臺灣博碩士論文加值系統

本研究採用觀測系統模擬實驗（Observing Systems Simulation Experiment；OSSE）的研究概念，透過四維變分資料同化（4DVAR）的方法，以探討颱風渦旋初始化（Typhoon Initialization）的相關問題；此外亦評估一些衛星資料在颱風渦旋初始化程序中所扮演的角色。本研究的重點主要包含兩個方向：1.探討衛星資料所反演之氣象場（例如：雲導風、海面風場、水汽場…...）對於颱風渦旋初始化研究的潛在價值、2.探討虛假資料同化（Bogus Data Assimilation；BDA）颱風渦旋初始化方法的相關研究。
本研究選取1996年薩恩颱風（Zane）作為本OSSE的研究個案。研究進行的程序如下：首先使用MM5兩巢的網格設定進行模擬，藉以產生較佳解析度之理想大氣狀態，然後選取內巢積分第12小時的模擬結果定義為最佳初始資料（BIC）；接著將外巢的模擬結果降低其解析度，以獲得與全球分析資料解析度相似的氣象場（FIC）。而後進行數種資料改善的程序（包括：直接取代或同化BIC中的特定氣象變數、BDA程序中同化由經驗公式決定的軸對稱渦旋結構、以及同化模式積分所產生的非軸對稱渦旋結構），藉以改進上述的FIC。最後再進行60小時的數值模擬，以檢測各種資料改善的程序對颱風初始結構與數值模擬之影響。
首先關於特定衛星資料所反演的風場在颱風渦旋初始化的應用評估上，本OSSE研究結果顯示，即使能有達30公里水平解析的海面風場（如QuikScat風場）及中、高對流層風場（如雲導風）的情況下，並無法直接透過資料同化模式同化這些資料來重建原有的颱風中心結構，而其路徑、強度的模擬也由於其初始結構的不完整，因而產生很大的偏差。而在衛星資料所反演的水汽場評估方面，研究結果顯示，當模式的初始場中具有較正確的水汽分佈時，將造成颱風中心在模擬初期產生明顯的降水，因而造成颱風中心有更多的潛熱釋放，使得颱風強度在初期便明顯的增強，進而改善颱風強度的模擬。雖然水汽場可以加快模式中颱風發展的趨勢，但渦旋初始的強度同樣無法藉由同化水汽場的過程而回復，進而導致模式無法獲得理想的強度模擬。
在現行BDA颱風渦旋初始化方法的探討方面，研究結果顯示，BDA同化軸對稱渦旋結構的程序中，同化風場資料比同化質量場更能獲得良好的路徑模擬及強度演變，此結果乃反應此OSSE研究架構的渦旋系統中，地轉適應的趨勢是由質量場趨近風場的特性。此外，我們認為模式背景場中羅士比變形半徑大小的差異，乃是導致現行BDA相關研究中（Xiao et al. 2000a；Pu and Braun 2001）不一致結論的關鍵所在。當背景場中的颱風位處較高的緯度、有較強的氣旋式風場時，其羅士比變形半徑將較小，則地轉適應的過程將由質量場所主導；反之，對緯度較低、強度較弱的渦旋系統而言，則具有較大羅士比變形半徑，地轉適應的過程將由風場所主導。此外，本OSSE研究個案顯示，藉由經驗公式決定的軸對稱渦旋結構並無法詮釋渦旋本身所具有的非軸對稱特性，尤其是代表颱風移動的指標---風場。我們發展新的BDA程序來解決此一問題，即透過模式積分所產生的非軸對稱渦旋結構來進行同化。
我們亦發現不論是在同化軸對稱、非軸對稱渦旋結構的實驗中，其初期的路徑模擬均呈現過慢的現象，這最主要是因為模式在初期無法掌握正確的駛流所導致。這樣的結果顯示，能否掌握颱風初始移動的駛流，將是決定模式路徑模擬的關鍵因素。因此藉由本OSSE的研究，我們發展透過4DVAR模式來解決此問題的方法，即在同化的過程中將同化時間變長，並且給定不同的颱風位置作為同化過程的約束條件，則能有效克服模式在初期無法掌握正確駛流的問題。
本研究亦透過OSSE的研究策略，檢驗Soden et al.（2001）與Tuleya and Lord（1997）研究中凸顯環境流場於颱風數值模擬的角色。此外，亦藉由其他模式來進行恆等孿生（identical twin）問題的檢驗，藉以確認我們的研究結果。

摘要 -----------------------------------------------I
致謝----------------------------------------------III
目錄-----------------------------------------------IV
圖表說明-------------------------------------------VI
第一章 前言與研究目的-------------------------------1
1.1 前言--------------------------------------------1
1.2 研究目的----------------------------------------5
第二章 研究策略與工具-------------------------------6
2.1 研究策略----------------------------------------6
2.2 研究工具----------------------------------------7
2.2.1 MM5模式---------------------------------------7
2.2.2 MM5-4DVAR模式---------------------------------7
第三章 OSSE研究流程與實驗設計----------------------10
3.1 OSSE研究流程-----------------------------------10
3.1.1 產生最佳觀測資料-----------------------------10
3.1.2 定義BIC與FIC---------------------------------11
3.1.3 資料同化-------------------------------------12
3.1.4 數值模擬-------------------------------------12
3.1.5 定義觀測資料---------------------------------13
3.2 實驗設計---------------------------------------16
第四章 特定資料對颱風渦旋初始化之影響探討----------20
4.1 模式側邊界條件之敏感性探討---------------------20
4.2 MM5-4DVAR模式結果探討--------------------------20
4.2.1 4DVAR之表現情形------------------------------20
4.2.2 背景、觀測協方差矩陣之敏感性探討-------------22
4.3風場、氣壓場於颱風結構中之地轉適應--------------23
4.4 似衛星風場資料對颱風渦旋初始化之影響探討-------25
4.5 似衛星水汽場資料之影響探討---------------------27
4.6 GPS/MET折射率資料之影響探討--------------------29
4.7 討論-------------------------------------------31
4.7.1 四維變分資料同化-----------------------------31
4.7.2 衛星遙測資料與颱風渦旋初始化-----------------32
4.7.3 地轉適應與颱風渦旋初始化---------------------34
第五章 BDA颱風渦旋初始化方法之研究探討與改進-------37
5.1 現行BDA颱風渦旋初始化方法之探討----------------37
5.2 BDA颱風渦旋初始化方法之改進--------------------40
5.3 颱風初始移動駛流之問題與修正-------------------42
5.4 颱風渦旋初始化方法之比較-----------------------43
5.5 真實個案之應用探討-----------------------------44
5.6 討論-------------------------------------------47
5.6.1 BDA相關研究的地轉適應問題--------------------47
5.6.2 BDA颱風渦旋初始化方法之改進------------------49
第六章 環境資料對颱風模擬影響之探討----------------50
第七章 WRF模式之恆等孿生(identical twin)問題檢驗---52
第八章 結論----------------------------------------57
參考文獻-------------------------------------------63
附圖-----------------------------------------------69

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