大氣中水氣(Water vapor)的量測，對於氣象、氣候模式的準確性有舉足輕重的影響力，過去在水氣資訊的取得上，多使用空間解析度與時間解析度低的探空氣球，或是運行與保養成本較昂貴的水氣輻射儀(Water Vapour Radiometers, WVRs)。近幾年更可利用GPS 訊號反推大氣的水氣含量(Bevis,1992)。
GPS訊號在經過大氣層時會產生時間上的延遲，一般延遲量分為對流層延遲與電離層延遲，後者可透過無電離層線性組合來消除，本研究利用GAMIT 10.7進行天頂向對流層延遲量 (Zenith Tropospheric Delay, ZTD)的解算。對流層延遲量可分為靜力延遲與濕延遲，前者雖然佔總延遲量約90%，但其日變化幅度約僅介於0.1%至0.2%之間，很少有超過0.5%變化幅度的情況，後者則與大氣中所含的水氣量幾乎成比例，佔總延遲量雖約只有10%，但可在一天內達50%至100%的變化幅度。因此本論文利用ZTD的變化貢獻來源主要為濕延遲量的特性，來做進一步的研究。
本研究演算法透過一定的ZTD上升斜率門檻與固定值門檻來生成預估接下來三小時內會發生降雨的預測點，並與實際雨量做統計與定性分析。由事件一長時間ZTD對比單站累積雨量的結果來看，山區並不適用於本研究的預測方法，這是因為地勢較高的地區多了地形抬升作用以及因氣溫較低導致達飽和所需的水氣較其他地勢平坦的測站少的原故。配合事件二臺灣本島168個GPS站與507個氣象站的統計結果發現，3小時內達5mm/hr的ZTD斜率門檻有最好的結果。在進行定性分析後發現，東部與高山區預測點多有誤判、高估的情況發生，說明本研究的預測方法亦不適用於東部地區。研究指出在非東部與接近山區的區域，利用一定門檻的ZTD變化來作為一種監測水氣短期充足與否甚至其動向的工具是可行的。

When Global Positioning System (GPS) signals travel through the atmosphere and the ionosphere, the signal velocities will be delayed due to the medium change. These delays in troposphere in the zenith direction, namely zenith tropospheric delays (ZTD), can be divided into hydrostatic (dry) and wet components, which are called ZHD (zenith hydrostatic delay) and ZWD (zenith wet delay), respectively. The wet delay is proportional to the water vapor along the signal path (Hogg et al.1981; Askne and Nordius 1987).
Since the variation of ZHD is relatively small than that of ZWD, in this study, we consider the variation of GPS-derived ZTD is mainly controlled by the ZWD. By using the increasing slope and the pass-threshold value of ZTD, a prediction point of precipitation is obtained to represent the possibility of rainfall. We investigated two different kinds of cases to assess the feasibility of applying ZTD on short-term rainfall events. For case 1, we chose 5 GPS stations in Taiwan that are separated widely during July and August 2018 for the long-term time series analysis. Results show that about 70% rainfall events are located within the GPS-derived ZTD prediction range except those events occurred at the high-altitude area. For case 2, 178 GPS stations and 168 meteorological stations were chosen to analyse the spatial statistics and the qualitative analysis. Results show the totally good agreement of ZTD prediction area and the rainfall area, but still have some overestimation of prediction points around the east side of Taiwan where is the lee side of Prevailing wind in summer. According to our results from these 2 rainfall events, the GPS-derived ZTD is feasible for the water vapor monitoring.

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