颱風雙眼牆形成動力研究──非平衡動力的延伸探討

Abstract

衛星觀測研究顯示，發展至一定強度的熱帶氣旋除了內側眼牆外也常在生命期中經歷外眼牆的生成。自此觀點出發，外眼牆的生成動力與熱帶氣旋內部動力的連結成為研究關注的議題，其中邊界層內的非平衡動力機制為近年熱門的研究議題之一。本研究借由WRF-EnKF模式進行兩個部分的研究工作：1) 探討軸對稱風速剖面的資料同化量對辛樂克颱風(2008)雙眼牆模擬的影響，2) 重新檢視邊界層非平衡動力機制在高模式水平解析度(1.67 公里)模擬實驗中的角色。 第一部分的研究，模式的水平解析度為5公里，風速剖面只在有飛機穿越觀測前後的2-3小時同化至模式中，此風剖資料同化量較少的實驗無法成功模擬辛樂克颱風中的雙眼牆結構，但能夠合理呈現內眼牆的增強過程，顯示在模式水平解析度不足的情況下，外眼牆相較於內眼牆對風剖資料同化的依賴程度更甚，颱風軸對稱風剖的最佳資料同化量值得後續研究加以釐清。 第二部分的研究採用較高的水平解析度(1.67 公里)進行數值實驗，使用第一部分研究於9月10日03 UTC以及15 UTC產生的數值資料做為兩個模擬實驗(EXP1003-4D及EXP1015-4D)的初始場，初始時間較早的實驗無法成功產生雙眼牆結構，初始時間較晚的實驗則能夠模擬雙眼牆的生成與發展。和先前5公里解析度的實驗比較，這兩個提高解析度的數值實驗皆提供渦旋演變過程中更細部的結構特徵，渦漩的非軸對稱部分明顯、眼牆處的位渦出現多角形結構，顯示非軸對稱動力在高解析度模擬中可能扮演不可忽略的角色，其對雙眼牆形成的影響仍需要更進一步的研究。雙眼牆出現的個案呈現明顯的外眼牆結構，且超梯度風在外眼牆形成的半徑區間發展，並於外眼牆建立前發展出第二個顯著的極值。由提高解析度的EXP1015-4D實驗結果可知，當模式的水平解析度足夠時，即使颱風軸對稱風速剖面的資料同化量減少，此架構下的模式動力能夠提供外眼牆形成及發展的有利渦旋內部動力條件。

Observational studies based on satellite data have shown that mature tropical cyclones (TCs) often experience secondary eyewall formation (SEF) during their lifetime. This brings concerns for the essential role of intrinsic dynamics of a TC vortex in SEF. Among many proposed intrinsic dynamical mechanisms, of particular interest in this study is the role of unbalanced dynamics within and just above the boundary layer, a recently-presented dynamical pathway to SEF. A modified version of WRF-EnKF is used to answer two key questions about the SEF problem: 1) what is the impact of the amount of data assimilation that is involved in the operational parameter of the axisymmetric tangential wind profile on simulating SEF in Typhoon Sinlaku (2008), and 2) what is the role of unbalanced dynamics associated with boundary layer processes presented in numerical experiments with higher horizontal resolution (1.67 km). In the first part of this study, wind profiles based on C-130 flight observations were assimilated into the model within a time window of 5-6 hours around the observing time. This experiment, with a reduced data amount of assimilated wind profiles, is unable to reproduce the secondary eyewall structure of Sinlaku, while intensification of the inner eyewall is reasonably captured. This result infers that the outer eyewall appears more sensitive to the reduced amount of assimilated wind profiles when coarser model resolution of 5 km is adopted. The optimal data assimilation amount of the TC wind profiles to be able to simulate SEF remains an interesting issue to be further investigated. In the second part, numerical data carried out in the part one study are used to provide the initial condition for two data-denial experiments with higher model resolution (1.67 km). The two experiments are run from 03 UTC and 15 UTC 10 September, named EXP1003-4D and EXP1015-4D, respectively. Compared with the 5-km-resolution experiment, the two simulations present more details of the evolved TC vortex. The asymmetry of vortex is more distinct and active; for instance, the inner eyewall ring has a polygonal structure of potential vorticity, quite different from the 5-km simulation. This suggests that the asymmetric dynamics needs to be further addressed for the SEF problem, particularly in model simulations with high resolution. The structure of the outer eyewall the high-resolution simulation (EXP1015-4D) is robust, and supergradient forces develop and establish a secondary maximum over the radii where SEF occurs, supporting the recently-proposed unbalanced pathway to SEF. The result of EXP1015-4D also implies that even though the amount of assimilated data is limited, given adequately fine model resolution, the model dynamics can capture the formation of the outer eyewall.