GPS observations of PW during the passage of a typhoon

The Global Positioning System (GPS) is used to provide hourly measurements of precipitable water (PW) in Taiwan during the passage of tropical cyclones. Typhoon Zeb, which caused serious damage in the Philippines, Taiwan, and Japan in mid-October 1998, is used as an example in this paper. GPS data are analyzed from the Central Weather Bureau's (CWB's) three weather stations in Taiwan, and from a site in Tsukuba, Japan. Bernese version 4.0 software is utilized to solve GPS signals for total delay due to the neutral atmosphere at the three CWB sites. Wet delay is obtained by subtracting surface pressure derived dry delay from total delay. Wet delay is then converted to PW through a simple calculation. GPS-observed PW time series demonstrate that PW is, in general, high before and during the occurrence of the typhoon, and low after the typhoon. PW increased from about 5 cm on DoY 285 (October 13) to near 8 cm or so on DoY 288 (October 16) when the typhoon was striking Taiwan, and, then, decreased to 2-3 cm after passage of the typhoon. In addition, GPS-observed PW depletion from 8 cm on DoY 288 to about 3 cm on DoY 290 is found to be consistent with radiosonde observations acquired at the Taipei weather station.