熱帶氣旋生成類型及其增強速率之探討

Abstract

基於切向風收支方程，較小的最大風速半徑（RMW）會造成較高的熱帶氣旋（TC）增強速率。再者，RMW內持續的對流會導致RMW收縮。因此，了解影響對流分布與RMW的因素，對於探討TC的增強速率至關重要。前人研究已顯示對流分布受TC生成類型的影響。然而，對TC生成的主觀分類難以代表其資料分布。本研究開發一種新的客觀方法，利用ECMWF Reanalysis v5（ERA5）資料中挑選數個大氣參數，再運用K-means 分群演算法對TC的生成類型做分類。為了比較增強速率和RMW之間的關係，本研究計算了每個個案的生命期最大增強速率（LMIR）。 本研究結果顯示LMIR與RMW之間接近反比關係，與前人研究一致。另外，在成為熱帶風暴（TS）時，具有較大RMW的TC通常具有較低的LMIR，代表初始RMW也會影響LMIR。K-means分群分析顯示四種TC生成類型：（i）季風匯流（MC）、（ii）東風波（EW）、（iii）季風風切（MS）、（iv）季風低壓（MD）。相較於前人研究較不顯著的對流分布，此新方法的分類結果顯示，每個TC生成類型的對流分布呈現明顯差異，代表此新方法在區分TC生成時的結構上更有效。本研究的分類結果顯示，EW的對流僅出現在中心周圍，MC跟MS的對流雖然也集中在中心周圍，但MC外圍的對流往南延伸，而MS外圍的對流則是往東西兩側延伸，MD的對流最為分散。由於MD的環流較大且對流分散，其RMW明顯較大，而LMIR較低，與EW相比具有統計顯著差異（Conover's test）。相反地，在EW個案，比濕較高和亮溫較低的區域均集中在中心周圍，這解釋了為何EW具有較小的RMW。即使MC和MS的RMW大小介於EW和MD之間，但由於集中在中心的對流，它們的LMIR與EW相近。 在準理想實驗中，EW較小的RMW明顯造成更高的渦度，因此根據切向風收支方程，EW的增強速率高於MD。儘管MD的RMW不斷收縮，但仍然具有較大的RMW。在RMW一邊收縮，強度一邊增強的過程，其RMW周圍的切向風徑向曲率也不斷變大，代表RMW位置與其內側的風速差異變大，限制了RMW內側的風速上升至超過RMW位置風速的機會，造成MD的RMW無法繼續收縮。

Higher tropical cyclone (TC) intensification rates are affected by smaller radius of maximum wind (RMW) based on the tangential wind tendency equation. Moreover, continuing convection within the RMW can cause RMW contraction. Thus, understanding the factors affecting convection distribution and RMW is crucial for characterizing TC intensification rates. Previous studies have shown that convection distribution is affected by TC genesis type. However, subjective classification of TC genesis does not rely on data distribution. In this study, a new objective method is developed to classify TC genesis type based on K-means clustering algorithm of critical atmospheric parameters available in ECMWF Reanalysis v5 (ERA5) data. For comparison between intensification rate and RMW, the lifetime maximum intensification rate (LMIR) in each case is also examined. The result shows a nearly inverse proportion between the LMIR and RMW, which is consistent with previous research. In addition, TCs with larger RMW upon becoming a tropical storm (TS) usually have lower LMIR, implying that the initial RMW can also affect LMIR. The K-means cluster analysis shows four TC genesis types: (i) monsoon confluence (MC), (ii) easterly wave (EW), (iii) monsoon shear (MS), and (iv) monsoon depression (MD). The convection distribution shows a distinct difference in each genesis type, which is not so significant in previous studies, implying that this new method is more effective in distinguishing the structure of TCs. As a result of this new classification, EW has the most aggregated convection. Although MC and MS also have aggregated convection around TC center, the convection in outer region extends southward in MC and extends eastward and westward in MS. In contrast, MD has more scattered convection than others. Owing to larger circulation and scattered convection, MD has a significantly larger RMW and lower LMIR than EW (Conover's test). In contrast, EW cases have higher specific humidity and lower brightness temperature only around the center, explaining why EW has a small RMW. Although both MC and MS have medium RMW sizes between EW and MD, their LMIR is as high as that in EW because of aggregated convection similar to EW. In the quasi-idealized experiment, the smaller RMW of EW contributes to evidently higher vorticity, thus EW has a higher intensification rate than MD based on the tangential wind tendency equation. Despite the continuous RMW contraction in MD, it still has a larger RMW than EW. During the period of RMW contraction and intensification, the radial curvature of tangential wind around RMW also increases. This indicates that the wind speed difference between RMW and its inner side increases, limiting the probability of wind speed inside RMW surpassing that at the RMW. Consequently, it stops the RMW contraction in MD.