| 摘要: | 台灣及大陸東南沿岸的風場、降水、雲量隨著季節的改變都有很明顯的日夜變化存在，而五、六月為台灣的梅雨季節，時常有鋒面通過影響，容易與大尺度環流及海陸風產生交互作用，影響風場與降雨日夜的變化情形，本研究將08-12年梅雨時期分為梅雨前期(5/15-5/31)、梅雨中期(6/1-6/15)、梅雨後期(6/16-6/30)，分別討論隨季節改變的環流與降水變化。這三個時期的海陸以及地形之強迫作用在不同的大尺度環境下(盛行風場、風向)有些差異，且大尺度的環境流場、穩定度、水氣、鋒面位置和綜觀天氣系統在各時段都不盡相同，因此風場、雲量、降水的區域在梅雨期都有不同的影響。梅雨時期鋒面的平均位置從台灣西南部移動到了台灣的西北部，後期則逐漸遠離台灣，使梅雨前期降水發生在台灣東部外海及西南部外海、中期降水主要發生在台灣西南部外海與西北部外海，後期降水則發生在台灣的西南部外海區域。
台灣海峽受到兩岸海陸風的交互作用下有明顯的變化產生，夜間兩岸的陸風作用增強，使台灣海峽區域夜間低層輻合作用增加，相較於白天有更多的機會降雨，而下午容易受到海風、上坡風與地形抬升作用的影響使降水發生在台灣內陸及山區，夜間則容易降水在台灣沿岸。梅雨中期西南氣流增強，台灣常受到地形效應的影響容易在台灣的西北部沿岸有地形急流作用，與盛行風場產生輻合並且引發降水，而西南氣流的增強，有更多的氣流能夠越過山脈，使台灣東部背風側的熱力效應有著明顯的變化，在下午受熱時段中央山脈背風側容易有背風低壓及沉降作用發生。台灣本島的局部海陸風環流則是隨著大尺度環流的轉變有不一樣的變化，前期降雨主要在東北部的迎風面，中期西南氣流增強降水則是在西南部的迎風面上，並且在下午容易有降水極值發生，後期降水則主要以熱對流的作用為主。因此在梅雨時期受到鋒面、地形效應、盛行風場和海陸風交互作用的影響，使梅雨時期台灣與鄰近區域環流與降雨日夜變化更複雜也更顯著。
;During Mei-Yu season of Taiwan (May-June), fronts often pass through Taiwan region and interact with the large-scale circulation and land-sea breezes which would also affect the diurnal cycle of wind field and precipitation. This study divides the Mei-Yu season of 2008-2012 into pre-Mei-Yu (5/15-5/31), mid-Mei-Yu (6/1-6/15), post-Mei-Yu (6/16-6/30) in order to discuss the seasonal changes of the circulation and precipitation. The large-scale circulation, stability, moisture, front position and the synoptic weather systems are different during each period. Therefore, the wind field, cloud amount and precipitation distribution are different during each period of the Mei-Yu season. The land-sea breeze circulation and the forcing effects of the topography during the three periods also exhibit differences under different large-scale circulation (e.g. prevailing wind speed and direction are different from mid-May to June). From the pre-Mei-Yu season to the mid-Mei-Yu season, the average position of the fronts moves from southwest of Taiwan to northwest of Taiwan. During the post- Mei-Yu period, the Mei-Yu front moves away from Taiwan to southeastern China. Thus, during the pre-Mei-Yu season, the precipitation associated with fronts occurs over ocean southwest of Taiwan. The precipitation primarily occurs over ocean northwest and southwest of Taiwan during the mid-Mei-Yu season, and over ocean southwest of Taiwan during the post-Mei-Yu season.
Under the seasonal change condition, the wind field, precipitation, cloud amount of Taiwan and southeastern China have significant diurnal change. Taiwan Strait area is under the effect of the land-sea breezes on both Taiwan and southeastern China sides. At night, the land breezes from both sides are significant so the low-level convergence is evident, causing a higher chance of precipitation within the Taiwan Strait than during daytime. In the afternoon, due to the impacts of sea breezes-upslope flow and orographic lifting, the rainfall occurs inland or in the mountain areas, whereas, occurs at shore at night over Taiwan. During the mid-Mei-Yu season, the southwesterly flow strengthens. The orographically induced barrier jet usually occurs on the northwest coast of Taiwan, and it would converge with the prevailing winds producing precipitation in some cases. If the southwesterly flow strengthens, more prevailing airflow can pass across over the mountains, causing significant thermal effects at lee-side of the east of Taiwan. In the afternoon, prevailing flows over southwestern Taiwan is prone to passing through the Central Mountain Range (CMR) when the terrain is heated. Thus, lee-side low and subsidence effects are more significant. The rainfall distribution over Taiwan will be different with the transformation of large-scale circulation. The rainfall occurs mainly at the windward side of northeastern Taiwan during the pre-Mei-Yu season under northeasterly flow at lowest level. During the mid-Mei-Yu season, the rainfall mainly occurs at the windward of southwestern Taiwan in the afternoon under low-level southwesterly flow. During the post-Mei-Yu season, the rainfall is mainly caused by thermal convection. To sum up, during Mei-Yu season, the interaction among fronts, orographic effect, prevailing winds and land-sea breezes are complicate which also affect the diurnal cycle of circulation and precipitation over Taiwan and the surrounding area. |
