梅雨鋒面系統經常在每年五?六月的梅雨季中形成，往往滯留在台灣與華南地區，造成連續性降水，當中常有豪大雨發生。連續性的降雨及豪大雨往往給台灣地區帶來重大的災害。 本研究將1979~2007，共28年的台灣北部地區（北緯24.18°以北）梅雨季豪大雨個案作整理分類，利用中央氣象局雨量站的累積雨量資料、日本氣象廳天氣圖與NCEP/DOE（National Center for Environmental Prediction/Department of Energy）Reanalysis-II的資料做為分析之用，藉以瞭解影響台灣北部地區梅雨季豪大雨降水的主要天氣系統。再藉由合成分析方法探討各豪大雨個案天氣類型的主要特徵，並利用尺度分離方法討論豪大雨事件中影響降水之大尺度與綜觀尺度的動力、熱力與降水機制。由於影響梅雨季豪大雨產生之因素眾多，因此將各豪大雨個案以主要天氣特徵分類成F、FJ與FP三種類型做進一步討論。其中F代表梅雨鋒面、J代表低層噴流（>10m/s）與P代表低壓帶。 F類型主要受到梅雨鋒面的影響，從氣象場與風場分佈可以看出台灣地區位在黃海高壓、太平洋副熱帶高壓外圍環流與微弱低壓帶之交界處，配合梅雨鋒面之風切，因此提供低層輻合良好之發展條件。 FJ類型則主要受到梅雨鋒面與低層噴流之影響，台灣北方高壓強度增強並向南移動，其外圍風場與西南氣流在台灣北部地區造成強風切，使得梅雨鋒面的風切與輻合情況加強，因而引發豪大雨。 FP類型則是受到低壓帶與梅雨鋒面的影響，低壓帶的氣旋式環流與西南氣流在台灣北部地區造成輻合的情形，且梅雨鋒面系統在豪大雨當日移進台灣北部地區，使得輻合情形更加強。而台灣地區剛好處於水氣輻合極大值之區域，由此而激發豪大雨。 Mei-yu front systems usually form in the area extending from Japan to Taiwan and subtropical China during May and June. This season is called the Mei-yu season. The occurrence of continuous and heavy rainfall has a great relationship with Mei-yu fronts in the Mei-yu season. The rainfall frequently causes severe damages in Taiwan. In this study, heavy rainfall cases occurred in northern Taiwan area (24.18°N) during the Mei-yu season from 1979 to 2007 are selected based on the daily accumulated rainfall data of Taiwan Central Weather Bureau. Major weather systems causing heavy rainfall are identified based on weather maps of Japan Meteorological Agency and reanalysis-II data of National Center for Environmental Prediction/Department of Energy (NCEP/DOE). Composite analysis method is adopted to investigate the characteristics of different types of heavy rainfall cases. Scale separation method is also applied to diagnose the roles of large and synoptic scales of circulation on the dynamic and thermodynamic field as well as precipitation mechanisms. A classification of all heavy rainfall cases is made according to the major weather characteristics with F, J and P represent the Mei-yu front, low-level jet and low-pressure band, respectively. The F type cases are mainly affected by Mei-yu front. For this type, Taiwan locates in the borders of the high pressure in Yellow Sea and the Pacific subtropical high. The accompanied weak low-pressure band and strong wind shear of the Mei-yu front provide a favorable condition in forming the low-level convergence. The FJ type cases are mainly under the influence of the Mei-yu front and low-level jet. The southward movement of the high pressure to the north of Taiwan and the southwesterly flow enhance the wind shear of the Mei-yu front in the northern Taiwan. The strong wind shear provides suitable conditions in causing heavy precipitation. For the FP type cases, Taiwan is covered by a low-pressure zone and affected by an approaching Mei-yu front. The cyclonic circulation associated with the low pressure zone is accompanied with southwesterly flow, which causes strong convergence in northern Taiwan area. The approaching Mei-yu front makes the convergence even strengthened. This provides a favorable environment for heavy rainfall to occur.
