氣候變遷下颱風活動的變化 – HiRAM模式模擬路徑群集分析以及理想實驗模擬

Typhoon Activities under the Changing Climate – Track-Cluster Analysis of HiRAM Projections and Mechanism Study based on Idealized WRF Model Simulations

李宜臻(Yi-Chen Li)

指導教授 ： 吳俊傑

國立臺灣大學/理學院/大氣科學研究所/碩士(2022年)

https://doi.org/10.6342/NTU202202029

摘要

本研究中，使用群集分析的方法討論High Resolution Atmospheric Model (HiRAM) 模擬的西北太平洋颱風在氣候暖化下的變化。除了缺少足夠的較強颱風數量，HiRAM的颱風活動模擬表現良好，生成位置與季節性大致與觀測相符，特別是登陸在菲律賓北部、台灣、中國東部沿海及日韓地區的C1颱風，以及在菲律賓或南海生成影響南海附近地區的颱風，兩者數量相當接近觀測。在RCP8.5 (Representative Concentration Pathways 8.5) 的暖化情境下，HiRAM模擬結果顯示未來二十一世紀末的颱風數量明顯減少，且颱風生命期間最大強度的最大值提升，表示更強颱風出現的可能性。透過群集分析將現在與未來的颱風一起分成六類，檢視各類的強度、路徑、生成位置及移動速率等變化，每一類對氣候暖化的反應程度並不相同。其中，C1颱風和生成位置及活動範圍較靠近中太平洋的北轉颱風的平均生命期間最大強度顯著增加，平均增強速率顯著增加與幾乎不變的增強時間為主要原因。
考慮到模式表現以及對人類活動的影響程度，選擇C1颱風的增強速率增加為分析對象，並檢驗其主要增強區域平均環境場的改變。在HiRAM的未來模擬中，因為大氣-海洋的熱力不平衡明顯增加，西北太平洋各處的PI增加，代表颱風可從熱力環境得到更多的能量。但中層大氣的熵減少，使得乾空氣透過風切阻礙颱風發展的影響提升，環境變得不利於颱風發展。為了瞭解颱風的結構變化與其對發展過程的影響，利用WRF (Weather Research and Forecasting) 模式做較高水平解析度的理想實驗。在未來C1颱風主要增強區域的環境條件下，颱風初期的增強速率提高，生命期最大強度中，地表最大風速提升3-5 m s-1，中心氣壓下降5-8 hPa。結構方面出現較垂直的眼牆及較高暖心位置，環流結構也從低層延伸至更高的範圍，眼強活躍的對流活動持續時間增加。雖然垂直風切為中等風切，但颱風雨帶於整個模擬時間並無明顯的不對稱性，風切導致乾冷空氣逸入的不利影響可能因為地表通量較高而抵銷。定性來說，C1颱風於主要的增強區域，未來的氣候環境場相比於當今的氣候環境場，更有可能使颱風發展至更強的強度。

關鍵字

颱風活動 ； 氣候變遷 ； 群集分析 ； 增強速率 ； 理想模擬

並列摘要

The responses of tropical cyclones (TCs) in the western North Pacific (WNP) to changing climate are investigated based on clustering analysis of TC tracks simulated with a modified High Resolution Atmospheric Model (HiRAM). TC activities in the present climate are well simulated, except for a lack of intense TCs. Under the Representative Concentration Pathways 8.5 (RCP8.5) scenario, HiRAM projects that the number of TC declines significantly and their lifetime maximum intensity (LMI) distribution extends toward higher intensity in the late 21st century. Furthermore, the mean LMI increases significantly in two TC clusters. In these two clusters, an enhanced mean intensification rate, together with a barely-unchanged mean intensifying duration, account for the elevated mean LMI in the warmer future. One of them (hereafter C1) contains strong TCs affecting a wide range of the coastal regions of East Asia and the northern Philippines. Examinations of a variety of TC metrics show that HiRAM particularly well simulates TCs in C1.
Considering the model performance and the great impact to human activity, we investigate the physical links between the TC responses in C1 (e.g., the increased mean intensification rate) and the environmental changes in the main intensifying region of C1 TCs. In the future projections, TC’s potential intensity becomes higher all over the WNP, due to the increase in thermodynamic air-sea disequilibrium. However, lower entropy air in the mid-levels provides more detrimental conditions for TC intensification. To investigate how the TC vortex structure changes, numerical experiments in specific idealized settings are conducted by using the Weather Research and Forecasting (WRF) Model. With future environmental conditions, TC experiences enhanced intensification and reaches stronger maximum 10-m wind speed by 3-5 m s-1 and lower minimum central pressure by 5-8 hPa. As for the structure, straighter eyewall, higher altitude of warm core and great vertical extent of the cyclonic circulation help the vortex maintain its intensity, accompanied by consistent active convection. Stronger vertical wind shear doesn’t cause obvious asymmetric rainband, given that its ventilation effect may be counteracted by more surface heat flux. In conclusion, the environmental conditions in the main intensifying region of C1 under warmer climate facilitates TCs development and stronger intensity.

並列關鍵字

Tropical cyclone activity ； Climate change ； Cluster analysis ； Intensification rate ； Idealized simulation

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