## 4 Validation of the STAR COSMIC-2 Temperature and Water Vapor Profiles
### Global Temperature and Water Vapor Comparisons with RS41 and RS92  
The COSMIC-2-RS41 pairs and COSMIC-2-RS92 pairs with the criteria within 3 h and 1 degree of latitude/longitude in October 2019 are collected and compared in Figure 8. Because the radiosondes may suffer from residual solar-zenith angle radiative correction [15], we separate our comparisons into daytime and nighttime, respectively. The daytime RAOB data are collected for solar zenith angles less than 80 degrees, whereas the nighttime RAOB data are from solar zenith angles larger than 80 degrees. Because all the refractivity information is used for temperature retrievals in the stratosphere and upper troposphere, we only compare COSMIC-2-RS92 and RS41 from altitudes of 13 km to 30 km. Figure 8 depicts that the COSMIC-2-RS92 and RS41 are less than \(-0.15\) K from 13 km to 30 km altitudes with a std of 2 K. There are no obvious COSMIC2-RS41 temperature differences during the daytime and nighttime; however, the RS92 temperature seems about 0.1 K higher than the COSMIC-2 from 22 km to 30 km during the daytime than those at nighttime. This indicated the possible incomplete residual solar-zenith angle radiative correction for daytime RS92 data. As discussed above, RO refractivity sensitivity to temperature information (see temperature averaging kernels) varies vertically. Although not shown in Figure 6, the magnitude of the temperature averaging kernels is highest in the upper and lower stratosphere (close to one, around 8-30 km altitude). It decreases to the surface primarily due to increased water vapor contribution below 8 km. The magnitude of the temperature average kernels decreases from an altitude of 25 km and above owing to the increase in measurement noise above 25 km (see [14]). Figure 8 demonstrates that the STAR 1D-Var temperature retrievals are very close to those collocated RS92 and RS41, which are consistent with those from the UCAR 1D-Var results shown in [15].  
Figure 7: The same as Figure 6 but for RO sensitivity to the water vapor.  
Figure 9 depicts the STAR COSMIC-2-RAOB specific humidity difference in October 2019 (about 1600 matchups total). The global (45\({}^{\circ}\)N-45\({}^{\circ}\)S) mean difference between STAR 1D-Var and RS41 water vapor specific humidity is \(-0.26\) g/kg with a std of 1.05 g/kg. The COSMIC-2 retrieved moisture systematically dryer than RAOB at the lower troposphere. The negative difference is about 1 g/kg near the surface in all latitude zones and then decreases with altitudes to above 4 km. The standard deviation is an order of 2 g/kg near the surface. The overall difference and standard deviation, averaged in the 0-12 km altitude range, are about \(-0.25\) and 1.0 g/kg, respectively. The magnitude and altitude dependence of RO-RAOB water vapor difference for COSMIC-2 is close to what has been obtained from the COSMIC-1 mission for different radiosondes [4,15]. Figure 9 demonstrates that because the COSMIC-2 refractivity is very sensitive to water vapor information in the troposphere (i.e., water vapor averaging kernels are close to 1 near the surface), the STAR 1D-Var water vapor retrievals are very close to those collocated RS41.