Comparison of precipitable water observations in the near tropics by GPS, microwave radiometer, and radiosondes

The sensing of precipitable water (PW) using the Global Positioning System (GPS) in the near Tropics is investigated. GPS data acquired from the Centr al Weather Bureau's Taipei weather station in Banchao (Taipei), Taiwan, and each of nine International GPS Service (IGS) stations were utilized to det ermine independently the PW at the Taipei site from 18 to 24 March 1998. Ba selines between Taipei and the other nine stations range from 676 to 3009 k m. The PW determined from GPS observations for the nine baseline cases are compared with measurements by a dual-channel water vapor radiometer (WVR) a nd radiosondes at the Taipei site. Although previous results from other loc ations show that the variability in the rms difference between GPS- and WVR -observed PW ranges from 1 to 2 mm, a variability of 2.2 mm is found. The i ncrease is consistent with scaling of the variability with the total water vapor burden (PW). In addition, accurate absolute PW estimates from GPS dat a for baseline lengths between 1500 and 3000 km were obtained. Previously, 500 and 2000 km have been recommended in the literature as the minimum base line length needed for accurate absolute PW estimation. An exception occurs when GPS data acquired in Guam, one of the nine IGS stations, were utilize d. This result is a possible further indication that the rms difference bet ween GPS- and WVR-measured PW is dependent on the total water vapor burden, because both Taipei and Guam are located in more humid regions than the ot her stations.