| 摘要: | 在2005-2010年夏季(7、8月)期間未受綜觀擾動影響下，台灣西南部地區(≦23.5°N，中央山脈以西)共出現130次大雨日(≧50 mm/day)，其中有5天(≧130 mm/day)的降雨系統是從西部山區生成，移到西部沿海及台灣海峽且維持10小時以上之生命期。由於雷達資料的限制，因此本研究僅針對2006年8月18日、2008年7月9日和2008年8月6日探討此類型對流系統產生機制、維持長生命期之條件及其移動機制。 利用National Centers for Environmental Prediction Global Final Analyses (NCEP FNL)和中央氣象局資料分析以上三個個案的對流系統發展概況：(1)個案一(2008年7月9日)台灣的綜觀環境為盛行西南風，濕福祿數約為0.28。午後在西部斜坡形成的對流系統朝台灣海峽發展(Flow Regime I, Chu and Lin 2000)。隔日02 LST顯示受到台灣海峽中層大氣的下沉運動影響，因此對流系統在西南部沿海只持續到隔日清晨(~06 LST)後消散；(2)個案二(2006年8月18日)綜觀天氣顯示受Wukong颱風和低壓影響，台灣海峽中北部(南部)盛行西北風(西風)，濕福祿數約為0.16。對流系統在上午從西部平地形成，隨後往山區發展，並在傍晚時從山區移至台灣海峽(Flow Regime I)。當對流系統移到海上後，由於台灣海峽南部低層有西北風和西風產生的中尺度輻合、上升運動區以及相當潮濕的環境，可使對流系統在海上到隔日早上(~09 LST)仍繼續發展；(3)個案三(2008年8月6日)綜觀天氣資料顯示台灣受到季風槽影響盛行東、東南風，濕福祿數約為0.09，而下午和晚上在台灣西部和台灣海峽有低層輻合。午後對流系統在西部山區形成，隨盛行風往台灣海峽移動。當對流系統移到海上後，由於台灣海峽中部低層有東南風和東風產生的中尺度輻合、上升運動區及較低的自由對流高度(~925 hPa)，可支持對流系統到隔日清晨(~07 LST)仍持續發展。 利用水平解析度27、9和3公里的WRF模式模擬以上三個案之降雨特性，可歸納出以下結論：(1)對流系統的產生機制：從個案一和個案三模擬結果顯示在早上地表加熱使向岸風環流發展，而在近地層水氣充分混合並配合上坡風，使水氣往山區輸送。持續的上坡風在山區產生上升運動，有利低層氣塊舉升到自由對流高度使對流系統開始發展。個案二早上由於盛行風受地形效應減速在西部平地產生輻合，因此對流系統在平地發展；(2)對流系統維持長生命期的機制：(2.a)降雨產生的冷空氣與盛行風之輻合：由個案一和個案二的模擬結果顯示在濕福祿數較低時，降雨所產生的近地層冷空氣會往台灣海峽移動，其冷空氣移速約與對流系統移速相當，再配合冷空氣和向岸風之輻合可加強系統前緣對流胞發展，使整體對流系統從山區往台灣海峽發展(Flow Regime I)且維持長生命期。個案二早上的對流系統由於近地層沒有冷空氣產生，因此對流系統隨盛行風往山區移動。另外藉由不考慮雨水蒸發的敏感度測試，個案一和個案二的模擬結果顯示對流系統在山區產生後，由於沒有近地層冷空氣產生，因此不會在對流系統前緣激發出新對流胞，故無長生命期的對流系統往台灣海峽發展。(2.b)背風輻合：由個案三模擬結果顯示盛行風繞過中央山脈南北端在台灣西部及台灣海峽產生輻合，以維持受盛行風西移之對流系統的發展。(2.c)台灣海峽中南部的中尺度輻合：由個案二、三模擬結果顯示當對流系統移出陸地後，受到台灣海峽中南部低層中尺度輻合及上升運動，可維持在海上之發展，有利對流系統維持其長生命期。 During the summer season (July-August) from 2005 to 2010, there are 130 heavy rainfall (≧50 mm day-1) events over southwestern Taiwan (≦23.5°N, and west of the center mountain range) under weak synoptic-scale forcing. Among there 130 events, there are five days with accumulated daily rainfall greater than 130 mm. The convective systems in these five days form in the west central mountain range (CMR) and then move toward western coast and Taiwan Strait. The life-span of the convective systems persist over 10 hours. Due to the limitation of radar data, we only focus on three cases, namely, 18 August 2006, 9 July 2008 and 6 August 2008. The objective of this study is to investigate the mechanisms of formation, maintenance and movement of these convective systems. The analysis results from the observational data of the National Centers for Environmental Prediction Global Final Analyses (NCEP FNL) and Central Weather Bureau (CWB) found that: (1) Case 1 (9 July 2008): The synoptic environment at 850-hPa level shows that Taiwan was affected by prevailing southwesterly flow with moist Froude number (Fw) around 0.28. The afternoon convective systems formed in the western slope of the CMR and moved towards Taiwan Strait. This simulation results similar to flow regime I (Chu and Lin 2000). In the next day before dawn, the convective systems dissipated in this morning because of sinking motion at middle troposphere in Taiwan Strait. (2) Case 2 (18 August 2006): The synoptic environment at 850-hPa level shows that Taiwan Strait is affected by the Typhoon Wukong and a low system. The prevailing northwesterly (westerly) flows were in the northwest (south) Taiwan Strait with Fw around 0.16. The convective systems developed