## 4 Conclusions  
The variability and trends in global PWs derived from COSMIC GPS RO retrievals and IGRA radiosonde observations during 2007-2016 are studied in this paper. Generally, PW shows apparent latitudinal and seasonal variations. The averaged PW peaks in the tropical region, with the value of 40 mm or higher, and then decreases apparently to the polar region, less than 10 mm. PWs are larger in warm seasons and smaller in cold seasons for both hemispheres. The variations in PW indicate atmospheric water vapor is closely related to the source of water vapor (occans and seas) and heat (surface temperature). However, the contributions of these two aspects to water vapor in the upper atmosphere are still not very clear, which makes PW variabilities complicated.  
The two observations agree quite well with PW variations, but with some differences in magnitude. Generally, IGRA PW is larger than COSMIC PW. In the tropical area, the IGRA PW is at least 5 mm larger than the ones derived from COSMIC. However, for the high latitudes in the Southern Hemisphere, COSMIC PWs are 1 mm larger. Moreover, around 30\({}^{\circ}\) N in the Northern Hemisphere, the COSMIC PWs are about 5 mm larger than the ones derived from radiosonde. The differences in two observations may result from the samplings of COSMIC RO profiles and the IGRA stations and the calculations of two observations. The averaged latitudinal PWs from COSMIC contain the information of both ocean and land. On the other hand, the averaged IGRA PWs are mainly the results from land. Regarding the calculations, the radiosonde measurements detect the real atmospheric humidity, while the COSMIC PW is retrieved from the refractivity profiles. It is noteworthy that these may both bring some uncertainties to the results.  
The global PWs have been changing in the past 60 years and, to some extent, strongly in recent 10 years. Based on the analysis of the long-term accumulation of radiosonde observations, it is found that PWs at Australian stations present statistically significant decreasing trends in the last 60 years, while PWs at most stations in the US have increasing trends, which is consistent with previous studies. In light of the good performance of the comparisons between two observational data sets, COSMIC data can be a complement to the radiosonde observations due to world-wide measurements. We pay close attention to the global PW changes in the most recent decade. It is found that PWs show uptrends over land and downtrends over oceans in the last 10 years, implying the widespread increase of water vapor in the troposphere over land. The long-term increasing trends in PW for the US region are assumed to be consistent with the increases in temperature [8]. In addition, the variations in some of the regional PW time series indicate that the known modes of variability such as NAO [18] and  
Figure 9: Trends in PW from IGRA and COSMIC for the period of 2007–2016 (stations with bold black circles are stations with significant trends).  
ENSO [19] may also explain a portion of PW variability. However, these are the previously published results on some local regions. More investigation about the variability in global PW is needed.  
[PERSON] and [PERSON] conceived the idea, analyzed the data and wrote the paper; [PERSON] and [PERSON] discussed the results and contribute helpful interpretation.  
This work was jointly supported by Jiangsu Provincial Research Foundation for Basic Research (BK20161531) and Meteorological Research Open Foundation of Huaihe Basin (HRM201604). We are grateful to the COSMIC Data Analysis and Archive Center (CDAAC) and the National Climatic Data Center (NCDC) for providing observation data.  
The authors declare no conflict of interest.