梅雨研究在東亞地區為一重要議題，季風肇始後，南海水氣增加，輻合增強，環流改變促成多雨的環境條件，即形成梅雨要因之一。然而梅雨降水年際變化顯著，本研究著重於台灣乾濕梅分析。首先以台灣降水為依據，將各年區分為平均乾濕梅個案、大於0.8標準差乾濕梅個案以及小於0.8標準差乾濕梅個案：再利用美國環境預報中心每日全球分析資料，進行大氣環流及水氣傳輸與台灣乾濕梅之間的差異比較研析。 經由分析研究結果得知，乾梅時梅雨帶偏北，自南海傳送之西南風微弱，輻合中心北偏，太平洋副熱帶高壓脊越過120。E伸及台灣上空。濕梅時則有較強西南風傳送，輻合中心偏南，太平洋副熱帶高壓明顯東退，大陸熱低壓得以伸展。進一步以擾動量分析，乾梅時台灣附近具反氣旋式擾動，並處於輻散氣流之中。濕梅期間則位在氣旋式擾動的水氣輻合環境，有利於降水的顯著增多。最後探究梅雨期間降水之日變化發現，所有個案皆於午後呈現極值，唯濕梅時具雙峰結構，且降水集中於晨間及午後。乾年少雨時偏差風場呈現往島內輻合，而濕年的多雨水則促使偏差風場易於向外輻散。 Meiyu is one of the major research topic in the East Asian meteorological community. The intensified low level convergence environment in East Asia together with increased abundant water vapor in South China Sea after the East Asian monsoon onset are claimed to be the primary cause of the Meiyu occurrence. According to the long-term precipitation history record from Central Weather Bureau of Taiwan, a significant interannual variation of Meiyu rainfall is clearly standout. Based upon this Meiyu rainfall norm, relative dry/wet seasons are separated into six regimes by adopting the 0.8 standard deviation of precipitation variance as a guideline. Followed by using NCEP daily global analyses data, the atmospheric circulation and water vapor transport were extensively investigated to understand the differences among these six Meiyu rainfall regimes. It is interesting that the rainband shifts northward during the dry Meiyu seasons when the water vapor transport driven by weak southwesterly flow converges toward the northward shifted convergent center. The ridge of Pacific subtropical high extends westward passing through Taiwan and beyond 120。E. During the wet Meiyu seasons, relative strong water vapor transported by southwesterly flow converges at the southward migrated convergent center. It appears that the southward shifted Meiyu rainband is due to the eastward withdraw of the Pacific subtropical high and eastward extent and well-developed Southeast Asian monsoon trough. Affected by the Meiyu rainbelt, Taiwan has relative more rainfall occurred. With regards to the rainfall diurnal variation, an afternoon precipitation maximum appears in both dry and wet Meiyu seasons. However, another rainfall peak exists in the early morning during the wet seasons.