from the western plain to slope in the daytime, and then moved toward Taiwan Strait in evening (Flow regime I). When the convective systems arrived at Taiwan Strait, the meso-convergence that caused by prevailing northwesterly and westerly flows produced low-level upward motion. In addition, moist troposphere existed over Taiwan Strait which can support the long-lived development of the convective systems over sea in the next morning. (3) Case 3 (6 August 2008): The synoptic environment at 850-hPa level shows that the monsoon trough was to the south of Taiwan. Easterly to southeasterly flows with Fw around 0.09 were over Taiwan. Low-level convergence was in the Taiwan Strait and western Taiwan. The afternoon convective systems form in the western slope, which moved toward the Taiwan Strait. When the convective systems arrived at the Taiwan Strait, same as case 2, the meso-convergence caused by prevailing southeasterly and easterly flows produced ascending motion. Becides, low level of free convective (LFC) was found. These favorable conditions support the development of convection to next morning. In this study, the WRFV3.2.1 model (horizontal resolution of 27 km, 9 km and 3 km) is used to help understand that the detail evolution of convective systems. Model results show that: (1) The generation mechanism of convective systems: The simulated result of three cases show that the development of sea breeze circulation in the early morning and the well-mixing layer of water vapor developed in the near surface near noon. Upslope winds transport water vapor to mountain area. In addition, the upward motion over sloped area is caused by orographic lifting helped the convective systems develop in the western slope of CMR. (2) The mechanism of sustaining long-lived convection: First, the interaction between the cold-air outflow of rainfall and prevailing wind is examined. The result of the case 1 and case 2 depict that the cold-air outflow in the western slope of CMR moved toward Taiwan Strait when the Fw was small. The movement of cold-air was consistent with that of convective systems. When the cold-air outflow and onshore convergenced, the convective systems will strengthen the front of edge through the growth of of storm cells, and then convective systems moved toward Taiwan Strait (Flow regime I) and sustained long-lived. Besides, in the sensitivity test study without evaporative cooling (NEV), the mountain-induce convective systems cannot trigger new cells over the western plain and Taiwan Strait. Therefore, the convective systems will not become long-lived. Secondly, the leeside convergence is investigated. From case 3, the low-level convergence in the western Taiwan and Taiwan Strait was caused by the prevailing southeasterly flow bypassed the CMR. The low-level convergence maintained the long-lived westward-movement convective systems. Thirdly, the meso-convergence in the southern central Taiwan Strait is studied. From case 2 and case 3, the low-level convergence and upward motion developed in middle southern Taiwan Strait due to northely winds and southwesterly monsoon flow. The convective systems were able to maintain long-live when it arrived in the low-level convergence area of Taiwan Strait. |
