41 RAOB data. (**b**) Shows the number of collocated Spire-RAOB temperature profiles, and (**d**) is the number of Spire-RAOB water vapor profiles. Above a 4.2 km altitude, the humidity difference (Spire-RS41) between the UCAR Spire retrievals and RS41 RAOB is relatively small, equal to \(-\)0.04 g/kg (Figure 16c). Below 4.2 km, the UCAR Spire humidity retrievals deviate from the RS41 observations with increasing negative humidity differences approaching the surface, and the mean difference below 4.2 km reaches \(-\)0.19 g/kg. This may indicate systematic dry biases in the RO humidity retrievals due to the refractivity biases caused by the super-refraction near the Planetary Boundary Layer (PBL) when a high temperature and moisture gradients are present. Table 5 also shows the mean humidity differences (uncertainties) over two height ranges (below and above 4.2 km) between the Spire retrievals and RS41. Generally, over the height region between 8 km and 16.5 km, the Spire temperature profiles match RS41 very well, with a mean temperature difference of \(<\)0.02 K. In the altitude range from 17.8 to 26.4 km, the average temperature difference is \(-\)0.01 K. However, small fluctuations can be seen in this height range, particularly a negative temperature difference (Spire temperature \(<\) RS41 temperature) over the height from 18 to 24 km. Because the quality of RS41 may be affected by the solar zenith angle (SZA) (see [5]), we compare the Spire retrievals versus the RS41 observations for different SZA groups. The Spire-RS41 temperature/humidity differences are divided into three SZA groups for daytime (SZA \(<\)= 80\({}^{\circ}\)), dusk/dawn (80\({}^{\circ}\) \(<\) SZA \(<\)100\({}^{\circ}\)), and nighttime (SZA \(>=\) 100\({}^{\circ}\)). Figure 17 shows the height-dependent mean temperature differences and uncertainties over the three SZA groups. Table 6 lists the mean temperature differences and uncertainties over three height ranges (8-11 km, 12.5-16.5 km, and 17.8-26.4 km) and three SZA groups. The number of Spire and RAOB collocations over the dusk/dawn zone is relatively small compared to the daytime and nighttime groups. Figure 17: (a) Height-dependent mean temperature differences (K) of UCAR wetPf2 versus RS41 RAOB in the zones of daytime (SZA \(<\) 80\({}^{\circ}\)), nighttime (SZA \(>\) 100\({}^{\circ}\)), and dusk/dawn (80\({}^{\circ}\) \(<\) SZA \(<\) 100\({}^{\circ}\)) in the upper troposphere and lower stratosphere. (b) The height-dependent profile numbers for the analysis. \begin{table}
\begin{tabular}{c c c c c c} \hline
**Spire** & **μ(\(\Delta\)T) (\(\sigma\)(\(\Delta\)T)) (K)** & **μ(\(\Delta\)T) (\(\sigma\)(\(\Delta\)T)) (K)** & **μ(\(\Delta\)T) (\(\sigma\)(\(\Delta\)T)) (K)** & **μ(\(\Delta\)H) (\(\sigma\)(\(\Delta\)H))** & **μ(\(\Delta\)H) (\(\sigma\)(\(\Delta\)H))** \\
**Retrieval** & **(8–11 km)** & **(12.5–16.5 km)** & **(17.8–26.4 km)** & **(g/kg) (below 4.2 km)** & **(g/kg) (4.8–8.4 km)** \\ \hline UCAR wetPf2 & 0.02(1.13) & 0.00(1.22) & \(-\)0.01(1.40) & \(-\)0.19(1.02) & \(-\)0.04(0.45) \\ \hline \end{tabular}
\end{table} 
Table 5: Mean temperature biases (uncertainties) (K) and mean humidity biases (uncertainties) (g/kg) between Spire RO retrievals and RS41 RAOB observations over different height regions. From Figure 17 and Table 6, over the height range of 8-26.4 km, the temperature between Spire and RS41 during daytime and nighttime is consistent and within 0.06 K. However, a warm difference, i.e., Spire being warmer than RS41, in temperature is shown at different altitudes during the daytime. In contrast, a cold difference in temperature is shown at nighttime, which can come from the temperature correction scheme implemented for RS41. We also analyze the humidity biases between the Spire retrievals and the RS41 RAOB observations according to the daytime, dusk/dawn, and nighttime SZA groups.