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    Please use this identifier to cite or link to this item: http://ir.lib.ncu.edu.tw/handle/987654321/76492


    Title: 垂直風切對通過地形熱帶氣旋路徑及降雨量之影響:理想化實驗
    Authors: 呂健瑜;Lu, Chien-Yu
    Contributors: 大氣科學學系
    Keywords: 垂直風切;路徑偏折;Vertical wind shear;Track deflection
    Date: 2018-07-25
    Issue Date: 2018-08-31 11:24:57 (UTC+8)
    Publisher: 國立中央大學
    Abstract: 前人研究主要討論風切對熱帶氣旋內部動力的影響,但大部分的研究都是建立在無地形的情況下,因此本篇研究希望利用WRF理想化模式探討不同強度的東風風切下熱帶氣旋在通過地形時路徑和降雨量的差異。在研究中進行一系列的敏感度測試,包括將熱帶氣旋初始位置往北移動4個網格點(N4案例)、環境駛流速度由-6 ms-1提高到-8 ms-1(ST8案例)以及將熱帶氣旋最大風速由40 ms-1提高到50 ms-1 (V50案例)。
    在實驗中我們仿照Gu et al. (2015)的研究,利用Hovmöller圖來分析垂直速度在不同象限隨時間的變化,結果發現在無地形情況下,熱帶氣旋上、下風切分別由上升及下降運動主導,而最大上升運動會先出現在下風切左側,隨後會逆時針旋轉到上風切左側,取代原先在上風切左側的下降運動;而有地形時的垂直速度在前12小時變化和無地形時相似,而當熱帶氣旋登陸時因為受到地形抬升作用,所有象限幾乎都是上升運動,但在下風切右側的上升運動會因為風切增加減弱。
    氣旋路徑在無風切情況下幾乎為一直線,但加入風切後氣旋路徑在地轉平衡調整完成時將會開始往南偏折,而偏折的程度和風切大小成正比;此外,熱帶氣旋的移動速度同樣也會和風切大小成正比。而在有地形的情況下,氣旋靠近地形時其路徑會往北偏折,對於ctl、N4、ST8案例來說,當風切增大時此北偏似乎會被抑制,但在V50案例中沒有此現象。由於熱帶氣旋會朝向位渦趨勢波數一最大值方向移動,因此在論文中將位渦趨勢分項來看,了解颱風前進及偏折主要是受到哪項的影響。對於ctl_sh-0案例而言,在氣旋登陸前3小時的北偏作用主要是由水平平流項和非絕熱加熱項作用所造成,而垂直平流項則會做負貢獻使氣旋南偏;當風切增加時,水平平流項的北向貢獻會減少,造成“當風切增大時北偏的偏折程度會降低”的現象發生,但在V50案例中,水平平流項的北向貢獻不像ctl案例會因風切增加而減少。。
    降雨量方面,在沒有地形的情況下,當風切到達-8 ms-1或更高時,降雨會集中在路徑左側,此現象符合前人的研究結果。而在有地形情況下風切大小為0、-4 ms-1個案的降雨主要集中在地形東北方以及西南方;當風切增加到-8 ms-1時,降雨主要集中在地形西南側,而在地形東北側仍有些微降雨;但當風切增大到-12 ms-1以上時,地形東北側幾乎沒有降雨,而原先出現在西南山區的降雨移動到西南沿海。地形東北側降雨因風切增加而減少的原因可能有兩個,第一是低層風速波數一不對稱,第二則是因為中高層的沉降運動將地形抬升作用抑制在低層。而N4、ST8、V50案例的降雨分布皆和控制組類似,當風切較小時氣旋降雨主要集中在地形東北方以及西南方;但當風切增大後,地形東北方降雨減少而西南方降雨由山區往沿海方向移動。各個敏感度實驗與控制組在降雨量方面的差異主要在於總累積降雨量的多寡,N4案例由於氣旋登陸位置較為北側,所以地形東北方的降雨量減少而地形西南方的降雨量增加;ST8案例中,氣旋受到強烈環境駛流場影響導致氣旋快速通過地形,使得地形東北方及西南方總累積降雨量相對較少;而V50案例則是因為氣旋強度較ctl案例強因此地形東北方及西南方總累積降雨量較多。
    ;Previous studies have focused mainly on the interactions between tropical cyclone (TC) dynamics and vertical wind shear (VWS), but most of studies simulated the ocean-only condition. Thus, this paper tried to use an idealized WRF model to discuss the evolution of track and rainfall when the tropical cyclone-like vortices passed through the mountain under different VWS. To understand the potential factors in affecting track and rainfall, a series of sensitivity experiments are conducted, including the change of the vortex initial position, the environment steering flow and the vortex strength.
    The vortex track seems to exhibit northward deflection when the vortex is close to terrain, and the upstream northward deflection would increase with the increasing VWS. The results of wavenumber-1 potential vorticity (PV) budget analysis indicate that the advection term dominates the translation direction of vortex, while the vertical advection term and diabatic heating term would drive the cyclone southward and northward, respectively. The rainfall occurs on the northeastern and southwestern sectors of the terrain for VWS < -4 m s-1. If the VWS is less than -10 m s-1, the rainfall mainly concentrates on the southwestern terrain, with little rainfall on the northeastern terrain. However, the rainfalls on the northeastern terrain almost disappear, and the rainfalls on the southwestern terrain appear near the coastal region instead of the mountain for VWS > -10 m s-1. The Hovmöller diagram shows that positive vertical velocity occurs on all the quadrants when the vortex makes landfall, but the uplifted-motion decreases as the VWS increases. This may be the reason for decreasing rainfall on the northeastern terrain under strong VWS.
    Appears in Collections:[大氣物理研究所 ] 博碩士論文

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