Precipitable water observed by ground-based GPS receivers and microwave radiometry

The sensing of absolute precipitable water vapor (PW) by the Global Positio ning System (GPS) and a Water Vapor Radiometer (WVR) is presented. The GPS approach requires a priori knowledge of the relationship between the weight ed mean temperature of the atmosphere and surface temperature whose regress ion relationship is derived based on ten-year climatological data observed by radiosonde and surface meteorological instruments. Similarly, the WVR sc heme needs a priori information of the relationship between sky brightness temperature and PW whose regression relationship is characterized based on the same set of climatological data. GPS-derived PW are compared with those observed by WVR and radiosondes. The GPS and WVR data were collected at th e Taipei weather station of Taiwan Central Weather Bureau (CWB) from March 18 to 24, 1998. To obtain the estimates of absolute PW at the Taipei site, GPS data acquired from Tsukuba, Japan, at a distance of 2155 km from Taipei were utilized. It is found that GPS-derived PW agrees reasonably well with observations by the WVR and radiosondes. The average of GPS-derived PW is 3.38 cm with a standard deviation of 0.39 cm. The difference between the av erage GPS-derived and WVR-observed PW is 0.27 cm with a bias of -0.04 cm, w hile the difference between the average GPS-derived and radiosonde-observed PW is somewhat larger, 0.36 cm with a bias of -0.42 cm. These differences are larger than differences reported at higher latitudes in regions with lo wer average humidity.