WISHE機制在颱風快速增強的角色

Abstract

海表面熱量通量為颱風發展的重要能量來源，本研究探討了與海表面熱量通量相關之wind-induced surface heat exchange (WISHE) 機制在颱風快速增強時的角色。首先，使用實際個案梅姬颱風(2010)進行探數值模擬，包含系集模擬；另外，也採用理想化實驗模擬，以探討在不同條件之下WISHE機制的重要性。在以上各數值實驗中，於計算海表面熱量通量時進行調整，以檢驗不同程度WISHE回饋機制對於颱風強度的影響。 在梅姬颱風個案之模擬中，若針對海表面熱量通量與風速的回饋過程進行限制，將導致較弱的颱風巔峰強度、被延遲的快速增強開始時間，甚至沒有發生快速增強。在快速增強開始之前，渦漩結構較弱且對流分布較不軸對稱，在環境初始的對流可用位能被消耗後，WISHE可維持對流不穩定度，並且提供足夠海表面熱量通量，促使氣塊舉升並向上發展，使得具有較明顯WISHE之渦漩具有較強且活躍的對流，其所伴隨之非絕熱加熱將有效地使颱風次環流增強，進一步回饋於主環流，使渦漩更快增強，並進入快速增強的過程。另外，較強的WISHE促使低層相當位溫較快增加，導致原先相當位溫較低的上風切象限之邊界層也逐漸增暖並發展對流，使得颱風對流逐漸軸對稱化，並且形成渦度環，颱風將更有效率地增強。在快速增強階段，颱風中至高對流層的對流更為軸對稱化，促使颱風進入發展更快的明顯增強時期(較連續增強)，並達到其巔峰強度。較強的WISHE機制將因為有較強的內核對流而產生較多的非絕熱加熱，進一步導致相對較強的次環流與非絕熱加熱，提升颱風之最大強度；同時，內核對流強度也影響颱風中心之下沉運動，進而造成颱風中心之中高層暖心結構強度及海平面中心氣壓之差異。另外，未限制WISHE回饋之所有成員皆經歷快速增強，但會因初始渦漩結構之差異，造成不同的快速增強起始時間。 理想化實驗的結果中，即使在WISHE機制較弱的條件下，仍能達到快速增強的標準，但時間明顯被延後。在渦漩較弱的時期，WISHE機制的角色較相對較式微，各實驗之間的發展並無太大差異；在渦漩達一定強度後，各實驗之颱風內核區熱量通量差異將逐漸顯著，進一步造成發展的快慢差異，並影響快速增強啟動時間以及颱風最大強度。 綜合而言，本研究顯示WISHE機制會影響颱風快速增強時間以及增強的幅度，並影響颱風在啟動快速增強的前後的發展，特別是在具有環境垂直風切的真實大氣中，更具其重要性。

This study examines the role of surface heat fluxes, particularly in relation to the wind-induced surface heat exchange (WISHE) mechanism, in the rapid intensification (RI) of tropical cyclones (TCs). Real case study with ensemble simulations and idealized simulations are both adopted in this study. To evaluate the role of WISHE, the surface heat fluxes from the ocean are artificially adjusted. In order to suppress the WISHE feedback, the values of surface wind speed for calculating surface heat fluxes are capped. In the real case simulations, both the thermodynamic environment and the convective-scale processes are clearly affected when capping the WISHE, leading to delayed RI and weaker peak intensity. Before RI, the vortices are relatively weak and the convection distribution is asymmetric, caused by the environmental vertical wind shear (VWS). WISHE could help the TC to overcome some effect from the downdrafts and provide the convective instability, resulting in more active convection and broader wind field. With stronger WISHE, TCs could intensify to a certain strength earlier, followed by further intensification. After the early stage, earlier increase of θe is found in the lower level in the experiments with more WISHE, which could earlier trigger the convection in the upshear quadrant. Therefore, earlier axisymmetrization of the storm vortices are found with larger upward vertical mass flux. After the onset time of RI, the convection in the higher levels then becomes axisymmetric, leading to effective vortex spinup. More WISHE leads to more diabatic heating in the inner-core region and causes relatively strong inflows and efficient spinup (more diabatic heating in the region with high inertial stability). Thus, stronger peak intensity is consequently found in the experiments with more WISHE. In addition, the downward motion in the vortex center is also more active with more WISHE, leading to stronger warm core and lower minimum sea level pressure in TC center. In addition, all ensemble members without capping the WISHE undergo RI, while the onset time of RI diverges in different experiments. Analyses show that the vortices structure, such as the distribution of convection or the wind field, could also be important issue that affects the TC evolutions before RI stage. This suggests that except for WISHE, other processes are also important and need to be considered. Moreover, similar results are found in the idealized simulations, including the delayed RI onset time and weaker TC peak intensity when capping the WISHE. Different from the real case simulations, RI could be identified in all the simulations, including the experiments with highly suppression on WISHE. In the early stage, since there is no environmental VWS and the surface wind speed is still weak, capping the WISHE does not cause significant difference among all the experiments. When TCs intensify to a certain strength, capping the WISHE causes larger difference than during the previous stage, especially the surface heat fluxes in the inner-core region, leading to different RI onset time and peak intensity. To sum up, WISHE could affect the TC evolution both in the pre-RI stage and during the RI stage, especially in the real-case framework.