- 學位論文
西北太平洋與北大西洋雙眼牆颱風研究
Concentric Eyewall Typhoons in the Western North Pacific and North Atlantic Basins
摘要
本研究利用微波衛星產品為基礎，搭配最佳路徑資料，觀察1997年至2012年西北太平洋(WNP)颱風和北大西洋(NATL)颶風強度與結構變化，特別是雙眼牆形成和其後的眼牆置換過程會有特殊的變化情形。僅大西洋和東太平洋有對熱帶氣旋例行性的機載觀測，過去研究在大西洋上使用K-Vmax (動能-最大持續風)圖，K來自機載觀測之計算、Vmax表示強度，能同時展現颶風環流尺寸伴隨強度的消長，在西北太平洋上則使用T-Vmax (對流強度-最大持續風)圖，T來自雲頂亮度溫度(TB)以彌補機載觀測之不足。 楊憶婷(2012)博士論文提出用TB影像對雙眼牆(Concentric Eyewall, CE)客觀定義：(1)有moat結構、(2)moat足夠明顯、(3)外眼牆對流夠深、(4)軸對稱化程度超過5/8方位、(5)外圍環流為外眼牆而非螺旋雨帶，並將雙眼牆分為ERC (Eyewall Replacement Cycle)、CEM (Concentric Eyewall Maintained)和NRC (No Replacement Cycle)，另也依據海洋聖嬰指數(ONI)將個月分分成暖期、冷期和正常時期(Warm, Cold ,and Normal Episodes)進行雙眼牆的統計分析。本研究對16年WNP颱風以TB外其它方式或微波衛星資料補充之，找出氣候平均T-Vmax迴歸線，發現正常時期、CEM個案距平值(ΔT)為正；CE個案雲中液態水含量((CLW) ‾)比非雙眼牆(non-CE, NCE)個案高，不同CE種類間(CLW) ‾則是CEM個案依序比NRC、ERC高；上層海洋SST、T100A和UOHC 3種熱力參數顯示CE形成前、後分別是NRC、CEM個案海氣交互作用最大。 由於微波衛星資料資料涵蓋範圍遍及全球，提高比較各大洋熱帶氣旋觀察的方便性，本研究以K’-Vmax圖和T-Vmax圖觀察Wilma颶風雙眼牆過程之強度、結構尺寸和對流強度的消長；以T時間序列和內眼牆半徑、moat寬度、外眼牆寬度觀察Frances颶風5次雙眼牆過程之強度和結構變化，顯示ERC使颶風結構向外成長，NRC則否。對NATL雙眼牆個案進行不同季節、不同ONI分類的ENSO狀態和不同CE種類之行經路徑、個數、強度變化和結構統計的分析，發現兩大洋之統計比較主要特徵有：(1) NATL雙眼牆個案十分不均勻地集中在9月；(2) NATL雙眼牆路徑統計和生成頻率在不同ENSO狀態下彼此區別比WNP不明顯，可能受ENSO影響較小；(3) WNP雙眼牆個案強度比NATL大，不同CE種類當中WNP、NATL分別是CEM類、ERC類強度最強；(4) WNP雙眼牆個案結構尺寸比NATL大，而兩大洋不同雙眼牆個案尺寸都是CEM類最大，都是的無因次化快速帶狀化區域寬度皆占moat寬度變異度50%以上，都是NCE和其它CE的T-Vmax曲線特徵差異明顯。
並列摘要
In this study, microwave satellite products and best track data were used to observe the changes of intensity and structure of the Western North Pacific (WNP) typhoons and the Atlantic (ATL) hurricanes in particular concentric eyewall cases (CEs) from 1997 to 2012. Around the globe, only over the Atlantic and Eastern Pacific basins do routine airborne TC observations be carried out. Previous study had used airborne observations to design K-Vmax diagram, which can simultaneously show maximum sustained wind (Vmax) and circulation size determined from accumulated kinetic energy (K). In this study, similar plots for WNP typhoons were made, which are T-Vmax diagram where T is from cloud-top brightness temperature (TB) to indicate convective activity (CA) and to compensate the lack of airborne observations. CE was objectively defined by 5 criteria: (1) moat exists, (2) most is clear enough, (3) outer eyewall is strong enough, (4) circulation is axisymmetric enough, and (5) outer eyewall can’t be a spiral-out rainband. CEs were divided into Eyewall Replacement Cycle (ERC), Concentric Eyewall Maintained (CEM), and No Replacement Cycle (NRC). Each month was discerned into warm, cold, and normal episodes based on Ocean El Nino Index (ONI). In this study, climate average linear regression of T was obtained, and the anomaly is symbolized as ΔT. Over the WNP basin, CEM and normal episode CEs have positive ΔT. There is more cloud liquid water content ((CLW) ‾) in CEs than in non-CEs (NCEs), and among CE categories, ((CLW) ‾) is higher in CEM cases than NRC and ERC. 3 upper ocean thermal parameters, SST, T100A, and UOHC, showed that before (after) CE formation, there is more air-sea interaction in NRC (CEM) cases. In this study, hurricane Wilma (2005) was analyzed by K’-Vmax diagram and T-Vmax diagram to observe intensity and structural change. 5 repeatedly-formed CEs of Hurricane Frances show hurricane size expands after ERC rather than NRC. Followings are comparisons between CEs over WNP and NATL. First, the number of NATL CEs concentrates almost only in September. Second, NATL CE tracks and formation frequencies in different episodes have smaller variation than WNP, which may due to smaller influence by ENSO. Third, WNP CEs have higher intensity than NATL, and among CE categories, CEM (ERC) cases have the highest intensity over the WNP (NATL) basin. Forth, WNP CEs have larger size than NATL. Over both basins, CEM cases have the highest size of all CEs, rapid filamentation zone width of each CE category determines at least 50% of variation of moat width, and obvious characteristic difference of T-Vmax curve between CE and NCE.