Observed PW Depletion after 1998 Typhoon Zeb
Figure 1 shows the surface measurements of (a) pressure and surface pressure estimated ZHD, and (b) precipitation collected at ahtye three sites of interest in Taiwan from DoY 285 ( October 13) to 292 ( October 20), 1998. the geographic locations of the three sites are form the southernmost sites of henc ( Hen-Chung ) to , a central Taiwan area of Chia ( Chia-Yi), and to the northernmost sites of banc ( Taipei ). From the pressure curves, it is evident that the center of he Typhoon Zeb was approaching the specific area between DoY 287 and 289 when the dips of the pressure curves occur. About the same period of time, heavy rainfall occurs at all sites due to the influence of the typhoon. Note that the dips are seen in the time sequence of Hen-Chung, Chia-Yi, and Taipei. This sequence is consistent with the northward path of the Typhoon Zeb while it was striking Taiwan.

Figure 2 shows GPS observed ZWD and the corresponding standard deviation while the ZWD solutions are sought, and radiosonde observations of ZWD at the Taipei site from October 13 to 20, 1998.it is an observable trend that ZWD starts to increase before the typhoon, reaches its maximum during typhoon's striking, reduces t a minimum right after striking, and finally recovers to tit nominal value a few days after striking. We notice that the quality of GPS solutions is bad for some periods of time. The solutions are defined as bad if ZWD standard deviation is relatively large. Hence, quality control ( OC) must be performed to assure good quality of ZWD solutions form PS. Note that it has been suggested that the accuracy of GPS observed PW possibly scales with total water burden ( Liou et al., 1999). Besides, the atmosphere could be extremely inhomogeneous when typhoon is closed by. We also observe that cycle slips seem to occur periodically from the standard deviation signatures, approximately within 2 hours after midday. The answer for this periodical slip shall be of great interesting to the literature, while we do not have it so far.

To acquire good quality in GPS estimates of ZWD, a threshold of 5 cm in the standard deviation is set when ZWD is determined. The resulting solutions of ZWD are then converted to PW. Figure 3 presents GPS observed PW at athe three CWB sites and radiosonde measurements of PW at the Taipei sites from October 13 to 20, 1998, there major characteristics can be seen. First, PW signatures show an expected trend that they mimic those of ZWD. Second, the sequence of PW depletion occurs from south to north. The time delay in PW depletion between the southernmost Hen-Chung location ot the northernmost Taipei site is about 8 hours, which is resulted form a time delay corresponding to the northward movement of the typhoon. Clearly, the PW depletion after Typhoon Zeb is captured by the PGS sensing technique. Third, GPS observed PW generally match radiosonde observations. Both readings increase from 5 cm on DoY 286 to 7-8 cm on DoY 288. then, they decrease to the minimum on DoY 290 although the minimums are 3.5 cm for radiosonde observations and observations and 2 cm for GPS measurements at the Taipei site. Finally, they return to their nominal values of 5 cm on DoY 292. We attribute this large discrepancy between two measurements near the dip at the Taipei site partly to the ihomogeneity of the atmosphere. Note that it is a general proactive to assume a stratified atmosphere to solve GPS data for ZWD. Apparently, this assumption should be carefully handled when the typhoon is nearby .

Conclusions
GPS is used to monitor PW dynamics in Taiwan due to the influence of Typhoon Zeb in mid-October 1998. GPS solutions demonstrate that PW is in general high before and during the occurrence of the typhoon, becomes low right after the typhoon, and finally recovers to its nominal values of few days latter. They show that PW increases from about 5 cm on DoY 285 ( October 16) when the typhoon was striking Taiwan, and , then decreases to 2-3 cm after. These signatures match those observed by radiosonde sounding. In addition, GPS-observed PW depletion by about 5cm 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 . clearly, the depletion in PW right after typhoon is accurately captured by GPS.

Acknowledgments:
The authors appreciate much the National Space Program Office grant NSC 87-NSPO (A)-PC-FA07-05. they also thank CWB for providing GPS data.

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