尺度交互作用對夏與秋季颱風強度發展之影響
Authors: 陳冠杰 陳冠杰 , Kuan-Chieh Chen 鄒治華 
Published: September 2022

<p>本研究利用綜觀尺度擾動(SSE)動能方程式，定量分析尺度交互作用對颱風強度發展的影響。研究結果發現，強烈颱風(C3-C5等級，最大強度&ge;96 knots)的移動速度(發展時間)較弱颱風(C1-C2等級，最大強度64~95 knots)慢(長)，增強率較大。分析強颱移動速度較慢的原因，發現弱颱風與強烈颱風主要受到大尺度副高環流場的導引而移動。然而，強颱伴隨的駛流場較弱，其可能與副高環流場減弱及季風槽和季內震盪(ISO)氣旋式環流的增強有關。此外，強(弱)颱往西北方向(往西以及往北轉向)移動。而強颱增強率較強的原因為其東南-西北向的移動路徑，經過西北太平洋海溫最高、對流層頂溫度最低、垂直風切場最弱，以及ISO擾動動能最大的區域，有利強颱的強度發展。</p>
<p>SSE動能方程式的研究結果顯示，在颱風從生成增強至最大強度的過程中，正壓能量轉換(CK項)與斜壓能量轉換(CE項)均是颱風強度增強的能量來源。CK項中的CKS-M 和CKS-ISO兩項均有正貢獻，即季節平均環流與ISO均傳送能量給颱風發展。然而，在颱風發展後期，強颱與弱颱CKS-ISO差異大，強颱自ISO 獲得較多能量。CKS-ISO差異主要來源為CKS-ISO中的-(〖u_s^&rsquo; v〗_s^&rsquo; (&part;u_I^&rsquo;)/&part;y) ̅ 項，該項與強颱伴隨ISO氣旋式環流(-(&part;u_I^&rsquo;)/&part;y>0)的增強及強颱 〖u_s^&rsquo; v〗_s^&rsquo;向北傳送動量較多有關。因此，當ISO與颱風增強，強颱將自ISO 獲得更多能量。此外，因強颱與伴隨強颱的ISO氣旋式環流移速較慢，在颱風發展後期，強颱與ISO氣旋式環流仍位在暖洋面上。而ISO氣旋式環流南側西南氣流所提供的水氣，亦有利強颱的潛熱釋放，將強颱可用位能轉換成更多的強颱動能。此正回饋效應，使強颱得到較多能量而強度較強。因此，尺度交互作用是颱風強度發展的重要機制。</p>
<p>&nbsp;</p><p>This study quantitatively analyzed the influence of scale interaction on typhoon intensity by using the synoptic-scale eddy (SSE) kinetic energy equation. Our research showed that speed of movement, development time, and intensification rate of intense typhoons (categories 3&ndash;5 with maximum sustained wind speeds greater than 96 knots) are slower, longer, and larger, respectively, than those of weak typhoons (categories 1&ndash;2 with maximum sustained wind speeds between 64 and 95 knots). By analyzing the reasons for the slower speed of movement of intense typhoons, we discovered that weak and intense typhoons are mainly steered by large-scale subtropical high circulation during the intensification process. However, the steering flow of intense typhoons are much weaker which may result from the weakened subtropical high circulation, the enhanced monsoon trough and strengthened intraseasonal oscillation (ISO) cyclonic circulation. In addition, intense typhoons were steered northwestward, while weak typhoons moved westward or northward recurving. The northwestward propagation of intense typhoons experienced the highest sea surface temperature (SST), lowest tropopause temperature, weakest vertical wind shear, and largest ISO kinetic energy region over the western North Pacific (WNP). These large-scale environments were favorable for the growth (a larger intensification rate) of intense typhoons.</p>
<p>Further diagnosis of the SSE kinetic energy budget suggested that the enhancement of typhoon intensity is contributed by both barotropic (CK) and baroclinic (CE) energy conversions during the intensification process. The positive contributions of CKS-M and CKS-ISO in the CK term indicate that both seasonal mean circulation and ISO flow provide energy to typhoons. However, intense typhoons gain more eddy kinetic energy from the ISO flow during the late period of typhoon development. This CKS-ISO difference is dominated by the -(〖u_s^&rsquo; v〗_s^&rsquo; (&part;u_I^&rsquo;)/&part;y) ̅ term associated with the strengthened ISO cyclonic circulation (-(&part;u_I^&rsquo;)/&part;y>0) and the greater momentum transport (〖u_s^&rsquo; v〗_s^&rsquo;) of intense typhoons. Thus, as ISO and typhoons intensified, intense typhoons gain more energy from ISO. In addition, both the intense typhoons and their accompanying ISO cyclonic circulation are still located over the warm ocean due to their slower speed of movement. The moisture provided by the southwesterly flows at the southern flank of this ISO cyclonic circulation was also favorable for the latent heat release of intense typhoons and converted more typhoon available potential energy into typhoon kinetic energy. This positive feedback causes intense typhoons to receive more energy and further results in more intensification of intense typhoons. This research indicates that scale interaction plays an important role in the development of typhoon intensity.</p>
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