| 摘要: | 熱帶氣旋強度常被觀測到有突然增強的情況且可能會對經濟民生造成嚴重影響。目前已有許多研究探討熱帶氣旋強度突然改變的原因。除了熱帶氣旋本身的動力機制外，也有些研究顯示海洋裡的暖渦對熱帶氣旋強度改變有很明顯的貢獻。本研究的目的是由氣候觀點探討熱帶氣旋強度突然改變的現象，主要研究區域為西北太平洋。同時我們也期待這次的研究成果能為氣候模式在劇烈天氣的模擬與掌握上提供可以比對的統計資料。這份計畫可分為兩部分：第一部份著重在分析熱帶氣旋強度改變的詳細情況。我們將使用數個預報中心提供的最佳路徑資料來分析並比較差異。目前我們的初步結果為使用Joint Typhoon Warning Center (JTWC) 的最佳路徑資料作分析，由初步結果得知在西北太平洋的增強熱區會隨季節南北移動，其中最令人感興趣的結果是此增強熱區的空間分布於1977年左右有明顯的改變，與十年際震盪有很高的相關。由於颱風強度資料在早期並不是很精確，我們首先將採用數個不同資料來驗證此現象。並進一步做更細緻的分類及統計。此計畫的第二部分為探討造成颱風強度變化有十年際變異的成因。由初步研究結果來看，颱風強度改變與太平洋十年際震盪(PDO)指數有明顯相關。我們將會檢視數個關鍵的氣候變數，例如低層渦度場、垂直速度場、中層水氣量、高層輻散場、海溫等資料，找出可能造成此長期變異的原因。未來若有長期且高解析度的氣候模擬資料，也可應用此分析方法來評估氣候模式在颱風強度變化趨勢的模擬上是否表現合理。在此研究中，我們試圖解釋的科學問題有：(1)熱帶氣旋的活動存在有哪些氣候變異度？(2) 什麼機制或過程造成此種變異？(3) 氣候模式是否能模擬出此種變異及是否透過正確的機制產生。
The intensity of tropical cyclone (TC) often changes dramatically and may cause damages. There are several processes that may cause such intensity change, for example, through the reorganization of the concentric eye wall. It is also found that the intensification can have contribution from oceanic warm eddies in the recent years. This project aims to explore this issue from the climate point of view. The main motivation of this research is to study the climate variability on TC climatology, especially low-frequency variation. We also expect the result of this study can provide statistical information for comparison with climate model simulation on extreme weather. The work we propose can be divided into two parts. The first part investigates the TC intensification rate, including tropical depression stage, during 1945 to 2010 using best track data from different warning agents. We will first explore the nature of the intensity change in general in the western North Pacific during the typhoon season. From our preliminary result using the Joint Typhoon Warning Center (JTWC) best track data, we have found that there is a hot zone that migrates seasonally with sun. The most intriguing result is that the spatial pattern of the hot zone shows a phase change around 1977, coincident with Pacific Decadal Oscillation (PDO). Since it is well known that the intensity data is not accurate in early period. It is necessary to use data from different warning agents to confirm this phenomenon. The second part will be analyzing the possible mechanisms that cause the decadal variability on TC intensity change. From our preliminary result, the correlation between PDO index and TC intensity change indicates that during the PDO warm phase, the low latitude hot zone tends to be smaller, and the weakening of TCs over higher latitudes becomes less significant as well. In other words, the TC intensity tends to change less during the PDO warm phase on average. We will look at some essential climate variables, such as low-level vorticity, vertical wind shear, mid-level moisture, upper-level divergence, sea surface temperature, and so on. If long-term and high resolution climate simulation is available, it is also possible to apply the same analysis on simulated data and evaluate the model performance. The scientific questions we will answer are (1) What climate variability is found in TC activity; (2) What mechanisms or processes are responsible for producing this variability; (3) Does climate model simulate this variability well and through the right processes? |
