- 學位論文
颱風外核結構如何影響兩種眼的形成－深化成眼和雲捲成眼
How does the Outer-core Structure Affect the Two Types of Typhoon Eye Formation: Deepening Formation and Banding Formation?
摘要
颱風快速增強時伴隨最顯而易見的特徵當屬颱風眼形成，透過衛星觀測，我們將眼的形成過程分為兩類，由中心密集雲區形成的深化成眼(Deepening Formation, DF)和由彎曲雲帶形成的雲捲成眼(Banding Formation, BF)；過去研究探討影響颱風眼形成的因素時，多只集中於綜觀環境，颱風自身的初始結構角色仍不明朗。因此本研究目標為分析DF和BF伴隨的颱風特徵差異，探討颱風初期結構差異如何導致兩種不同的颱風眼形成過程，並分析眼形成時內核結構的改變。 本研究第一部分首先以衛星資料將2007至2019年西北太平洋的颱風區分為DF和BF兩類，並使用最佳路徑資料分析其特徵差異。結果顯示，DF數量為BF數量的3倍，DF相較BF在颱風眼形成時有顯著較高的強度和增強速率、較小的眼，颱風眼形成前則有較小的暴風半徑，颱風生成和眼形成的位置略為偏東和偏南，移動方向則顯著偏西。此外，DF和BF分別好發於秋季和夏季。 第二部分以WRF模擬2015年颱風蘇迪勒(Typhoon Soudelor) 不同的外核風場結構，觀察颱風初期結構差異如何影響颱風眼形成。結果顯示所有敏感性實驗均為DF，但外核風場較大者(A04)，仍具有更接近BF的特徵，與外核較小者(A08)相較，增強速率小，最大風速半徑(Radius of Maximum Wind, RMW)內縮較慢，颱風成熟時具有更大的RMW和因此而更大的眼。A08外核的慣性穩定度小，低層輻合及其引發的上升對流位置有更大一部份落於RMW以內，讓潛熱加熱更有效率，激發眼牆更強烈的對流，另外也讓RMW內縮較快並形成較小的眼。眼牆更活躍的對流過衝現象，使得低平流層接受更多補償性下沉增溫，被增溫的空氣隨著對流過衝本身的輻散流向颱風中心並下沉。A08更小的眼區面積，會進一步產生更強烈的下沉及絕熱增溫，最終使A08暖心增暖更快，地面氣壓下降更快。另外，A08的DF特徵使得眼區有更多水相粒子被蒸發，會干擾中層暖心的建立。
並列摘要
Eye formation is the most distinct structure transformation of tropical cyclone (TC) associated with rapid intensification (RI). Eyes can evolve from the central dense overcast (CDO) or the curved band patterns. In this study we define the two types of eye formation as “Deepening formation (DF)” and “Banding formation (BF)” respectively. Synoptic environment-related factors of TC eye formation have been identified in recent studies. However, the roles of TC structure-related factors remain unclear. The objectives of this work are to identify the TC characteristic associated with the DF and BF processes; to investigate how initial TC structure leads to the two different eye formation processes; and to examine the evolution of the inner-core structure during eye formation. In the first part of this study, the satellite imagery and best-track dataset are used for the observational analysis of typhoons named in western North Pacific from 2007 to 2019. Results show that the number of DF cases is three times more than that of BF cases. TCs with DF have significantly higher intensity, higher intensification rate and smaller eyes during eye formation, smaller size before eye formation and with more westward tracks, and are more likely formed to the southeast region. Meanwhile, DF and BF TCs tend to occur in autumn and summer, respectively. In the second part of this study, sensitivity experiments of Typhoon Soudelor (2015) with various idealized outer-core structure are conducted. Storm with stronger outer core (larger storm, e.g., A04) behaves more BF-like features even though all storms can be categorized as DF in simulated IR. The smaller storm (e.g., A08) has higher intensification rate, higher contraction rate of radius of maximum wind (RMW) during RI, and smaller RMW and eye in mature stage. The convergence and diabatic heating in A08 are more concentrated inside the RMW, which may lead to stronger secondary circulation, faster contraction of RMW, and thus leading to a smaller eye. With more active convective overshooting in the eyewall of A08, the lower stratosphere beyond the eyewall significantly warms due to the compensating subsidence near the overshooting convection. The warm air flows into the storm eye with the divergence flow of the overshooting convection, and subsides to troposphere. The closer updraft area to the center in A08 limits the extent of eye, and further enhances the subsidence and adiabatic warming in the upper-level center, resulting in a smaller and stronger warm core, eventually with a higher intensification rate. On the other hand, the evaporation and sublimation of hydrometer in the denser anvil cloud in A08 can offset the adiabatic warming and eddy radial potential temperature advection, hindering building the mid-level warm core.