## 1 Introduction  
As one of the most significant constituents in the atmosphere, water vapor has been a major concern for decades. Variations in the water vapor concentration are closely linked to changes in global radiation budget, which may have impacts on the atmospheric temperature structure, the characteristics of rainfalls and droughts [1]. Accurate and consistent water vapor measurements in the troposphere are significant for studying the water vapor feedbacks on the clouds, precipitation and other parts of the hydrological cycles, which is still one of the largest uncertainties in understanding the global warming mechanism. Precipitable water (PW), also known as tropospheric water vapor and integrated column water vapor, is a quantitative measurement of water vapor in the upper atmosphere for numerical weather model and climate studies [2]. Nowadays, there are various kinds of methods to obtain PW from observations, and they are usually divided into two types: ground-based and space-based measurements. Among these observations, radiosondes and satellites are two popular instruments to detect water vapor in the atmosphere. Radiosondes have provided detailed measurements of global atmospheric water vapor since 1938 [3] and now there are over 2700 stations distributed all over the world, which provides us the chance to study the characteristics of PW for global climate change and weather prediction. In 1963, [PERSON] [4] calculated the regression equation of monthly mean surface dew point and monthly mean PW using 15 stations' observations in America, which provided a new method to access PW with the limited observations. In 1981, [PERSON] and [PERSON] (2018) used the relationship between PW and dew point temperature to study the PW under the control of the India monsoon and found the maximum correlation between dew point temperature at 850 hPa and PW. However, the regression equation of the relationship changes with the latitude, altitude and other climate elements of the regions and the method only provides the approximation of the PW value. In 1996, [PERSON] and [PERSON] (1996) pointed out the fact of the bad performance of earlier radiosonde sensors over 300 hPa, even 500 hPa in middle and high latitudes. In addition, in [PERSON] and [PERSON]'s later research in 2000 ([PERSON] and [PERSON], 2000), the increase of PW during 1973-1995 in the Northern Hemisphere, except Europe, was found after identifying and excluding stations with the problems of inhomogeneity and incomplete records. After that, [PERSON] et al. (2018) updated the trends of water vapor of the Northern Hemisphere through 2006 and widespread significant increases of tropospheric water vapor from the surface to the highest observing level in the Northern Hemisphere were presented. With the development of sensor techniques and launches of satellites, space-based observations with global coverage and a higher resolution of space and time are possible. In 2010, [PERSON] et al. (2010) compared specific humidity profiles obtained by the global positioning system radio occultation (GPS RO) technique with radiosonde observations retrieved by different sensors and European Center for Medium Range Forecasts (ECMWF) reanalysis data; comparisons show overall agreements among these data sets and the usefulness of the new GPS RO water vapor profiles retrievals. Similar results can be found in [PERSON] et al. 's (2018) research of comparisons in PW with three data sets of GPS RO, ground-based GPS observations and NCEP reanalyses. [PERSON] et al. (2010) studied the variations in the lowest perigee height of the wet profiles by using GPS RO observations and a special sensor microwave imager (SSM/I) and the result demonstrates that the variation itself does not bring systematic errors to PW differences. In [PERSON] et al.