| 摘要: | 梅雨季節的降水與台灣地區的經濟活動、農業以及人民的日常生活息息相關，梅雨期間的雨量佔了台灣全年雨量的四分之ㄧ左右。因此水資源的分配與利用往往取決於梅雨季節降雨量的多寡，但梅雨季的連續性降水性質及間或伴隨雷雨和豪大雨的特性，卻也時常為台灣帶來相當嚴重的災害。 本研究將最近20年的中南部梅雨季豪大雨個案做整理分類，使用了地面及高空天氣圖的氣壓場與風場分析，以了解影響中南部梅雨季豪大雨的降水個案發生的天氣系統。另外利用了合成處理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探討豪大雨事件中影響降水之大尺度及綜觀尺度的動力、熱力及降水機制。由於影響中南部地區梅雨季降水的天氣系統特徵組成相當複雜，將各個案分類後，在此僅就FSWH型及FSW型兩種類型進一步討論。其中F代表梅雨鋒面，SW代表西南氣流，H則代表西太平洋副熱帶高壓。 FSWH類型主要受到梅雨鋒面、西南氣流以及西太平洋副熱帶高壓的影響，從氣象場及風場分布可觀察到西太平洋副熱帶高壓在事件當日增強，其外圍環流與西南氣流合流後造成台灣中南部地區的西南風場加強，且從動力場分布看出梅雨鋒面處的輻合及正渦度皆有增強情形。另外從熱力場及水氣場的分布可以得知台灣中南部地區的低層大氣處於對流及條件不穩定的狀態，而西南氣流會將孟加拉灣、中南半島及南海附近溫暖潮濕的氣流帶往台灣附近，加上西太平洋副熱帶高壓外圍環流與西南氣流合流。因此，在兩者交會處形成寬廣的水氣輻合區域，加上以上動力及熱力條件皆顯示有利於豪大雨的發生。 FSW類型則主要受到梅雨鋒面及西南氣流影響，從風場分布可明顯觀察到沿著梅雨鋒面的東南及西北側皆有數個反氣旋環流系統排列，使得鋒面處的風切更為增強。從動力場分布也可看出鋒面處的輻合及正渦度在豪大雨事件當日有增強情形，且在鋒面東南及西北側皆為很強的輻散及負渦度區，使得梅雨鋒面系統得以維持和發展。而熱力場及水氣場的分布皆顯示西南氣流會將孟加拉灣和中南半島附近的溫暖潮濕氣流帶往梅雨鋒面，因此造成水氣在台灣中南部地區輻合而引發豪大雨。 Precipitation during the Mei-Yu season is closely related to the economic activities, agriculture and human daily life in Taiwan. Because the rainfall during Mei-Yu season comprises about a quarter of the Taiwan's annual rainfall amount, the management and the use of water resources often rely on the amount of the rainfall during this period. However, the Mei-Yu rainfall is frequently continuous and sometimes accompanied with thunderstorms and severe heavy rainfall which often cause serious damages in Taiwan. In order to understand what weather systems triggering heavy rainfall in central and southern Taiwan areas during the Mei-Yu season, this study attempts to classify the weather types which producing heavy rainfall events in the past 20 years by the use of surface and upper-air weather maps. Besides, we use the composite analysis method to reveal the main features of the various weather types in association with the heavy rainfall events, and the scale-separation technique to study the roles of large-scale and synoptic scale atmospheric circulations in producing heavy rainfall from the dynamic, thermodynamic and precipitation mechanism viewpoints. Because the weather systems involved in the Mei-Yu heavy rainfall events occurring in central and southern Taiwan areas are rather complicated, only two of the weather types will be discussed in this study, namely type FSWH and type FSW, in which F stands for the Mei-Yu front, SW stands for the southwest airflow, and H stands for the western Pacific subtropical high pressure. Type FSWH is mainly affected by the Mei-Yu front, the southwesterly flow, and the western Pacific subtropical high pressure. The meteorological and wind fields depict that, on the day of heavy rainfall event, the western Pacific subtropical high is strengthening and its outflow converges with the southwesterly flow causing strong southwesterly wind in central and southern Taiwan areas. The kinematic fields also show enhancements of convergence and positive vorticity along the Mei-Yu front. The thermodynamic and moisture fields further reveal convective and conditional instability in the lower troposphere over central and southern Taiwan areas. The southwesterly flow will bring warm and moist air to the vicinity of Taiwan from Bay of Bengal, Indo-China Peninsula and South China Sea. It converges with the outflow of the western Pacific subtropical high, which will cause a widely moisture-convergence region. All of the conditions mentioned above are favorable of the occurrence of heavy rainfall. Type FSW is mainly affected by the Mei-Yu front and the southwesterly flow. From the wind field, we can find that a series of anti-cyclones located to the southeastern and the northwestern sides along the Mei-Yu front, which make the frontal wind shear even enhanced. The kinematic fields also show enhancement of convergence and positive vorticity along the Mei-Yu front on the day of heavy rainfall event. The strong divergence and negative vorticity on the southeastern and the northwestern sides of Mei-Yu front help the development and maintenance of the front. The thermodynamic and moisture fields further show that the southwesterly flow will bring the warm and moist air to central and southern Taiwan areas from Bay of Bengal and Indo-China Peninsula, and then heavy rainfall occurs due to strong moisture convergence. |
