| 摘要: | 台灣地區冷月（九月?四月）主要是受到東北季風的影響；暖月（五月?八月）則主要是受到西南季風的影響（Chen and Chen, 2003；Chen T.-C. et al., 2004）。其中5月中旬到6月中旬的梅雨季，和夏及早秋之季，分別為台灣主要雨季發生之時期（Chen and Chen, 2003）。 利用EC資料及中央氣象局氣象資料來探討1997年?2004年五、六月梅雨季期間，同時去除熱帶擾動之影響。在這八年之中，再以日雨量（在台灣西南部）大於130 mm的雨量站至少有五個並且日雨量大於200 mm的雨量站至少有一個的條件下，挑選出13個個案。本研究選取2003年6月7日，其台灣西南部豪雨平均降雨率百分比為87.7％，而西南部日雨量在130 mm?200 mm的平均降雨率為30.2％，西南部日雨量在200 mm?350 mm 的平均降雨率為55.7％，地形水氣通量值（ ，其中 是指水平風向量、▽h是指山坡斜率、q是指地面水氣混和比）為16.8（m/s.g/kg），其值皆非常大，所以選取此個案來探討台灣西南部的豪雨特性。在此個案中最大降水觀測站為瑪家站（22.685°N、120.679°E）(379.5mm/ day)，最大降雨時間在0800LST產生71.5mm的大雨，而第二大降水量的尾寮山站（22.819°N、120.670°E）(353mm /day)。 分析此個案之綜觀天氣，可知在6日晚上（2000LST），因高層300hPa有短槽出現在福建、廣東（24°N、113°E），所以加深500hPa上短槽（24°N、112°E）的發展，使得850hPa在華南的低壓強度及低層噴流（LLJ）增強，相當位溫及相對濕度順勢增強，增加對流不穩定。在7日清晨（0200LST），300hPa 噴流中心在東海上面，台灣位在噴流中心入區右方，上升區在台灣海峽南部、南海北部，有利對流發展，850hPa上LLJ以涵蓋台灣南部，此時鋒面在台灣北部（約25°N）附近，LLJ軸與鋒面之間，有很明顯的上升運動，高θe及潮濕空氣也到台灣海峽南部。另外，由EC地面圖顯示，有一地面低壓中心（1002 hPa）在中國東南海岸附近（23.5°N、117°E），但此低壓未出現在JMA地面天氣圖上。在7日上午（0800LST）高層300hPa噴流中心在日本，噴流入區在華南、台灣地區，上升區從南海北部涵蓋整個台灣海峽一直到台灣東部，850hPa的低壓環流涵蓋到台灣東方海面，此時鋒面在台灣北部（約24°N）附近，LLJ軸與鋒面之間，有很明顯的上升運動，θe大於350 K及RH大於90％在台灣西南部，加上LLJ的影響下，台灣西南部降雨一定相當可觀。另外，由EC地面圖顯示，地面鋒面位置在苗栗、台中之間，且地面低壓中心移到台灣及台灣海峽，與JMA地面天氣圖位置一致，台灣西南部則受鋒面前的西南氣流和低壓的影響。7日下午（1400LST）時，從EC地面圖可看出鋒面位置在23°N附近，過了六小時（7日晚上2000LST），鋒面向南移至22.5°N。 從衛星雲圖上得知，在5日1300LST時，有雲系在28°N、105°E生成，與EC資料850hPa上5日1200LST低壓中心出現位置相符，此雲系隨著時間伴隨850hPa低壓向東南移動，在6日0200LST時，雲系向東南移動到24°N、110°E，也就是850hPa低壓東移增強的位置。且此雲系持續隨著850hPa低壓前緣向東南移動在07日0200LST進入台灣西南部繼續發展。此時850hPa上LLJ已涵蓋台灣南部，高θe及濕空氣已籠罩台灣，台灣南部也在上升區中，有利於對流發展。當降水系統隨著低壓從中國東南海岸地區經台灣海峽到台灣西南部，即06日1700LST到07日0800LST時雲系移進台灣西南部受地形抬升，對流系統增強造成降水。然後雲系於0800LST後繼續向東移動。而由雷達回坡顯示降水系統在中國東南沿海逐漸向東移動進入台灣海峽南部，當回波移到台灣西南沿海時，強度增強，接著向東南移動至高雄、屏東地區最後移出台灣。 為了想更進一步探討此中國東南沿海低層低壓之產生及其與豪雨之關係，以天氣預報模式（Weather Research and Forecasting Model, WRF） ，來研究此低壓與豪雨之關係。 模擬結果顯示，受到高層中緯度槽及西風之影響，在850hPa青康藏高原東側生成低壓，而此低壓在廣東福建增強，再受到300hPa及500hPa華南短槽的影響，加深在廣東福建的低壓。同時在此低壓東南緣（中國東南沿海）在700hPa及850hPa及地面產生新的低壓。700hPa低壓中心雲水產生之潛熱有利地面低壓之加強。07日清晨在台灣海峽300hPa、500hPa及700hPa的短槽增強以及700hPa潛熱加強低層低壓之發展。降水系統隨著新的低壓環流移到台灣西南部，因此在沿海地區有豪雨及大豪雨產生。當降水系統移到斜坡，受到LLJ及地形影響產生超大豪雨。 在模擬降雨結果分析上，台灣西南部降雨分佈位置與觀測大致相同，最大降雨位置也與觀測一致，最大降雨測站之日雨量為382.68mm，達到超大豪雨標準，實際觀測為379.5 mm，最大時降雨量為45.7 mm，實際觀測資料為71.5 mm，只有觀測資料的63.9％，最大降雨時間與觀測大致相同。 Precipitation in Taiwan is influenced by the north-easterly monsoon (September-April) during the cold season and the southwesterly monsoon (May-August) during the warm season. Mei-yu season (mid-May to mid-June) and late summer (mid-July to August) are two major rainy seasons over Taiwan. During Mei-Yu season, heavy rainfall evens are freguently observed in southwestern Taiwan. In this study, we investigate the characteristics of heavy rainfall in southwestern Taiwan from 1997 to 2004. The heavy rainfall events over southwestern Taiwan is defined as days in which more than 5 stations record hourly rainfall rate that exceeds 15 mm at least once and daily rainfall accumulation is greater than 130 mm. Besides, one or more rainfall stations record daily rainfall accumulation exceeding 200 mm. Based on rainfall data from about 360 Automatic and Mwtworological Telemetry System (ARMTS) around Taiwan, 13 cases are identified. The current study focuses on the heay rainfall event on 7 June 2003. Because this heavy rainfall event bears 88% heavy rainfall frequency over southwestern Taiwan. The frequency of daily rainfall amount between 130 and 200 mm over southwestern Taiwan is around 30.2%. The frequency of daily rainfall amount between 200 and 350 mm over southwestern Taiwan is around 55.7%. In addition, the orographic moisture flux over southwestern Taiwan is 16.8（m/s.g/kg）. This value is very large. In addition, ten minute time interval radar images over southwestern Taiwan are available. The synoptic analysis derived from European Center for Medium-Range Weather Forecasts (EC) data indicates that a short trough at 300-hPa located at the area near Guangdong and Fujian (24°N、113°E) at the 300-hPa level at 2000LST on 6 June. This short trough enhanced the development of short wave trough at the 500-hPa level. Meanwhile, upper level short wave trough also strengthened the intensity of the low pressure at 850-hpa level and Low Level Jet (LLJ) as well. Due to