颱風生成與梅雨鋒之交互作用:凱米(2018)個案分析

The Interactions between Tropical Cyclone Formations and Mei-Yu Fronts: A Case Study of Typhoon Gaemi (2018)

張玉來(Yu-Lai Chang)

指導教授 ： 楊明仁

國立臺灣大學/理學院/大氣科學研究所/碩士(2021年)

https://doi.org/10.6342/NTU202101995

摘要

在本篇研究中，我們參考Lee et al.(2006)的概念將生成於梅雨鋒面低壓槽的熱帶氣旋定義為鋒面旋生類型並針對其生成機制及其與環境的交互作用進行研究分析。首先，我們透過1979-2018年間的鋒面旋生個案分析大尺度環境場的重要特徵，並利用濾波分析分離出不同時間以及空間尺度的特徵。大尺度環流分析表明:強大的西南氣流、底層豐富的氣旋式渦度以及逐漸減少的環境斜壓性有利於熱帶氣旋在梅雨鋒面低壓槽的西南端生成。透過與合成分析的比較，我們驗證了凱米(2018)的代表性並分析他的渦旋尺度過程以及機制。強大的西南氣流促使鋒面區域產生活躍的對流及強水平風切，鋒面上的正壓不穩定產生擾動並且在鋒面低壓的位渦”滋養”下逐漸增強，覆蓋於其中一個渦旋上的中層短波槽協助中層渦旋的建立，最終此渦旋逐漸發展形成凱米(2018)。除了熱帶氣旋的生成外，伴隨的鋒消也是一個值得討論的現象。在凱米生成前，鋒前覆蓋的對流遮蔽太陽短波輻射導致該區域鋒消，斜壓性的減弱也有助於凱米發展為熱帶氣旋。在凱米生成後，其強大的位渦中心會扭曲破壞鋒面的位渦結構，凱米的次環流又會抑制鋒面對流的發展，使得第二類條件不穩定(CISK)被限制導致更進一步的鋒消。最後，我們提出了一個完整的概念模式解釋熱帶氣旋旋生及梅雨鋒面鋒消的過程及這些天氣系統之間的交互作用。

關鍵字

熱帶氣旋生成 ； 梅雨鋒面 ； 濾波分析 ； 位渦 ； 正壓不穩定

並列摘要

In this study, we followed the concept of Lee et al. (2006) to define that the tropical cyclones formed in the Mei-Yu frontal depression as the frontal-type tropical cyclone formation. The frontal-type cases during 1979 to 2018 were identified to uncover the essential signatures of the large-scale environments. The filtering method was applied to distinguish the signals from different temporal and spatial scales. The large-scale analyses suggested that the strong southwesterly flow, abundant low-level vorticity and reduction of baroclinicity fertilized the formation of tropical cyclone in the southwest end of the frontal depression. Furthermore, a representative frontal-type case, tropical cyclone Gaemi (2018), was analyzed to investigate the vortex-scale process and mechanism via the reanalysis and the numerical simulation. The southwesterly flow induced the active convection and strong horizontal wind shear in the frontal region. The perturbations were triggered via barotropic instability and the potential vorticity in the frontal depression nourished their intensification. Moreover, the mid-level trough was above one frontal vortex and helped the establishment of mid-level vortex. This perturbation finally developed into a tropical cyclon¬e ¬— Gaemi. In addition to the cyclogenesis mechanism, the frontolysis was an interesting phenomenon as well. The frontolysis was induced by the short-wave radiation prior to Gaemi’s development. The reduction of baroclinicity in this stage provided a suitable environment for Gaemi’s formation. After Gaemi enhanced, Gaemi’s strong potential vorticity distorted the frontal structure and Gaemi’s secondary circulation constrained the convection in frontal region. Thus, the Conditional Instability of the Second Kind (CISK) mechanism was restrained and led to the significant frontolysis. Last, a conceptual model was proposed to elucidate the interactions between the tropical cyclone formations and Mei-Yu fronts.

並列關鍵字

tropical cyclone formation ； Mei-Yu front ； filtering analysis ； potential vorticity ； barotropic instability

參考文獻

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