- 期刊
高層噴流於台灣地區梅雨期間之變化研究
The Evolution of Upper Level Jet Streaks during the Mei-Yu Season over Taiwan Area
摘要
冬季季風環流轉變為夏季季風環流期間之梅雨期是東亞地區主要降水來源之一，就台灣而言，梅雨期之降雨量平均為全年雨量的1/4以上，因此探討影響梅雨期間相關環流之演變為本研究之主要目標。此報告將著重於200 mb高層噴流之演化過程的了解。利用ECMWF/WMO analysis data sets選取1979年至1987年期間梅雨顯著年（1981，1979）及不顯著年（1985，1987）透過200 mb風場，流函數場，700 mb風場，500 mb垂直速度場，850 mb溼度場及200－500 mb溫度場等四個15日平均分佈，200 mb流函數場及200－500 mb溫度場的15日平均差及入梅出梅時前後共十天之熱力方程式的收支變化之估算等，了解高層噴流在台灣地區梅雨期間之演化過程，同時亦比較此四年所造成之梅雨顯著與不顯著年之高層噴流的位置及走向。綜合了所分析之四年資料顯示出高層噴流隨季節性的演變及其熱力原因，我們可得知：（1）入梅前，200mb噴流在110°E以西地區有兩個主軸位置，約是在青康藏高原南北兩側，110°E以東地區噴流範圍寬廣，此時期噴流大致呈東西走向。同時低層噴流也呈東西走向，200mb反氣旋中心位在20°N以南地區。（2a）當200－500mb溫度場顯示在塞北地區有增溫發生，日本附近地區有降溫發生時，200mb噴流北移且呈‘Ｖ’字形，即在125°E以東地區呈東北－西南走向，125°E以西地區呈西北－東南走向，噴流軸心通過120°E、30°N附近，此時即為華南、台灣地區之梅雨期，同時造成低層在東亞地區之30°N南、北處風場中各有較強之南、北風分量，使120°E、30°N附近形成較大溼度梯度值之不穩定大氣狀態，而有利豪雨發生。此時200mb反氣旋中心較前期偏向西北方。（2b）若200-500mb溫度場顯示在塞北地區有增溫發生，但在日本附近區域沒有顯著降溫發生時，200mb噴流中心將北移到35°N附近，雖亦是華南、台灣地區之梅雨期，但其走向呈東西向，且位置偏北，此時700mb上東亞附近地區之風場則無顯著南北風分量存在，將無法產生較大溼度梯度值，亦無較大之降水發生。（3）在35－40°N間，200－500mB全面增溫，造成200mb噴流完全北移到30°N以北，顯示華南、台灣地區梅雨期結束，此時700mb噴流亦北移，造成最大溼度梯度區及上升運動區在日本、華中附近地區建立，而200mb反氣旋中心則位在100°E、30°N以西處。
並列摘要
The transition period from the winter of northeast monsoon to the summer of southwest monsoon is the Mei-Yu season over East Asia. About 1/4 of the annual rainfall for Taiwan's area is falling during this period. Therefore, the study of the large scale circulation, which could affect the Mei-Yu rainfall is the main purpose of this research. We will emphasis on the study of the evolution of the 200mb upper level jet streak (ULJ). Through the analysis of 200mb wind field, streamfunction, 700mb wind field, 500mb vertical velocity, 850mb moisture field and the 15 days' average of 200-500mb temperature as well as 200mb streamfunction, and the thermodynamic energy budgets during the ten days' period of on set and break of Mei-Yu seasons, by using the ECMWF/WMO analysis data sets for the May and June period of the years of 1979, 1981 (Mei-Yu active years) and 1985, 1987 (Mei-Yu inactive years).The general features of the ULJ distribution during the analyzed periods may be summarized as the following:(1) Before the on set, on the eastside of 100°E the ULJ is westerly and located over broader area, and the low level jet is also westerly, however, on the westside, the ULJ has two branches around the Tibetan Plateau. And the 200mb anticyclone center is located over south of 20°N.(2a) When 200-500mb temperature field has increased over northwestern China an decreased over Japan area, then we can find the 200mb jet becomes ”V” shaped and has moved northward with its jet axis passed through 120°E, 30°N. At the same time it will induce strong northerly, and southerly winds at the northern and southern side of 30°N respectively, and it will also cause large moisture gradient and unstable condition around 120°E, 30°N. The 200mb anticyclone, has now moved northwestward compared to (1). This is the so called Mei-Yu season over southern China and Taiwan area.(2b) If 200-500mb temperature field has increased over northwestern China but the temperature over Japan has not changed significantly, then the 200mb jet will move to the north of 35°N. At this movement, we can not find strong northerly and southerly winds around East Asia at 700mb, so it could not creat bigger moisture gradient and therefore the rainfall is greatly reduced. (3) When 200-500 mb temperature field has increased all over 35°-40°N, it will cause the 200mb jet shift northward as well as the 700mb jet. and the most unstable area is now located over central China and Japan areas. The 200mb anticyclone center is also moved to the west of 100°E, 30°N. This period corresponds to the break of Taiwan's Mei-Yu season.