the southwest flow, the equivalent potential temperature ( ) and relative humidity (RH) increased. Conseguently convective instability was enchanced. At 0200LST on 7 June, the upper level jet at 300-hPa was located over the East China Sea. The position of Taiwan was near the right side of the jet enfrance region. The ascenting motion was over southern Taiwan Strait and northern South Chine Sea. It would be beneficial to the development of convection. The LLJ at 850-hPa level was over all the south part of Taiwan. A significant upward motion was between the axis of LLJ and the Mei-Yu front which was located at north part of Taiwan. At this time, the high and moist air were observed in southern Taiwan Strait. Besides, the surface low pressure center (1002 hPa) obtained from the EC data was near the coast of south east China (23.5°N、117°E). In the morning (0800 LST) on 7 June, the jet core at 300 hPa was located at Japan. The ascenting area was from the Taiwan Strait to east of Taiwan. The circulation of the low pressure at 850-hPa was over Taiwan. At this time, the position of the Mei-Yu front was near the north of Taiwan (about 24°N) and the surface low (1002 hPa) was in central Taiwan Strait near western coast of Taiwan. Significant upward motion existed between the axis of LLJ and the Mei-Yu front. The was excending 350 K and RH was greater than 90% in southwestern Taiwan. The ascenting motion from low to upper level helps to produce heavy rainfall. Satellite images at 1300LST on 5 June show that cloud systems developed in the southwestern China (28°N、105°E), coincided with the low pressure at the 850 hPa level. At 0200LST on 6 June, the cloud system move southeastward to the area near 24°N、110°E where the low center at the 850-hPa level strengthened. Furthermore, the cloud system moved southeastward with the low center of the 850-hPa leve to the southwestern Taiwan at 0200LST on 7 June. At this time, the LLJ, the high and moist air arrived Taiwan. The upward area was also over southern Taiwan. At 0800LST on 7 June, the cloud system to the south of the low pressure in central Taiwan Strait moved to the southwest of Taiwan. From radar echo images, the precipitation system associated with the cloud system and the low pressure moved from southern Taiwan Straits eastwards to the southwest of Taiwan. The radar echo intensity was strengthened over sloped areas. In order to investigate the relationship between the low level pressure over southeast China coast and the heavy rainfall over southwestern Taiwan, the present study employs the Weather Research and Forecasting Model (WRF) to examine the formation of the low pressure and the formation of heavy rainfall southwest Taiwan. In the simulation, the middle latitudes trough at upper level and the westerly wind caused the production of the low pressure over east of the Tibeta plateau at 850 hPa. Later this low pressure was strengthened in Fujian and Guangdong. In the early morning the short wave trough at 300 and 500hPa occurred over southern China. These short waves produced a new low pressure at the 700-hPa level, 850-hPa level, and the surface over the southeast side of the existing low pressure at the southeastern China coast. The latent heat from the cloud systems associated with the low center at 700-hPa level amplied the low pressure near surface. Precipitation system associated with the new low pressure moved to the southwest of Taiwan and produced heavy rain over the coastal area. As the precipitation system moved to the sloped areas, the intensity of heavy rainfall incressed due to LLJ and topographic effects. The simulated rainfall pattern over the southwestern Taiwan was similar to that observed. The position and the magnitude of the simulated daily rainfall accumulation (382.68 mm, exceeding the definition of super heavy rainfall 350 mm/day) were also similar to that observed where the maximum daily rainfall accumulation (379.5 mm) was at MA-CHIA （22.685°N、120.679°E）. But, the simulated maximum hourly rainfall rate was 45.7 mm, only 64% of the observed value. The peak value of the simulated rainfall was one hour late than the observed value (at 0800LST on 7 June). |
