Comprehensive Precipitable Water Vapor Retrieval and Application Platform Based on Various Water Vapor Detection Techniques  
[PERSON]  
1 College of Geomatics, Xi'an University of Science and Technology, Xi'an 710054, China; [EMAIL_ADDRESS]  
[PERSON]  
1 College of Geomatics, Xi'an University of Science and Technology, Xi'an 710054, China; [EMAIL_ADDRESS]  
[PERSON]  
2 School of Aerospace Medicine, Key Laboratory of Aerospace Medicine of Ministry of Education, Air Force Medical University, Xi'an 710032, China 2*water vapor (PWV) is the indictor used to reflect the variation of atmospheric water vapor content, which is defined as the water vapor content per unit cross-sectional area from the surface to the top of the atmosphere [4]. Several conventional techniques have been used to retrieve PWV, which includes radiosonde (RS), water vapor radiometer (WVR), and numerical weather prediction (NWP). However, these techniques have their disadvantages and limitations, and cannot simultaneously obtain PWV with high precision and high spatio-temporal resolutions. For example, RS could provide PWV products with 30 m vertical resolution, but its spatio-temporal resolutions are low because the distance among stations is 200-300 km and the RS balloon are launched twice or four times a day; thus, the PWV derived from RS is usually used for data calibration [5]. WVR can provide PWV products with a high temporal resolution, but it is too expensive to apply widely and vulnerable to cloud and rainfall [6]. Remote sensing techniques could obtain PWV with high spatial resolution; however, temporal resolution and accuracy are relatively poor for polar-orbiting satellites [7]. Furthermore, the performance of satellites is also affected by bad weather in deriving PWV [8,9]. Although PWV can also be obtained based on global model NWP with a temporal resolution of one hour, the spatial resolution and accuracy are relatively low [10,11]. Apart from the above techniques, the emerging techniques, Global Navigation Satellite System (GNSS), the fifth-generation reanalysis dataset of the European Centre for Medium-Range Weather Forecasting (ECMWF-ERA5) and FY-3A and Sentinel-3A satellite have more potential for deriving PWV products. Since the conception of GNSS meteorology is first proposed and the PWV is calculated by [PERSON] et al. [12], the GNSS-derived PWV has been widely validated and used globally. It has the advantages of high-precision, real-time, low-cost and all-weather conditions [10]. [PERSON] et al. [13] generated a six-hourly PWV dataset over the period from 1999 to 2015 at more than 260 GNSS stations in China. [PERSON] et al. [14] updated this dataset and generated the hourly PWV dataset over the period of 2011 to 2017 at 249 stations in China, which improved the temporal resolution compared with that from [PERSON] et al. [13]. To obtain the PWV value, the zenith total delay (ZTD) and the zenith hydrostatic delay (ZHD) should be calculated, and then the zenith wet delay (ZWD). Therefore, the PWV can be calculated by multiplying the conversion factor by the ZWD, which plays a very important role in the progress of retrieving PWV from the ZWD of the GNSS [15]. Current researchers often use empirical models to obtain them quickly [16]. ERA5 is the latest generation of reanalysis data released by ECMWF at 14 June 2018, which enhances the horizontal spatio-temporal resolutions when compared with its previous version (ERA-Interim) and adopts the latest integrated forecast system (IFS) to reprocess a large number of assimilation datasets [17]. ERA5-provided PWV values were also validated at 268 GNSS stations worldwide over the period of 2016 to 2018 and the statistical result shows that the PWV derived from ERA5 agrees well with that from the GNSS, whereas the root mean square (RMS) and Bias are 1.84 and 0.67 mm, respectively [17]. FengYun-3A (FY-3A) and Sentinel-3A, have high spatial-temporal resolution in deriving PWV.