水氣的變異以及水氣的全球分布情形對天氣系統、氣候的變化皆有重要的影響。本研究主要目的為使用福爾摩沙衛星三號掩星資料(FORMOSAT-3/COSMIC GPS RO)反演之可降水量，探討全球或區域的水氣分布，同時與其它觀測比較以及驗證。
首先探討全球水氣分布，本研究就聖嬰暖事件與反聖嬰冷事件，比對 GPS RO的可降水量與其他水氣相關資料(降水、外逸長波輻射、海表溫度及垂直上升速度)的分布情形，顯示在熱帶地區具有相當高的相關性。
在海洋上，比較GPS RO 與SSM/I、AMSR-E衛星觀測資料的可降水量。分析發現，三者的全球水氣分布相似，唯GPS RO 資料的可降水量因低層水氣並非實際觀測，導致大部分地區有低估的情形，尤其，在南緯10度至北緯30度間，三者有較大差異。另外，比較三者在冷暖事件的距平分布，顯示GPS RO 資料在強事件時，與其他兩者的相位相當吻合，其結果也和相關研究結果一致。整體而言，GPS RO 資料提供相當可靠的全球水氣分布，有助於了解氣候的變化。
在陸地上，則探討地基GPS、GPS RO 資料二者可降水量的差異。在各緯度，兩者具有相當高的相關係數，但低緯度的相關係數較低一些；兩者的時間序列也是在中高緯度比較吻合，可能因為掩星水氣在熱帶地區低層的估計誤差較大、資料數量較少。另外，本研究亦針對兩者，比較海陸站點的差異。二者海陸的差異並不如緯度的差異大，但是陸地上的差異會呈現出明顯的季節變化。

The global variation and distribution of water vapor play an important role in weather system and climate change. In this study, we used the total precipitable water (TPW) retrieved from FORMOSAT-3/COSMIC GPS Radio Occultation (RO) to exhibit global or regional water vapor distribution. By comparing GPS RO data with other different data sources, we can identify the correlation of GPS RO data with others and thus explore the same climate signals as revealed by their coincidence.
We first focus on the comparison of global distribution between GPS RO TPW and all the other related data, including precipitation, OLR, SST and vertical velocity. We found that GPS RO data correlated well with all the other different variable data sets in tropical regions, regardless of opposite phases of the climate signals exhibited by El Nino (warm event) and La Nina (cold event) from 2007-2011.
Similar global distributions were found when comparing GPS RO TPW with water vapor over ocean retrieved from SSM/I and AMSR-E. But, the unrealistic water vapor of GPS RO data in lower troposphere results in underestimate in most regions, especially during latitude range from 10oS to 30oN, as compared to estimates by the other two satellites. Besides, for a strong climate event, GPS RO data appear to address a clearer picture in agreement with the other two data sets. In summary, GPS RO data provide reliable water vapor estimation and climate signals.
We also extended comparisons of the GPS RO TW with ground-based GPS TPW and found a very high correlation coefficient (>0.9) globally with latitudinal dependence. These comparisons highlight the value of the GPS RO water vapor retrieval over land and ocean.

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