影響颱風快速增強潛在機制之探討：多邊形眼牆及強對流區與颱風大小之日變化

Investigation of Potential Mechanisms for the Rapid Intensification of Tropical Cyclones: Polygonal Eyewalls and Diurnal Variation of the Convective Area and Eye Size

李在德(Jae-Deok Lee)

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(2019年)

https://doi.org/10.6342/NTU201800681

摘要

在合適的大環境條件下，颱風的快速增強(Rapid Intensification, RI)可能經由內部動力的交互作用而發生；而這些非線性的交互作用造成預報颱風快速增強的難度。本研究使用了數值模擬與觀測資料，加以探討颱風多邊形眼牆的發展、強對流區的日夜變化與颱風中心的大小變化在颱風經歷快速增強時的角色。
在模式模擬的部分，本研究使用全物理、高階析的Advanced Research Weather Research and Forecasting (WRF)模式模擬梅姬颱風 (2010)在快速增強前後的發展。在同樣使用Mellor-Yamada-Nakanishi-Niino 3.0-level (MN3)的行星邊界層參數化方法之設定下，經由使用不同的雲微物理參數化方法WRF single-moment 6-class (WSM6)和WRF double-moment 6-class (WDM6)進行敏感性測試，颱風的強度明顯產生不同的發展。將結合WDM6與MN-3 (WDM6-MN3)的實驗與結合WSM6與MN3 (WSM6-MN3)的實驗結果相比，前者在低對流層中有較乾的環境以及較強的下衝流 (downdraft)，而這兩個因素影響其初始渦漩的發展，並且限制了在特定波數下多邊形眼牆的維持，進而使WDM6-MN3實驗無法發生快速增強；相對而言，WSM6-MN3在快速增強時期，能在特定波數下維持較久的多邊形眼牆結構，並且在多邊形眼牆的頂點(vertex)具有較強的主環流與絕對角動量、較大的慣性穩定度、較大的位渦與海洋表面向上傳輸進入大氣的熱通量。
在觀測資料分析的部分，本研究使用向日葵八號衛星所觀測的影像，來探討2015至2017年西北太平洋27個曾歷快速增強的颱風中，活躍對流區域(Active Convective Area, ACA)以及颱風眼大小的日夜變化。ACA的面積通常在傍晚至深夜逐漸增加，並且在日間逐漸減少；然而，若在日間（daytime）期間，ACA在颱風的最大風速半徑(Radius of the Maximum Wind, RMW)內能維持較大的面積，將減少其下方輻射造成的冷卻過程，並使得雲層下方較暖的溫度得以維持到夜間，進一步導致雲層較高(冷)以及較低(暖)部分之間的溫差而產生不穩定性。在這樣的情形之下，將造成較活躍的對流。透過這樣的過程，颱風的強度將產生顯著的發展；本研究之分析資料顯示，巔峰強度達到三級(Category-3)或以下的颱風，可能只需經歷一次ACA的日夜變化；然而，若RMW內的ACA能維持更久的時間，則颱風的強度將發展到四級(Category-4)甚至五級(Category-5)的颱風強度，並伴隨較強的眼牆。 另外，統計資料顯示，颱風的快速增強通常發生在一個較緩慢的增強過程之後，這結果指出，颱風主環流及眼牆強度的同時增強，可能有利於之後颱風的快速增強。另一方面，一旦颱風產生了颱風眼（eye structure）的構造後，其大小將隨著ACA面積的發展而產生改變。

關鍵字

快速增強 ； 對流爆發 ； 低層多邊形眼牆 ； 活躍對流區域的日夜變化 ； 輻射冷卻造成之不穩定 ； 眼牆強度

並列摘要

The rapid intensification (RI) in tropical cyclones (TCs) likely occurs as a result of nonlinear internal interactions under favorable large-scale environments. For this reason, RI forecasts remain a great challenge. In this study, the role of polygonal eyewalls and the diurnal variation of the convective area and eye size associated with RI of TCs are investigated based on the results of numerical experiments and observational analyses.
A high-resolution numerical experiment is designed to examine the inner-core dynamics of Typhoon Megi (2010) using the Advanced Research Weather Research and Forecasting (WRF) model with full physics. From the sensitivity experiment, a significant difference in TC intensity evolution between the WRF single-moment 6-class (WSM6) and WRF double-moment 6-class (WDM6) microphysics with Mellor-Yamada-Nakanishi-Niino 3.0-level (MN3) planetary boundary layer schemes emerged since the onset of RI. Prior to RI, WDM6-MN3 exhibited relatively drier environment and stronger downdraft in the lower troposphere as compared with WSM6-MN3. These two factors can interrupt the initial development of convective cells and limit sustainability of the polygonal eyewall at a specific wavenumber in the lower troposphere. As a result, as compared with WDM6-MN3, WSM6-MN3 that exhibited a long-lasting polygonal eyewall at a specific wavenumber during the RI period shows more enhanced internal physical quantities such as the primary circulation, convective cells, inertial stability, potential vorticity, absolute angular momentum, and surface heat fluxes at each vertex of polygonal eyewalls. These differences in internal physical quantities could result in different intensity evolution.
Observational analyses using Himawari-8 satellite imagery examined the diurnal variation of the active convective area (ACA) and eye size associated with 27 RI TCs selected from 2015 to 2017 in the western North Pacific. The ACA generally increases between the late afternoon and midnight, while it shrinks significantly during the day. However, if the ACA remains inside the radius of maximum wind (RMW) during the day, it could significantly reduce radiational cooling beneath that cloud area. This condition can cause a significant destabilization by the contrast between the upper and lower clouds, and thus enhancing the convective activity during the nighttime. The category-3 storm and below showed that one diurnal cycle of the ACA inside the RMW seems to enough to trigger RI. However, when the ACA is prolonged inside the RMW, storms tend to evolve into stronger TCs such as category-4 and -5 storms together with strong eyewall strength. In statistical analyses, RI tends to begin after a slow intensification stage in the tropical storm stage. It indicates that a simultaneous enhancement of primary circulation and eyewall strength of the storm could be conducive to RI. Meanwhile, once the storm forms the eye structure inside the RMW during the RI period, its size tends to change against the ACA development.

並列關鍵字

rapid intensification ； convective bursts ； low-level polygonal eyewalls ； the diurnal variation of the active convective area ； destabilization caused by radiational cooling ； eyewall strength

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