臺灣梅雨季初期南亞高層反氣旋之演化研究

南亞初夏期間，大氣之大尺度環流有兩次急劇改變，分別為東南亞夏季季風及印度夏季季風的開始。前者正值台灣地區的梅雨季初期，此時大氣流場的特徵在高層有南亞反氣旋的迅速北移，在低層則有西南氣流的急劇增強現象。本文使用ECMWF之網格資料，分析1981年至1986年間之6年台灣地區梅雨初期高、低層流場的演化特徵。結果顯示，200 hPa之高層南亞反氣旋環流向北伸展的範圍，可影響台灣地區；而850 hPa之低層西南氣流隨西太平洋副高脊環流向東移，則自中南半島穿越台灣地區向東北延伸至日本南部。因此高層環流帶來西北方之較乾冷空氣，低層環流帶來西南方之較暖濕空氣，此高、低層的配置，可於台灣地區建立或維持大範圍的對流不穩定度。高層南亞反氣旋東側的流場與北方西風槽脊系統相互配合，則可形成高層分流的現象，加強中低層的對流運動；此兩者皆有利於降雨的發生。由1984及1986兩年梅雨季前非絕熱效應對高層南亞反氣旋影響之數值模擬得知，對稱於赤道上的潛熱加熱效應，可使赤道地區高層南風向北平流負行星渦度的效應加強，以維持赤道北側之反氣旋環流。中南半島的加熱效應，可直接增強高層輻散場而使反氣旋迅速增強。而青康藏高原的可感熱效應，則有利於反氣旋的北移，且因西風的平流效應而維持反氣旋東北側的環流。高層南亞反氣旋具有類似Rossby wave之向西移行特性，上述之中南半島的潛熱效應為加速高層南亞反氣旋西移最主要的因素。在不同的背景流場環境裡，高層南亞反氣旋對加熱效應有不同的反應，若背景場屬於大範圍帶狀分佈的反氣旋式流場，南亞反氣旋受環境牽制而較不易單獨發展或迅速減弱；若背景場屬於氣旋式流場，則較易發展並加速西移。

關鍵字

南亞高層反氣旋；梅雨；太平洋副高；數值模擬；非絕熱加熱；青康藏高原；可感熱

並列摘要

In early summer of the South Asia, there are two major changes of large scale circulations: first is the beginning of the southeast Asia monsoon, which is also the brginning of the Taiwan Mei-Yu season; and the second one is the onset of the India monsoon. The onset of the southeast Asia monsoon is often accompanied by a rapid northward movement of the upper - level South Asia anticyclone (ULSAA) and an intensification of the low - level southwestly flow.With the use of the ECMWF grid points data from 1981 to 1986, we have analyzed the evolutions of the upper and lower levels streamfunctions during the onset period of Taiwan Mei - Yu season. The result shows that the circulative domain of the ULSAA at 200 hPa can reach to Taiwan during its northward extension. At the same time the southwesterly flow from Indo - China, which is passing through Taiwan to Japan at 850 hPa is moving eastward along with the retreat of the Pacific subtropical high. Therefore, the upper level circulation bring the dry and cold air from northwest and the low level flow carries the wet and warm air from southwest thoese two air masses merge together around Taiwan area which will provide a favor conidition for large scale convection. From the numerical simulations of the diabatic forcings to the ULSAA during the onset period of the Mei - Yu season in 1984 and 1986 cases. We can find that: 1) the symmetric tropical convective heating could maintain the upper level anticyclone circulation on the tropics, 2) the convective heating over Indo - China can intensify the upper level anticyclone circulation, and 3) the Tibetan sensible heating could cause the northward movement of the upper level anticyclone.We also find that the reactions of the ULSAA to the diabatic heating in different background flows will be different. If the large scale background flow is in anticyclone circulation then the ULSAA can be well maintained and intensifed. However, if the background flow is in cyclone circulation then the ULSAA will be rapidly developed and moved toward west.

並列關鍵字

uppet-level South Asia anticyclone ； Mei-Yu ； Pacific subtropical high ； numerical simulation ； diabatic heating ； Tibetan sensible heating

國際替代計量

臺灣梅雨季初期南亞高層反氣旋之演化研究

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