策略性颱風觀測理論之動力分析與比較

Abstract

由颱風觀測計畫所衍生之策略性(自適性)颱風觀測，除能在實際應用上提供設定觀測策略之參考外，亦可作為分析與探討颱風動力問題上的有效工具。本研究以2006年發生於西北太平洋的19個熱帶氣旋為背景，以策略性颱風觀測理論之一的美國海軍全球預報模式奇異向量，深入探討熱帶氣旋與其周圍大尺度系統間的動力關係，結果顯示奇異向量敏感結果可分為兩大類，其最大敏感值分別出現在「熱帶氣旋之入流區」與「相對於氣旋中心之徑向風速極小的區域」，由合成分析發現，此兩大分類在不同季節以及不同熱帶氣旋生命期會顯現不同特徵，且與熱帶氣旋及其周圍大尺度系統之相對配置息息相關，其中「最大敏感值出現在徑向風速較弱區域」的結果在之前關於熱帶氣旋與奇異向量的研究中從未被提及，是一相當有趣與創新的發現，顯現出由於氣旋本身環流以及其他大尺度系統(例如副熱帶高壓系統與中緯度槽線系統)之環流相互作用下產生的合流區域在颱風移動預報上的敏感性。另外，透過奇異向量在垂直方向上的分析，可獲得動能與位能之結構分佈在初始與終端時間的變化，而對不同分類間的差異亦有詳細探討。 除奇異向量在熱帶氣旋移動上的動力探討，本研究亦統合比較目前應用於西北太平洋上幾種主要的熱帶氣旋敏感結果，在國際間的學術合作下，獲得6種不同模式同時針對2006年西北太平洋的19個熱帶氣旋，以「奇異向量」、「颱風駛流敏感共軛向量」、「系集平均深層風變異」與「系集卡曼濾波器」4種理論計算出之6種不同的敏感性結果。藉由定量上的統計方法比較此6種結果在綜觀尺度與颱風尺度上的相似程度，發現以不同模式計算之「奇異向量」彼此間有最高的相似性，而「系集平均風變異」與其他敏感性結果的比較則顯現低相似性，另外「颱風駛流敏感共軛向量」與「系集平均深層風變異」常在鄰近熱帶氣旋的區域出現敏感性，因而兩者在統計上顯現出高相似度。另外，研究中亦針對3個分別主要受到「副熱帶高壓系統」、「中緯度槽線系統」與「副熱帶噴流系統」影響之個案進行分析。整體結果顯示，由共軛模式技術計算之敏感性結果與可能影響颱風移動或發展之動力特徵的吻合度較高，而以系集技術計算之敏感結果較不容易掌握可能影響颱風之大尺度動力特徵。 上述研究結果除可以提供未來觀測計畫與颱風預報參考之外，同時亦藉由對與颱風基礎動力有關之特徵的瞭解，進一步增加模式對於颱風預報能力的掌握，為策略性颱風觀測理論的額外貢獻。

Targeted (adaptive) observation of tropical cyclones can not only provide the reference to the operational observing missions/experiments but be a useful tool to study the typhoon dynamics. In this study, the leading Singular Vectors (SVs) constructed by the Navy Operational Global Atmospheric Prediction System (NOGAPS) forecast and adjoint systems are used to examine and classify the dynamic relationship between tropical cyclones and synoptic-scale environmental features that influence their evolution through the composite techniques. The NOGAPS initial SVs show several different patterns that highlight the relationship between the TC forecast sensitivity and the environment during the western North Pacific typhoon season in 2006. In addition to the relation of the SV maximum to the inward flow region of the TC, there are three patterns identified where the local SV maxima collocate with low-radial-wind-speed regions. These regions are likely caused by the confluence of the flow associated with the TC itself and the flow from other synoptic systems, such as the subtropical high and the mid-latitude jet. This is the new finding beyond the previous NOGAPS SV results on TCs. The sub-seasonal variations of these patterns corresponding to the dynamic characteristics are discussed. The SV total energy vertical structures for the different composites are used to demonstrate the contributions from kinetic and potential energy components of different vertical levels at initial and final times. Besides the above dynamical analysis, this study also compares six different guidance products for targeted observations over the Northwest Pacific in 2006, and highlights the unique dynamical features affecting the TC tracks in this basin. The six products include 3 types of guidance based on total-energy singular vectors (TESVs) from different global models, the ensemble transform Kalman filter (ETKF) based on a multi-model ensemble, the deep-layer mean (DLM) wind variance, and the adjoint-derived sensitivity steering vector (ADSSV). The similarities among the six products are evaluated using two objective statistical techniques to show the diversity of the sensitivity regions in large, synoptic-scale domains, and smaller domains local to the TC. It is shown that the three TESVs are relatively similar to one another in both the large and the small domains while the comparisons of the DLM wind variance to other methods show rather low similarities. The ETKF and the ADSSV usually show high similarity because their optimal sensitivity usually lies close to the TC. The ADSSV, relative to the ETKF, reveals more similar sensitivity patterns to those associated with TESVs. Three special cases are also selected to highlight the similarities and differences between the six guidance products and to interpret the dynamical systems affecting the TC motion in the North western Pacific. The adjoint methods are found to be more capable of capturing the signal of the dynamic system that may affect the TC movement or evolution than the ensemble methods.