邊界層動力過程對颱風強度之影響

Abstract

本研究使用MM5模式探討拖曳係數(CD, drag coefficient)對於颱風結構、增強過程和穩定態強度的影響，並檢驗Smith et al. (2009)的颱風增強理論。模式中使用bulk aerodynamic邊界層模組。實驗設計分為控制實驗及敏感性實驗兩組，控制實驗(1CD)使用bulk aerodynamic模組診斷出的CD；敏感性實驗則是將模式診斷出的CD分別乘以0.5倍(0.5CD)以及2倍(2CD)。實驗結果發現，改變CD對於颱風的最大切向風風速影響不大(10%以內)，但對於颱風的結構則有顯著的影響；其中2CD的颱風的最大切向風風速半徑最小、有最強的軸對稱平均入流以及最弱的切向風風速。藉由梯度風分析發現，不論是在颱風增強過程或是穩定態期間，邊界層內皆為梯度風不平衡，而在不平衡下所伴隨的超梯度風可視為是颱風內核顯著而重要的特徵；敏感性實驗發現，當摩擦增加時，梯度風不平衡以及超梯度風程度變得更為顯著。因此，可推測梯度風不平衡以及超梯度風的出現應是自由大氣對於邊界層內摩擦的反應。在包含平均流以及擾動場的切向風動量收支分析發現，摩擦及軸對稱的徑向平流為收支分析中的兩大項，但其貢獻幾乎會互相抵消，因此，抵消後的剩餘項以及擾動項為造成切向風增加的主要兩項貢獻。本研究結果顯示，入流並非造成颱風增強的主要因素，摩擦耗散也扮演重要的角色。摩擦的作用一方面增強颱風入流，同時也增加耗散，抵消入流使切向風增加的作用。此研究的分析結果凸顯了摩擦對於颱風增強扮演雙重角色。

In this study, idealized simulations are conducted in MM5 model to investigate the sensitivity of tropical cyclone (TC) structure, intensification and steady state intensity to the surface drag coefficient (CD), and also to examine the new intensification paradigm (Smith et al. 2009). The bulk aerodynamic boundary layer scheme is employed. The experiments are standard CD (1CD, control run), half (0.5CD) and double (2CD) of the standard. The three experiments indicate that changing the surface drag coefficients leads to a small variation of intensity (within 10%) and a clear change of TC structure. The 2CD experiment shows the smallest radius of maximum tangential wind, strongest azimuthally-averaged inflow and the weakest tangential wind speed. The gradient wind balance analyses demonstrates that for both intensifying and steady state TCs, the gradient imbalance arises in the boundary layer and the accompanied development of supergradient wind is a salient feature in the inner-core region. The sensitivity experiments demonstrate that as the surface drag is increased, the imbalance of tangential wind from gradient wind balance becomes greater and the supergradient flow is more evident. It is inferred that the gradient wind imbalance and the development of supergradient wind speed is a response of the free atmosphere to the boundary layer surface drag force. The tangential wind momentum budget analyses include the contribution from the axisymmetric mean flow and eddy terms. The analyses show that the actual location of the tangential wind speed increase should be regions where the remaining contribution of the two dominant terms (friction and mean axisymmetric-radial advection) and the eddy terms are important to actual wind increase. This result reveals that the inflow is not the only critical factor in TC intensification and that frictional dissipation also takes a significant role. On one hand, the inflow provides the gradient wind imbalance to enhance the radial advection. On the other, the friction dissipates the momentum and inhibits TCs from intensifying. Our analyses provide new physical insight in highlighting the dual roles that friction plays in the intensification of TCs.