颱風路徑與強度敏感性分析–系集預報與標靶觀測觀點

Abstract

本研究使透過使用系集預報的標靶觀測方法及概念，將其應用於了解模式中物理現象對於初始場的敏感性；特別是探討以颱風路徑及強度相關的敏感性。 在路徑敏感性中，我們以2002年的鳳凰及風神颱風為個案，並使用Typhoon-Position-Oriented Sensitivity (TyPOS)進行分析。然而，當系集數量少於獨立變數數量時，TyPOS會有數學上的限制；本研究提出的解決方法是重新定義線性模型中的獨立變數，使每個變數可以代表重要的天氣系統，並降低獨立變數的數量。另外，除了使用由NCEP (National Centers for Environment Prediction) background error covariance 建立初始擾動場以外，亦使用渦旋植入方法使系集中颱風的大小產生變異。敏感性分析指出，在初期的敏感性是颱風周圍的渦度，此結果可能與該處較大的正壓不穩定有關。在中後期，影響風神颱風移動的主要因素是太平洋高壓的強度，而鳳凰颱風的大小亦會些微地影響風神颱風；影響鳳凰颱風移動的因素較為複雜，包含季風槽西側的擾動、風神颱風大小，及雙颱之間的擾動。檢驗雙颱互相作用結果顯示，風神颱風大小對於鳳凰颱風移動的影響較大；另外，環境擾動亦會影響雙颱的相對的位置，使得雙颱作用效果改變。 在強度敏感性中，我們模擬理想環境的颱風發展。分析方法使用淨相關，比較相同颱風強度下，各物理量與颱風增強速率的相關性。結果顯示，增強速率較大颱風的特徵，從動力場而言，包含較強的次環流、眼牆較高的慣性穩定度；從熱力場而言，包含1至3倍最大切向風半徑、2公里以下區域（敏感區）較高的相當位溫及眼牆處較多的潛熱及冰混合比。敏感區較高的相當位溫可以透過平流影響到眼牆邊界層上方，改變垂直穩定性，使得眼牆上方的垂直運動加強；較強的上升運動可使底層較大的絕對角動量平流至高層，進而使眼牆高層處的慣性穩定度增加，並增加加熱效率。較高的加熱效率及較多的潛熱釋放使得颱風中心氣壓可以有效地下降。我們亦發現次環流的增強通常伴隨著中層內流的增強，並會將環境較低相當位溫的空氣往內部傳送，並緩慢地向下逸入邊界層的內流區域，此過程的週期大約為12小時；水汽傳輸及軌跡分析結果指出，中層內流的逸入作用會使敏感區的相當位溫降低。另外，外圍雨帶的發展亦會使敏感區內的相當位溫降低。

In this study, the ensemble-based targeted observation technique is adopted to identify the sensitive factors in typhoon track and intensity. This work can be divided into two parts: (1) track sensitivity, and (2) intensity sensitivity. For track sensitivity, we focus on the sensitivity to vorticity perturbations. A new method based on TyPOS (Typhoon-Position-Oriented-Sensitivity) is proposed to investigate the sensitivity for tropical cyclone (TC) binary interaction, i.e., Fengshen and Fungwong (2002). In the original version of TyPOS, there are some mathematical constraints: when the ensemble size is smaller than the number of independent variables in the multiple linear regression model, the Moore-Penrose pseudoinverse gives the solution with an additional constraint, minimum norm, which may not be the correct solution. We redefine the independent variables in the new method, allowing these variables to better represent the weather system, which can also reduce the number of independent variables. In addition, we also utilize a new sampling strategy, which involves the perturbations based on the background error covariance from National Centers for Environment Prediction, and different TC sizes by TC bogus method, to generate the ensemble. The results show that, in the beginning, the vorticity perturbations surrounding around 500-800 km of the initial TC position are the most sensitive for both TCs, which is related to the barotropic instability occurring near the outer region of TC where the vorticity gradient changes signs. At a later time, the main factor controlling the movement of Fengshen is the strength of subtropical high, while the size of Fungwong also plays a minor role. The factors controlling the movement of Fungwong include the perturbations in the western part of the monsoon trough, the size of Fengshen, and the perturbations located between two TCs. For the interaction between two TCs, Fengshen (with larger size) has a larger impact on Fungwong (with smaller size); the environment perturbations can change the relative location of two TCs, which can also change the effect of binary interaction. For intensity sensitivity, we simulate the storm in an idealized environment condition. The partial correlation is adopted to find the correlation between several variables and the TC intensification rate under the same TC intensity. The results show that the larger intensification rate is positively correlated with stronger secondary circulation, larger inertial stability, larger equivalent potential temperature in 1-3 times of the radius of the maximum wind (RMW), below 2 km (called sensitive region), and larger latent heat release and ice mixing ratio in the eyewall region in mid-to-upper troposphere. The partial correlation between equivalent potential temperature and intensification rate in the sensitive region is about 0.7, which is the largest among all the variables. Further analysis shows that larger equivalent potential temperature in the sensitive region can cause larger conditional instability, and hence larger vertical velocity in the eyewall region. The air with larger vertical velocity can release more latent heat and bring larger absolute angular momentum to the inner core in the mid-to-upper troposphere, which in turn increases the inertial stability and heating efficiency. The larger latent heat along with the increasing heating efficiency helps TC to develop the warm core, and to reduce the minimum sea level pressure. We also find that strength of the mid-level inflow is positively correlated with the strength of secondary circulation. From the results of water vapor transport and trajectory analysis, stronger mid-level inflow may bring low equivalent potential temperature air into the boundary inflow layer, which can reduce the equivalent potential temperature in the sensitive region. The development of the outer rainbands also plays a role in reducing the equivalent potential temperature in the sensitive area.