||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.
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