## 3 Results and Discussion  
### Comparisons of COSMIC and IGRA  
In order to compare the PW derived from radiosonde observations with COSMIC satellite data in details, we selected eight stations for the case study, with consideration of latitude, elevation and the distance to the ocean. The locations are shown in Figure 1.  
The variations of monthly averaged PW at selected stations are shown in Figure 2. In general, PWs derived from two datasets show consistent patterns. PWs are larger in warm seasons and smaller in cold seasons for both hemispheres. In the polar regions, PWs are quite small (Figure 2a,h) and the differences between two observations are relatively small. PWs at the Antarctic are below 10 mm throughout the whole year. In summer, stations in the Arctic (Figure 2a) have relatively larger PWs than other seasons. Overall, PWs derived from COSMIC have smaller values in the middle and lower latitudes than the ones from radiosonde observations, except for two stations (Figure 2b,f). The PWs from the satellite are relatively larger than the ones from the radiosonde observations at the European station Schleswig (Figure 2b) throughout the year. For the Darwin Airport station (Figure 2f), which is located in the Southern Hemisphere, the IGRA PWs are smaller than the COSMIC ones in springtime. Figure 1: Distribution of selected stations ((**a**–**h**) are the stations selected in Figure 2; blue triangles are stations selected for trend analysis). Figure 2: Variations of monthly mean precipitable water (PW) of selected stations (**a**–**h**) retrieved from Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) and Integrated Global Radiosonde Data (IGRA) observations for the period of 2007–2016. It is noteworthy that two stations with elevation over 1 km (Figure 2c,g) have larger IGRA PWs, especially for the summertime, with differences larger than 5 mm or even higher. As presented in Section 2, PW values are accumulated from the ground to the highest observing level for the COSMIC data. Because of the existence of the lowest perigee point, the void lower levels (from the surface to the lowest perigee point) are filled with PW value of the lowest water vapor observation of water vapor. This process might cause an averaged 2 mm or less bias of PW, as [PERSON] et al. introduced in their research [19]. This might bring the differences between COSMIC and IGRA PW closer over stations, especially for those with higher elevations. According to Figure 2, it is easily found that the PWs increase from the polar region to the tropical region. However, not all stations in the tropical area have larger PW values. PWs in summer at the tropics are usually about 50 mm (Figure 2e,f). However, for station Windhoek (Figure 2g), summer PWs have not reached 30 mm. This station might be influenced by Benguela Current, which is a cold current that constrains the evaporation of the sea surface and brings less water vapor to the coastal area. This implies that the coastal regions may not always be assumed to have sufficient water vapor supply and PWs at these areas may be impacted by the ocean current. To further explore the differences in PW derived from COSMIC and radiosonde observations, the latitudinal monthly averaged PWs of both observations are compared in scatterplots (Figure 3). Note that the latitudinal mean values are obtained by the averaged values in 5\({}^{\circ}\) latitude width (36 bins). Two observations show a good agreement, and [PERSON]'s correlation is 0.98 (significant). When the PW values are smaller than 30 mm, two observations scatter around the \(y=x\) line evenly. However, when the PWs are larger than 30 mm, COSMIC PWs are systematically smaller than the ones derived from IGRA. This difference will be further explored in the following sections. Interestingly, there are a few spots with very small IGRA PW values scattered above the \(x\)-axis.