登陸颱風引發之海岸地形噴流及其對降雨和強度影響之觀測研究

An Observational Study of the Coastal Barrier Jet Induced by Landfall Typhoon and its Influence on Precipitation and Intensity Changes

高聿正(Yu-Cheng Kao)

指導教授 ： 周仲島

國立臺灣大學/理學院/大氣科學研究所/博士(2020年)

https://doi.org/10.6342/NTU202001825

摘要

颱風所引發的海岸地形噴流，是造成臺灣地區登陸颱風降雨結構及強度發生顯著改變的重要因子。本研究首度利用臺灣環島岸基都卜勒雷達網及地面觀測資料，針對2005年海棠颱風登陸過程中所引發之地形噴流、颱風降雨結構及強度變化特徵進行詳細分析，同時亦檢視伴隨其他5個西行颱風的海岸地形噴流特徵，並討論其與颱風雨帶之間的關聯性。
地形噴流的形成及其結構的演變是受到海棠颱風登陸過程中不同範圍的環流型態所控制。當海棠外圍環流接觸到陸地時，地形迎風面強烈的東北風因阻擋效應轉而向南，形成平行於海岸地形的低層噴流，地形噴流長約140公里、寬約25公里、持續時間約6小時，北支有較強的風速(最大風速值超過50 m s−1)、核心高度介於1至2.5公里；南支風速較弱(最大風速值約45 m s−1)、核心高度介於1至2公里。當海棠的核心區開始影響陸地時，因顯著氣流曲率的影響，導致地形噴流的南支向離岸方向偏離，地形噴流的偏北風氣流與地形東南側背風低壓所導引轉向的南風氣流在颱風中心西南側輻合，激發東西向的線狀雨帶發展（長約160公里、寬約20公里），隨後線狀雨帶氣旋式移動併入內眼牆，造成雙眼牆結構的崩潰(東側眼牆增強，西側眼牆消失)及強度的減弱，當海棠中心接近至離岸60公里處時，西側眼牆重新發展，隨後眼牆重整內縮，強度增強。
在不同的個案分析中均發現有地形噴流，噴流的位置、強度及持續時間受颱風路徑、強度及大小所控制。颱風路徑偏北(南)，噴流位於地形東岸北側，噴流風速較強(弱)，發展高度較高(低)；噴流風速與迎風面垂直於地形的風速分量成正比；噴流的持續時間則與颱風的環流大小成正比。除了低層地形噴流之外，在部分颱風個案中亦有觀測到多噴流及噴流舉升的訊號，其與颱風主雨帶的通過有密切的關聯性。伴隨強烈對流發展的雨帶，雨帶中層存在中層噴流。當雨帶移入沿岸陸地時，低層因地形阻擋形成低層噴流，因而呈現中低層雙噴流特徵。

關鍵字

沿岸地形噴流 ； 登陸颱風 ； 降雨結構改變 ； 強度改變

並列摘要

The coastal barrier jet (CBJ) induced in typhoon environment is a critical mesoscale phenomenon that causes significant precipitation structure and intensity change of typhoon prior to its landfall in Taiwan area. By using coastal Doppler radar and surface observations in Taiwan, CBJ and precipitation structure and intensity changes of typhoon Haitang (0505) before landfall are studied for the first time. In addition, different types of CBJ associated with the other five selected typhoons are examined and the relation between the CBJ and the principle rainband are especially investigated.
It is found that the formation and evolution of CBJ was controlled by different flow regimes associated with Haitang. When outer circulation touched the coastal terrain, CBJ formed. A persistent prevailing strong northeasterly wind impinging the mountain led to the formation of CBJ parallel to the shore. The CBJ persisted for almost 6 h and was approximately 140 km long and 25 km wide. The northern branch of the CBJ had a stronger wind (maximum wind speed > 50 m s−1), a greater vertical extent (the core of the CBJ was between 1.0 and 2.5 km in height), and a more persistent jet signal than the southern branch (maximum wind speed ~ 45 m s−1 and the core was between 1.0 and 2.0 km). After the core region reached the coastal terrain, considerable curvature of the core region circulation caused the southern branch of CBJ shifted offshore. The northerly CBJ and the southerly of the terrain-induced mesolow (located to the southeast of topography) converged at southwest portion of the storm center resulting an east-west line convection, approximately 160 km in length and 20 km in width, formation. The line convection moved cyclonically and merged into inner eyewall lead to the concentric eyewall breakdown and temporal weakening of the storm. When Haitang moved to approximately 60 km off the coast, the western-side eyewall recovered by nearshore convection, and followed eyewall contraction and intensification.
In addition to Haitang, other five westbound landfall typhoons are selected to investigate the characteristics of CBJ. The property of CBJ is controlled by the track, intensity, and size of the storms. The CBJ associated with north (south) track group locates to the northern (southern) section of east coast, and has a stronger (weaker) wind speed and higher (lower) core altitude. The maximum wind speed of CBJ is proportional to the normal-terrain component of upstream flow, and a positive correction exists between CBJ duration and typhoon size. Multiple jets or uplifted CBJ existed in some landfall typhoon cases. It closely links to the convective activities of the rainbands in the storm. The vigorous convective rainband associates band-paralleling mid-level jet. When it encountered coastal terrain, the low-level jet formed owning to terrain blocking, thus the multiple jets feature was revealed.

並列關鍵字

coastal barrier jet ； landfalling typhoon ； precipitation structure change ； intensity change

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