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研究生:余俊宏
研究生(外文):Chun-Hung Yu
論文名稱:斜坡上底邊界層的對流溫降與湧升機制-以澎湖水道為例
論文名稱(外文):Mechanism of bottom convection and upwelling over continental shelf
指導教授:曾若玄曾若玄引用關係
指導教授(外文):Ruo-Shan Tseng
學位類別:碩士
校院名稱:國立中山大學
系所名稱:海洋科學系研究所
學門:自然科學學門
學類:海洋科學學類
論文種類:學術論文
論文出版年:2018
畢業學年度:106
語文別:中文
論文頁數:69
中文關鍵詞:澎湖水道剪切對流底邊界層艾克曼傳輸渦流黏滯係數沿岸湧升流
外文關鍵詞:Ekman transportEddy viscosityCoastal upwellingPenghu channelShear-induced convectionBottom boundary layer
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本研究於2015年9月及2016年4月在澎湖水道利用垂直紊流剖面儀 (VMP) 進行實測,以及分析1999年~2000年於澎湖水道北側的底碇海流剖面資料來探討海流於上坡時對海床邊界層不穩定混合的降溫效應以及底層艾克曼傳輸現象。澎湖水道的潮型為M2潮,由VMP的觀測中也發現澎湖水道底邊界的紊流強度呈半日周期的變化,渦動能消散率 (ε) 明顯出現於漲潮最大流速之後,其受紊流影響的底邊界層厚度可達離底40米,最大ε可達2.29×10-5 W kg^-1,紊流能量主要來自強流與底邊界的剪切應力。從水文資料中發現在漲潮時水深80~105米之間密度出現強烈的水團轉置不穩定,退潮時則呈現均勻混合,另外於底碇上的溫度資料也顯示底層海水溫度在漲潮時普遍下降攝氏2~3度,表示在水團爬坡時會產生底層結構不穩定並混合降溫的現象,本研究將此現象定義為平行水道方向的湧升流,且底層週期性降溫的現象在夏天較為明顯。本研究也分析底碇流速資料與估算渦流黏滯係數,發現底邊界層的流向存在週期性的艾克曼偏轉,偏轉角度於漲潮轉退潮 (流速小於1 m s^-1) 時較為明顯,比較渦流黏滯係數與由VMP觀測到的渦流擴散係數,發現兩者的數值相近,夏天時渦流黏滯係數剖面在底邊界層內變化不大且分佈較平滑,有利於艾克曼現象的產生。綜上所述,夏天平行水道方向的湧升流,以及漲潮轉退潮時明顯的艾克曼傳輸效應,兩者連續性出現的週期性現象會加強澎湖地區的湧升流。
Turbulence, current, and stratification were measured in the northern section of Penghu Channel (PHC) during September 2015 and April 2016. Two datasets of current profiles and bottom temperature from bottom-mounted ADCP in summer and winter of 1999 at about 10 km north of PHC were used for bottom Ekman layer and eddy viscosity analysis. Our results indicate that the cycle of turbulence in the bottom boundary layer has a semi-diurnal period. Dissipate rate of turbulent kinetic energy, ε, is higher during the flood phase and reaches a maximum value of 2.29×〖10〗^(-5) W 〖kg〗^(-1). Turbulent layer extends from the bottom to a height of 40 m, is produced primarily by vertical shear of tidal currents. Density profiles show that stratification is eroded within depths of 80-105 m during the flood, and the bottom layer becomes well mixed during the ebb. Temperature observations from the bottom-mounted ADCP reveal periodic occurrence of temperature decrease of 2-3℃ during the flood, implying shear-induced convective cooling near the bottom during the upslope flow. This phenomenon is more prominent in the summer which contributes to the along-channel upwelling in the PHC. Ekman veering of the current profiles near the bottom occurs periodically with tidal oscillation and is more obvious when the flow speed is less than 1 m s-1 during the summer. A comparison of the profiles of estimated eddy viscosity and observed eddy diffusivity indicates that the orders of magnitude of these two quantities are close. Eddy viscosity has a maximum value at the bottom and decreases upward. In summary, the combining effect of along-channel, shear-induced convection and the cross-channel, bottom Ekman dynamics will act sequentially and periodically to promote upwelling in the PHC.
目錄
論文審定書 i
誌謝 ii
摘要 iv
Abstract v
目錄 vii
圖目錄 ix
表目錄 xi
參數及符號 xii
一、 前言 1
1-1 緒論 1
1-2 前人研究 1
1-2-1 海床邊界層的爬坡降溫效應 1
1-2-2 底邊界層的艾克曼效應 2
1-2-3 澎湖群島的沿岸湧升流 5
1-3 研究動機 6
二、 觀測方法與資料來源 7
2-1 垂直紊流剖面儀(VMP-250) 7
2-1-1 溫鹽度探針 (CT Sensor) 8
2-1-2 剪切探針 (Shear probe) 9
2-1-3 熱敏探針 (Thermistor, FP07) 10
2-2 航次實驗介紹 12
2-3 底碇歷史資料 13
三、 資料分析方法 15
3-1 渦動能消散率(Epsilon, ε) 與渦流擴散係數 (Eddy Diffusivity, Kρ) 15
3-2 渦流黏滯係數 (Eddy Viscosity, μ) 20
四、資料呈現 25
4-1 海研三號航次OR3-1887 25
4-1-1 斜坡地形造成的底層水團翻轉 26
4-1-2 底層的渦流擴散係數 (Kρ) 28
4-2 海研三號航次OR3-1923 30
4-2-1 斜坡地形造成的底層水團翻轉 32
4-2-2 底層的渦流擴散係數 33
4-3 夏季底碇資料 (wh11) 34
4-3-1 流速與潮汐分析 34
4-3-2 溫度變化分析 40
4-4 冬季底碇資料 (wh21) 41
4-4-1 流速與潮汐分析 41
4-4-2 溫度變化分析 44
五、 討論 46
5-1 平行水道方向的爬坡降溫 (Along Channel) 46
5-2 澎湖水道橫向湧升流 (Cross Channel) 48
5-3 澎湖群島的強烈降溫效應 50
六、 結論 51
七、 參考文獻 53
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