外核海表熱通量對於颱風快速增強的影響

Abstract

在2015年的颱風季，蘇迪勒颱風成為西北太平洋地區最強的熱帶氣旋，並且曾經歷快速增強(Rapid Intensification, RI)，在48小時之內其中心氣壓下降90hPa。本研究使用全物理(full-physics)的WRF模式，以1.67公里的高網格解析度成功重建蘇迪勒颱風的路徑走向及增強趨勢，並以此結果為控制組實驗。 為了評估颱風內核(inner core, 距颱風中心60公里以內上升運動明顯之區域)以外之海表熱通量對於颱風結構以及快速增強過程的影響，本研究以1m/s之地面風速，在計算中大幅限制外核區域海表熱通量，並於控制組實驗發生快速增強前24小時執行一系列之敏感性實驗模擬，檢驗不同區域之海表熱通量對於颱風快速增強的敏感性。實驗結果顯示，限制2.5倍內核半徑(150公里)以外之海表熱通量導致颱風在快速增強時期當中，有更大之增強速率、較小之颱風壯度及最大風速半徑。然而，同時限制1.0 – 2.5倍內核半徑(60 – 150公里)之海表熱通量則使颱風發展緩慢，無法經歷快速增強時期。 在敏感性實驗之颱風內核結構中，經歷快速增強的颱風有較強之中高層上升運動、較高之軸對稱程度，以及較大之加熱效率。此外，高層暖心在快速增強時期迅速建構，但是在未發生快速增強之颱風中心附近則無明顯之高層暖化現象。直覺上，人為大幅減少颱風外核區域之海洋能量供給對於颱風發展有負面影響，但於經歷快速增強的颱風內核中，則有更強之海表熱通量，歸因於地面風速也急遽增強，促進海氣熱量交換。由於限制海表熱通量區域上方之較乾空氣隨著邊界層內流以及中高層逸入作用入侵颱風內核外側區域(約距中心80公里)，使該區域穩定化，因此對流集中發展於內核區域，在增多之海洋能量供給下，發展更為旺盛，造成大量渦度集中產生於颱風內核，颱風內核風速急遽增大，導致颱風迅速增強。 綜合以上的研究結果，我們提出一個觀念：限制颱風所處海域之能量供給不一定使颱風增強速率減緩，取決於限制之海域與颱風中心的距離；換句話說，限制颱風外核區域之海表熱通量將可能導致潛熱更集中釋放於颱風內核，使颱風增強更迅速。

Typhoon Soudelor was the most destructive tropical cyclone (TC) in the western North Pacific in 2015, undergoing rapid intensification (RI) with the central minimum sea-level pressure (MSLP) drop of 90 hPa in two days. In this study, a 1.67-km convection-permitting full-physics model simulation is conducted with the track and intensification trend of Soudelor well captured. To investigate how the surface heat fluxes outside the inner core (IC, the inner core region within the radius of 60 km) affect TC structure and RI process, a series of numerical experiments with the surface wind highly capped at 1 m/s in the calculation of surface latent and sensible heat flux in different radial extent are performed. It is found that the intensification rate is larger than that of the control experiment (CTRL) during RI when the surface heat fluxes are suppressed in the area 150-km (2.5 times of the IC size) away from the TC center, while the TC is significantly weaker and does not undergo RI when the surface fluxes are also suppressed at 60 to 150-km radius (1 - 2.5 times of the IC size). In addition to intensity change, substantial reduction of surface fluxes outside the inner core leads to lower TC strength and smaller radius of maximum wind (RMW), indicating that the most violent winds concentrate in the inner-core region. As to the inner-core feature in each experiment, the RI cases show stronger mid- to upper-level updrafts, higher axisymmetricity and heating efficiency than that of non-RI cases during convective burst times before RI in CTRL. Furthermore, the upper-level warm core develops significantly during RI, while no evident upper-level warming is found in non-RI cases. Although the surface fluxes outside the inner core in these RI cases are substantially suppressed, stronger intensity and more consolidated inner-core structure than that of CTRL is identified associated with the abundant wind-induced surface heat exchange (WISHE) in the inner core. The stabilization of lower troposphere outside the inner core in RI cases leads to aggregation of deep convection and subsequent generation of potential vorticity near the TC center, concentrating the violent winds in the inner-core region. Special insight is identified that the limitation of surface heat fluxes does not always result in a reduction of TC intensification rate. In other words, if the surface heat fluxes are suppressed in the outer region, against one’s physical intuition, TC can possibly turn even stronger.