理想颮線受真實台灣地形影響之數值模擬研究

Abstract

2019年4月19日，一發展至典型弓狀回波 結構之颮線型中尺度對流系統由西向東侵襲澎湖，隨後抵達台灣本島；此颮線系統 造成台灣西南部多 處強陣風及短時強降雨，更有多處帳棚及鷹架倒塌，危害民眾生命財產安全。原本南北對稱之颮線系統接觸到台灣地形後，在地形迎風側形成南北不對稱性，並且於颮線南端有較強雷達回波。我們推論此颮線南北不對稱性是由於台灣地形北段雪山山脈山脊走向(東北-西南)與南段中央山脈山脊走向(正北-正南)不同所造成；而於地形背風側所產生之颮線不對稱性則來自於北段雪山山脈與南段中央山脈的長寬比(aspect ratio)不同。 為了驗證上述 假說，我們透過 WRF(Weather Research and Forecast)模式進行理想數值模擬 探討颮線系統結構受台灣地形之影響。模擬實驗結果顯示實際觀測個案與我們的理想模擬 結果有許多相似之處。我們並計算低層冷池內的福祿數(Froude number)來當作 颮線是否能通過台灣山脈之標準，發現-2 K的位溫擾動 等值線完全受到台灣 500公尺之等高 線所阻礙。北段雪山山脈之東北-西南走向與南段中央山脈之正北-正南走向是造成整體颮線結構南北不對稱之關鍵，而冷池之厚度與強度也是決定整體颮線強度之關鍵要素。另外，我們發現颮線於地形背風側不對稱性之形成 來自於水躍現象(hydraulic jump)於雪山山脈及南段中央山脈之發生位置與強度不同所致。 我們進行地形高度敏感度實驗以更了解台灣地形對於颮線扮演的角色，另外我們對於低層風切強度也做了一系列之敏感度實驗，嘗試了解不同風切強度對於颮線發展之影響為何，並透過RKW理論討論 颮線之結構演變如何受到地形影響 。

On 19 April 2019, a squall-line mesoscale convective system (MCS) with the characteristics of a leading convective line and trailing stratiform precipitation made landfall over Penghu and then Taiwan later, resulting in strong wind and heavy rainfall. The squall-line structure became asymmetric when it encountered with Taiwan topography with the southern (northern) part showing stronger (weaker) radar echoes. We hypothesized that the asymmetry might result from the impacts of Taiwan terrain on the squall-line MCS; specifically, it may be due to different orientations of the Central Mountain Ridge (CMR) and the Snow Mountain Ridge (SMR) on Taiwan. Additionally, the asymmetry on the lee side may result from the difference of aspect ratio between the northern SMR and the southern CMR. To verify the above hypothesis, a set of idealized numerical simulations using the Weather Research and Forecasting (WRF) model were performed to examine the impacts of realistic Taiwan topography on a squall-line MCS. Model results show many similarities between the idealized simulations and real-case observations for the squall-line system evolution. The low-level Froude number within the cold pool was estimated to determine whether the squall-line MCS was able to climb over the CMR or SMR. A cold pool with the magnitude of –2K was found to be completely blocked by the terrain with heights of 500 m or above. The north-south orientation of the CMR and northeast-southwest orientation of the SMR constrained the squall-line MCS to develop differently, causing it to become asymmetric and to have stronger radar echoes at the southern end of the system. The depth and intensity of the cold pool were also key factors in determining the squall-line structural evolution. On the other hand, the asymmetry on the lee side might result from the location and the intensity of the leeside hydraulic jump. Terrain height experiments were conducted to better understand the role of the Taiwan terrain height on the squall line development. Wind-shear experiments were performed to investigate how the low-level wind shear affects the storm evolution of the squall-line MCS.