凡那比颱風(2010)與臺灣地形交互作用─模擬路徑、強度及降雨不確定性之探討

Typhoon Fanapi (2010) and Its Interaction with Taiwan Terrain – Evaluation of the Uncertainty in Track, Intensity and Rainfall Simulations

林裕豐(Yu-Feng Lin)

指導教授 ： 吳俊傑

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/NTU202002410

摘要

本研究使用利用Advanced Research WRF模式結合系集卡爾曼濾波器(ensemble Kalman filter；EnKF)資料同化技術，同化2010年國際觀測計畫ITOP（Impact of Typhoons on the Ocean in the Pacific）實驗期間特別的觀測資料，藉由系集模擬以探討臺灣地形對於凡那比颱風(2010)路徑、強度及降雨不確定性的影響。模擬結果顯示，臺灣地形增加了模擬路徑及強度的不確定性，尤其在登陸期間。移動速度較快的系集成員受到地形影響時間較早，當颱風接近地形時出現路徑較早向南偏折及強度提早減弱的現象，造成模擬路徑及強度的標準差在登陸初期突然增加。此外，愈高(愈低)緯度的雨帶軸線分布對應愈高(愈低)緯度的颱風中心位置，颱風離陸時中心位置與雨帶軸線位置在緯度上有很高的相關性。系集成員時間延遲相關分析也顯示，登陸前颱風中心緯度可視為預測颱風離陸時位置的預報因子。然而，臺灣地區降雨模擬的不確定性主要來自於颱風雨帶、環流及地形之間的相互作用，尤其是颱風離陸期間雨帶的分布，該雨帶分布的不確定性與離陸時颱風中心所在的位置有關，同時地形高度引起颱風環流不對稱結構，也會直接影響颱風雨帶生成及發展的位置。另一方面，透過EnKF資料同化過程所獲得的初始場不僅改變了初始渦旋結構，也同化了大氣環境場，在模擬過程中大幅減小了颱風預報路徑及強度的不確定性。
此外，凡那比颱風離陸期間颱風南側雨帶的發展、維持機制及其引起之強降雨事件，可分為初始對流形成及對流胞移入陸地增強的過程。初始對流的形成主要是垂直風切提供位於下風切處的有利環境，配合低層暖濕氣流與下墊面冷池前緣的強迫舉升作用產生。然而，對流胞往陸地移動過程持續增強發展，主要是受到地形及垂直風切引起颱風的不對稱結構，進而造成不對稱氣流增強低層的氣流輻合作用，激發對流發展，同時雨帶上的強對流伴隨颱風環流移入陸地，並受地形強迫舉升的加乘作用，導致臺灣南部地區的強降雨形成。

關鍵字

凡那比颱風 ； 臺灣地形 ； 不確定性 ； 系集模擬 ； 系集卡爾曼濾波器

並列摘要

Using special data from the field program of “Impact of Typhoons on the Ocean in the Pacific” (2010) and an ensemble Kalman filter (EnKF)–based vortex initialization method, this study explores the impact of Taiwan terrain on the uncertainty in forecasting track, intensity, and rainfall of Typhoon Fanapi (2010) based on ensemble simulations. The results show that the presence of Taiwan topography leads to rapid growths of the simulation uncertainty in track and intensity during the landfall period, particularly at the earlier landfall period. The fast moving ensemble members show an earlier southward track deflection as well as weakening of intensity, resulting in a sudden increase of standard deviation in track and intensity. During the period of offshore departure, our analysis suggests that the latitudinal location of the long-lasting and elongated rainband to the south of the tropical cyclone (TC) center has strong dependence on the latitude of the TC center. In addition, the rainfall uncertainty in southern Taiwan is dominated by the uncertainty of simulated TC rainband, and the latitude of the TC track can be regarded as a good predictor of the rainband’s location at departure time. Considering the fact that the rainband impinging the high mountains in the southern Central Mountain Range generates the greatest accumulated rainfall, positions where the rainband associated circulation and its interaction with topography appear to offer an explanation on the uncertainty of the simulated rainfall. Meanwhile, with EnKF data assimilation, Fanapi’s structures are well developed and the initial environmental conditions show a small variation, leading to the reduction of track and intensity uncertainty as well as the simulated rainfall, i.e., much of the forecasting uncertainty strongly related to the model initialization can be effectively reduced.
In addition, the self-sustaining processes of the studied rainband during Fanapi’s departure period are analyzed in two perspectives. One is the formation of new convective cells to the southwest of TC center. These new convective cells are triggered by low-level flow convergence in the down shear side of vertical wind shear and upward lifting as an oncoming warm, moisture inflow met the leading edge of the cold pool. The other one is the continuous enhancement of convective cells as they move along the convective line toward the inland. These enhanced convective cells embedded in the rainband are developed by moisture jet stream, asymmetric low-level flow convergence induced by both vertical wind shear and Taiwan terrain, and TC circulation. Therefore, the productions of heavy rainfall over southern Taiwan are increased significantly as these convective cells move to the southwest of Taiwan and interact with Taiwan topograpgy.

並列關鍵字

Typhoon Fanapi ； Taiwan topography ； uncertainty ； ensemble simulation ； ensemble Kalman filter

參考文獻

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