海洋回饋對颱風眼牆置換過程之影響—哈吉貝颱風(2019)個案研究

Abstract

眼牆置換過程能夠造成熱帶氣旋強度與結構的快速變化，因此了解眼牆置換期間的內眼牆消散機制以及外眼牆增強過程是非常重要的研究議題。然而，過去大部分關於眼牆置換過程的研究僅探討大氣過程，並未考慮海洋回饋作用對眼牆置換的影響。因此本研究希望透過海氣耦合模式來模擬哈吉貝颱風（2019）的眼牆置換過程，探討海洋回饋作用如何影響此個案的眼牆置換過程。 本研究首先設計三組實驗，包含僅使用大氣模式的UC實驗、耦合海洋模式的C3D實驗，以及加入海洋冷渦的COE實驗。此三組實驗的分析結果顯示，對於此強度強且移速快的個案來說，耦合海洋模式的C3D實驗僅造成颱風的切向風場減弱，但對於整體風場結構以及眼牆置換過程沒有顯著影響，而COE實驗則造成眼牆置換過程延長了大約7小時。透過對流平面圖的分析，可以發現在COE實驗之中，颱風東側象限的對流受到冷渦冷海水的影響，較慢增強與重新組織，延長往下游傳遞並完成對流軸對稱化的時間。此外，從環狀區域平均的內外眼牆強度隨時間的變化也可以看到相同特徵，在C3D實驗之中的外眼牆增強較快，而COE實驗的外眼牆則增強較慢，此增強較慢的外眼牆讓內眼牆能夠透過進入內眼牆區域的內流持續維持，進而延長眼牆置換時間。最後，各象限平均的低層次環流同樣顯示COE實驗的外眼牆上升運動較弱，使得COE實驗在特定象限的內眼牆區域內流能夠維持比C3D實驗還要強的強度，說明較弱的外眼牆確實有利進入內眼牆區域的內流持續維持，並延長COE實驗內眼牆的生命期以及眼牆置換過程。 本研究也進行了植入不同強度的海洋冷暖渦的敏感性實驗，結果顯示，較弱的海洋冷暖渦對於眼牆置換過程的影響較不顯著，而較強的海洋冷暖渦皆能造成眼牆置換過程的延長。較強的海洋冷渦造成眼牆置換過程延長的機制和COE實驗相同，較強的海洋暖渦能夠延長眼牆置換時間則是因為海洋環境條件較有利內眼牆對流的發展，較強的內眼牆導致外眼牆不利發展，較弱的外眼牆讓內眼牆能夠持續維持，進而導致眼牆置換過程的延長。 以上結果說明海洋回饋作用能夠影響眼牆置換過程，而眼牆置換過程的時間取決於內外眼牆之間的競爭效應，因此海洋條件對於眼牆置換過程的影響也和熱帶氣旋受影響的區域密切相關。未來可以進行更多真實個案模擬或是理想實驗來定性、定量探討海洋回饋作用對於眼牆置換過程的影響，以對於此議題有更深入的了解。

An eyewall replacement cycle (ERC) can lead to rapid intensity and structural change of tropical cyclone (TC). Therefore, it is important to understand the inner eyewall dissipation and outer eyewall intensification mechanisms during the ERC. Most previous studies focused solely on atmospheric processes and were short of the investigation of ocean feedback on ERC. Hence, this work aims to discuss the impact of ocean feedback on the concentric eyewall in Typhoon Hagibis (2019), which was a fast-moving storm developed under a favorable ocean condition. Three numerical experiments are conducted in this study. The uncoupled run (UC) and the ocean-coupled run (C3D) are used to explore whether the ocean cooling effect is important to ERC in this strong and fast-moving case. Another ocean-coupled run (COE) with a cold ocean eddy imposed at the region where SEF occurs is designed to investigate the impact of cold ocean eddy on ERC. Results show that the experiment with ocean coupling (C3D) has rather limited impact on the storm structure and ERC duration in this strong and fast-moving case. The main difference among these three experiments is in the ERC duration between C3D and COE. The presence of cold ocean eddy in COE prolongs the ERC period for 7 hours. In COE, the outer convection in the east quadrants is strongly influenced by the cold sea surface temperature (SST) lying behind the storm track. Therefore, it takes more time for the outer eyewall convection to reintensify and move to the downstream quadrants, leading to a longer convection axisymmetrization process. Time evolution of eyewall intensity also shows great consistency with the above results. The outer eyewall in COE intensifies more slowly and leads to both longer inner eyewall maintenance and longer ERC duration. From secondary circulation analysis in different quadrants, it is shown that the inner eyewall in COE can be maintained through the asymmetric inflow at the quadrants in which the outer convection is weaker. Sensitivity tests with warm/cold ocean eddies at different strengths show that moderate warm/cold ocean eddies impose little impact on ERC duration, while strong warm/cold ocean eddies result in a prolonged ERC duration. The process by which the strong cold ocean eddy prolongs the ERC duration is the same with that in COE. On the other hand, the strong warm ocean eddy is more favorable for the inner eyewall intensification, whereas the stronger inner eyewall is unfavorable for outer eyewall development. Therefore, the inner eyewall can be further maintained, and the ERC is prolonged. This study shows that the ocean feedback can influence the ERC process, and the ERC duration depends on the competition between the two eyewalls. Moreover, the impact of ocean feedback on ERC also depends on the region influenced by the ocean. More real-case simulations and idealized simulations are needed to quantitatively and qualitatively evaluate the influence of ocean feedback on ERC.