乾旱是臺灣重要的天然災害之一，過去幾十年出現過多次大型乾旱事件，往往造成嚴重的經濟損失 與民生不便。本研究比較兩種近期發表的乾旱測定方法後，建構出一個較能反映實際乾旱發生時段的「臺 灣氣象乾旱指數」（Taiwan Meteorological Drought index，簡稱 TMD index)。TMD index 採用臺灣全島低 於或等於海拔150公尺的平地觀測站雨量資料，藉由90天窗區內連續不降水日數的計算，以超過歷史 數據一個百分化的標準，來測定在這個窗區內的乾期發生時間，用以計算「臺灣氣象乾旱指數」。 本研究計算1950年至2015年間臺灣的乾旱發生時段，並理解嚴重乾旱事件多發生於颱風季結束後 至隔年梅雨季前。為了分析嚴重乾旱事件發生時臺灣及附近區域的大氣環境場，研究中將乾旱時段區分 成秋冬季、春季及梅雨季三組。結果發現，嚴重乾旱發生時臺灣及附近區域為不利對流系統發展的條件， 因此少雨是可預期的。 此外，本研究也分析臺灣幾十年來乾旱的發生頻率變化，得知以秋季而言，1980年代後發生乾旱的 頻率增加，而冬春季則在1970年代前發生的頻率較高，後期乾旱發生機率變小。此情況可能是受到Pacific Decadal Oscillation (PDO)的影響，因而在1950年代至1970年代期間屬於少雨期。

Drought is one of important natural disasters in Taiwan. Numerous severe drought cases in the past few decades in Taiwan caused tremendous economic loss and great inconvenience in livelihood. After comparing with two other recent drought studies in Taiwan, a relatively reasonable definition, Taiwan Meteorological Drought index (TMD index), was established in this study. The TMD index considers rainfall data on the plains of Taiwan (altitude^ 150m). Within a 90-day moving window, the maximum number of consecutive dry days needs to be larger than the criteria which are the one percentile of the historical records to be identified as a drought case, and the actual dry days in the 90-day windows are marked as drought periods to calculate the TMD index. The 1950-2015 TMD index are calculated in this study and show the severe dry cases in Taiwan tend to happen from the end of typhoon season to the next spring before Meiyu season. In order to compare the dry events and the large-scale environment condition, the drought seasons are divided into three groups: fall and winter period (October - next January), spring period (February - April) and Meiyu period (May). The analysis of reanalysis data shows that the atmospheric environment around Taiwan during severe droughts is under unfavorable conditions for convection systems to develop, and that could benefit droughts to occur in Taiwan. The severe drought frequency is also discussed in this study. The frequency changed in the past decades. More severe drought cases in fall occurred after the 1980s, but more cases happened in winter-spring season before the 1970s. This may be related to the impacts of the Pacific Decadal Oscillation (PDO) from the 1950s to the 1970s.