颱風強度與路徑變化的模擬研究:渦旋初始化與地形作用之影響

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題名:	颱風強度與路徑變化的模擬研究:渦旋初始化與地形作用之影響
作者:	林柏辰;Lin, Po-Chen
貢獻者:	大氣科學學系
關鍵詞:	氣旋式偏轉;動力渦旋初始化;cyclonic deflection;dynamic vortex initialization
日期:	2023-07-21
上傳時間:	2023-10-04 14:35:48 (UTC+8)
出版者:	國立中央大學
摘要:	本研究使用WRF理想化模式探討兩個部分，第一部分模擬西行熱帶氣旋在逼近一中尺度理想(模擬台灣)地形時之路徑演變，其中包含不同初始位置以及不同角度地形；第二部分研究動力渦旋初始化對颱風路徑跟強度預報的影響。第一部分設定氣旋在十個不同出發位置以及四種不同地形角度進行模擬，可以涵蓋大部分颱風侵襲台灣的範圍。路徑產生偏轉主要受緯向初始位置、氣旋大小與有效地形長度(即長寬比或渦旋無因次參數)的比值所控制。在氣旋和地形尚有一段距離時，氣旋外圍環流受地形影響造成氣旋駛流出現變化使氣旋略微向南偏轉(與無地形實驗相比)。當氣旋足夠靠近地形時，大部分實驗在地形前方會出現向北偏轉，通過地形中心後向南偏轉的逆時針路徑，稱為氣旋式偏轉路徑。藉由位渦收支趨勢分析得知，主要由水平平流項主導，改變方向引導氣旋移動，垂直平流項與非絕熱加熱項只有在氣旋中心已經登陸的時間點才有明顯作用。從多數實驗來看，氣旋外圍環流被地形分割，當環流繞過地形，重新繞回氣旋本身環流時，輻合區對流增強，氣旋之水平環流變的不對稱，垂直剖面也顯示出內核東側垂直運動和風速對比西側有顯著差距，氣旋的不對稱結構對水平平流的變化緊密相關。位渦收支不對稱量分析發現在氣旋接近地形過程中，將有無地形的波數一不對稱(駛流)風場相減，會出現一對環流(gyre)，該環流提供氣旋移動分量並隨時間跟氣旋中心逆時針移動，使路徑先向北，再向西，最後向南移動，產生氣旋式偏轉。第二部份針對渦旋動力初始化(dynamic vortex initialization, 以下稱DVI)進行實驗探討，將已調整的渦旋植入單純駛流場預報當作自然實驗(模擬真實大氣颱風最佳路徑，以下稱為NR)，用平均法弱化已調整渦旋，當成控制試驗(模擬較觀測弱的颱風初始場，以下稱為CTL)，再以DVI方法強化渦旋最低氣壓或最大風速達初始場數值為止。結果顯示大部分實驗路徑差異不大，強度部分，CTL則逐漸增強，風速在第36小時、氣壓在第60小時趕上其他實驗；經DVI後的實驗強度會先維持或稍稍增強，之後也會慢慢衰退。對實驗的氣旋軸對稱平均剖面進行不同時段分析，主要差異體現在氣旋內核區。在初始時間，經DVI後的渦旋徑向風、切向風和垂直速度比CTL更強。最後以WRF區域模式模擬真實個案颱風莫蘭蒂探討渦旋初始化的影響，最大風速達標之路徑預報有良好改善。實驗結果與理想化結果類似，DVI修正初始颱風結構，可以得到長達60小時的強度預報。;This research includes two parts using an idealized WRF model. The first part simulates the track evolution of a tropical cyclone moving westward at different departure positions toward an elongated mesoscale mountain (mimicking Taiwan) at different orientation angles. The second part examines the impact of dynamic vortex initialization on typhoon track and intensity forecasts. The first part involves simulations of cyclones departing from different initial positions toward the terrain at different angles to cover a range of typhoon impacts on Taiwan. The track deflection is primarily controlled by the meridional departure position as well as the ratio of the vortex size and effective terrain length (the aspect ratio, or the nondimensional vortex size). When the cyclone is still somewhat distant away from the terrain, the outer circulation of the cyclone is influenced by the terrain, causing a slightly southward deviation in the track (compared to the experiments without the terrain). When the cyclone approaches the terrain, a “cyclonic deflection” of track is observed where the track deviates northward upstream of the terrain and then southward after passing through the center of the terrain. The wavenumber-1 vortex flow and potential vorticity (PV) budget analyses have helped explain the track deflection that is dominated by horizontal PV advection in comparison to both vertical PV advection and diabatic heating that somewhat modulate the tracks in the vicinity of the mountain. The circulation has already bypassed the topography and recirculated back to the cyclone. The horizontal circulation of the cyclone′s center becomes asymmetric. The vertical profile also reveals significant differences between the eastern and western sides, with stronger vertical motions and wind speeds on the eastern side. The adjustment of the cyclone′s asymmetric structure is related to the horizontal PV advection. Subtracting the wavenumber-1 asymmetric wind fields with and without topography results in a pair of gyres. The gyres move counterclockwise with time and the cyclone center, providing the components of cyclone′s direction of its movement. Consequently, the cyclone moves northward initially, then westward, and southward finally, thus presenting a cyclonic deflection. The second part will focus on the dynamical vortex initialization (DVI). A modified vortex is inserted into a basic uniform flow as a nature run (representing the best track of a real typhoon, denoted as NR). The vortex after DVI is weakened to produce a control run (representing the initial field for forecasting, denoted as CTL). The CTL vortex then can be strengthened using the DVI method to match the minimum sea-level pressure or maximum wind speed of the NR vortex. The results show that most of the tracks for the experiments are similar. CTL gradually intensifies, with the wind speed catching up with other experiments at 36 simulation hours and the pressure catching up at 60 simulation hours. The experiments using the DVI can well maintain or slightly enhance the vortex intensity at early forecast times. The vortex in all the DVI experiments, however, becomes weakened after 24 hours. The azimuthal-mean transverse circulation across the typhoon center for the DVI experiments reveals significant differences are primarily produced in the inner core region. The impact of DVI is also investigated for the real case of Typhoon Meranti (2016) using the WRF model. The track forecast can be significantly improved when the DVI is conducted with the matching of maximum wind speed. Similar to the idealized experiments, the DVI for this real case can effectively intensify the initial typhoon vortex and thus results in better intensity forecasts up to 60 hours.
顯示於類別:	[大氣物理研究所 ] 博碩士論文

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