| 摘要: | 2019年5月17至19日，一道梅雨鋒面滯留於東海南側，並於20日開始向南移動並經過臺灣。該期間臺灣附近綜觀天氣條件有利對流發展，因此發生三起豪雨事件(5月17、18及20日)。本文使用觀測資料、再分析資料及數值模式研究此三日受同一道梅雨鋒面影響下，三起強降雨事件的發生機制以及地形效應的影響。除此之外，低層噴流(low-level jets, LLJs)對此三日的降雨影響甚大，本文亦探討各種低層噴流與對流發展之間的關係。
WRF(Weather Research and Forecasting)模擬結果顯示，5月17日臺灣海峽北部靠臺灣一側的地形急流和靠華南一側的偏西風輻合，中尺度對流系統(mesoscale convective system, MCS)於海上不斷發展，此時臺灣海峽上對流層中低層(~700 hPa)存在綜觀尺度低層噴流(synoptic system-related low-level jet, SLLJ)，MCS受SLLJ影響下向東延伸至西北臺灣造成高強度降雨(~325 mm/7hr)。5月18日盛行風結合上坡風受地形抬升造成白天於中央山脈累積大量降雨(~423 mm/day)。5月20日原先滯留的鋒面開始向南移動，鋒面接近臺灣北部時，地形急流與鋒面低壓槽前氣旋式氣流導致的西北風輻合，產生對流系統並影響台北(~200 mm/4hr)，直到鋒面經過台北後，隨著乾冷偏北風移入，對流系統才開始消散。
本研究亦使用WRF進行地形作用的敏感度測試，移除以及增高地形實驗用來了解臺灣地形於降雨事件中所扮演的角色。在移除臺灣地形的實驗中，5月17及20日由地形阻擋效應產生的地形急流消失，降雨皆明顯減少；18日由於失去地形抬升作用，山區的降雨幾乎消失。地形增高實驗中將臺灣地形增高成1.5倍，結果顯示5月17及20日由於地形阻擋效應更明顯，地形急流強度、寬度及厚度增加，降雨發生的位置及強度因此改變；18日由地形阻擋效應產生於臺灣西部的高壓脊增強，盛行風遇高壓脊於西部沿海輻合抬升並造成降雨。
;Heavy rainfall events during early summer rainy season over Taiwan is an important issue because it usually causes severe disasters. During 17-20 May 2019, the synoptic wearher patterns are favorable for convections developing and a mei-yu front which is stranded in the southern part of the East China Sea moves south and passes through Taiwan. There are three heavy rainfall events with daily accumulative rainfall exceeding 200 mm, which occurred in northwestern Taiwan on 17 May, central mountainous area on 18 May and northern Taiwan on 20 May respectively. This study focuses on the formation and development mechanisms of these three heavy rainfall events and the interaction between low-level jets (LLJs) and convection systems. In addition, the role of terrain effects will also be discussed in this study.
On 17 May, the barrier jet over northwestern Taiwan converges with the westerly wind in front of the mei-yu front over northern Taiwan Strait, as a result, the mesoscale convective system (MCS) develops over the ocean. With the synoptic system-related low-level jet (SLLJ) exists around 700 hPa, the MCS extends to the northwestern area of Taiwan, causing the heavy rainfall event over Taoyuan and Hsinchu coast areas. On 18 May, the prevailing wind combining with upslop wind is lifted by central mountains range (CMR), resulting in heavy rainfall over the mountainous area during daytime. On 20 May, as the front moves closely to Taiwan, the barrier jet converges with the northwesterly wind and convections are present over Taipei and causing considerable precipitation.
Several sensitivity experiments of terrain effect are performed. With the model terrain removed (NT run) and increased (1.5H run), the rainfall distribution and the amount of the accumulated rainfall are evidently distinct. On 17 and 20 May, the barrier jet over northwestern area of Taiwan is disappeared in NT run and stronger in 1.5H run. On 18 May, the orographic lifting effect by CMR is disappeared in NT run and the orographic blocking effect of CMR is obvious in 1.5H run. The accumulative rainfall on 17 May is weaker in NT run and slightly shifts northernly in 1.5H run; it decreases and shifts to eastern Taiwan in NT run and shifts to western coast in 1.5H run on 18 May; rainfall over northern Taiwan is not present in NT run and decreases in 1.5H run on 20 May.
In summary, the barrier jet plays an important role for heavy rainfall events over northern Taiwan on 17 and 20 May cases and the SLLJ influences the movement of the MCS on 17 May case. 18 May case shows the localized diunal circulation is important for the precipitation over mountainous area. During 17-20 May, a boundary layer jet (BLJ) along South China coast over South China Sea is present, playing an important role on moisture transport. In addition, as the upstream wind for Taiwan, the intensity of the barrier jet might depend on it. Consequently, it can be researched in future probably. |
