高層槽對颱風強度影響的機制探討

Abstract

本研究利用TCM4 (Wang 1999, 2001) 針對高層槽對颱風強度的影響進行一系列理想數值實驗及機制與敏感性探討，當颱風在達到不同強度時加入不同大小範圍和不同強度的高層槽，分析各組實驗颱風強度和結構對高層槽的反應，結果發現範圍越小、強度越弱的高層槽對颱風強度的發展越有利。而相對較弱的颱風而言，在颱風強度較強時與高層槽作用初期大部份均會呈現減弱的現象而比較沒有快速增強的情形出現。 當颱風與高層槽產生交互作用時，在颱風高層均有明顯外流噴流的產生，而隨著高層槽往颱風接近，也會帶來垂直風切的增強，但於颱風外流和槽之間的交互作用下，高層槽在較高層的部份尺度會變小，強度會減弱，而在其下的部分甚至會與颱風有合併的現象，因而使垂直風切強度減小進而減弱其對颱風的破壞力。而高層槽所伴隨的垂直風切也會使颱風出現明顯波數一不對稱的結構，並在部份實驗中導致較強的中、低層增溫。且當颱風在與高層槽作用後，檢視其暴風半徑和強度(strength)的發展可發現其均比在沒有高層槽影響下的颱風來得大，這些結果與過去文獻的結論一致。 本研究中在部分實驗雖有較大垂直風切的存在，但颱風強度仍能夠繼續增強，主要可能是由於高層槽其氣旋式環流的結構會額外帶來渦流角動量的傳送而抵消較大垂直風切對颱風發展的負面影響。在各項實驗中渦流角動量通量輻合增強的區域，外流也會變大，當高層外流被增強，則有利於低層內流的增加，因此有助於颱風強度的增強。但值得注意的是並非每次渦流角動量通量輻合增加都能使颱風增強，在一些實驗中有大範圍渦流角動量通量輻合為正值的情況下，颱風強度並未增強，這可能是與渦流角動量通量輻合所增強的外流位置有關。而對照高海表面溫度的實驗結果可發現若要使颱風產生快速增強的現象，除了高層要有適宜高層槽的存在之外，海表面溫度的高低也是影響此一現象產生的重要關鍵。而對平穩狀態的颱風而言，似乎沒有適宜的高層槽可使颱風強度增強，高層槽的加入均會使已達平穩狀態的颱風產生減弱。

This research utilizes Tropical Cyclone Model Version 4(TCM4, Wang 1999, 2001) to conduct a series of ideal numerical experiments and investigate the mechanism and sensitiveness of the effect of upper-layer trough on the intensity of tropical cyclones (TCs). In these experiments, the TC with different intensity encounters the upper-layer trough of varied size and magnitude. The analyzed results of this research shows that weak upper-layer trough with small scale is favorable to TC development. Contrary to weaker TCs(category 1), interacting with upper-layer trough makes almost all stronger TCs(category 3) weaken initially and no rapid intensification occurs. The vertical wind shear increases as the upper layer trough approaches. After interacting with the upper layer trough, it tends to decrease, and this is favorable for TC development. Upper layer PV meandering decays while approaching the TC, and then outflow-jet aloft forms at north and northwest of the TC center. After the TC-trough interaction, mid- and high-troposphere cools while low troposphere warms and both the vertical velocity and rainfall rate reveal wave-number one structure, and thus affects the TC intensity. In all numerical experiments, the outflow in intense eddy flux convergence of relative momentum (EFC) region often increases, but it does not guarantee TC’s intensification due to the position of the outflow. The result of high SST’s experiment reveals that although the SST is an important factor, the upper-layer trough also plays an important role in the TC intensification. In the cases of TCs of the steady state, there is not favorable upper-layer trough that can induce TC intensification. The intensity of the TC that reaches steady state often weakens when it interacts with upper-layer trough.