近岸海洋波浪對海面粗糙度之影響; On the Gravity Wave and Sea Surface Roughness Relationship in Coastal Ocean

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Title:	近岸海洋波浪對海面粗糙度之影響;On the Gravity Wave and Sea Surface Roughness Relationship in Coastal Ocean
Authors:	苑瀞丰;Ching-Feng Yuan
Contributors:	水文與海洋科學研究所
Keywords:	拖曳係數;近岸海域;海面粗糙度;粗糙長度;海面平均坡度;Coastal ocean;Sea surface roughness;Drag coefficient;Roughness length;Mean square slope
Date:	2011-07-18
Issue Date:	2012-01-05 14:11:34 (UTC+8)
Abstract:	海面粗糙度(sea surface physical roughness)是海洋表面幾何起伏程度，受到海洋表面波浪狀態與大氣邊界層動力(Aerodynamic)影響，是大氣-海洋交互作用中決定動量、熱量與水汽等交換之主要因子。海面粗糙度特性在近岸海洋(coastal ocean)受波浪淺化效應影響，與深海有顯著的差異，了解近岸海洋海面粗糙度特性對於氣候變遷、水循環、碳循環、洋流模式推算風暴潮或海岸溢淹、海洋遙測技術開發以及離岸風能評估運轉等十分重要。以往多以大氣邊界層特性參數，拖曳係數(Drag coefficient, Cd)和粗糙長度(Roughness length, Z0)來間接描述海洋表面之粗糙度，但尚未能完全反應波浪所造成的影響。本研究為探討於近岸海洋表面粗糙度特性，嘗試使用海面平均坡度(Mean square slope, MSS)作為指標來描述海面粗糙度。本研究採用觀測為手段，針對大氣邊界層海氣通量、海面波浪與海流、岸基微波雷達三項目，於桃園縣新屋鄉中央大學臨海工作站近岸海域進行同步觀測。時間為東北季風盛行時期：2011年1月14日至2011年1月31日共18日，觀測週期為20分鐘，共1244筆資料。資料分析方面，拖曳係數的估算是將收集到的三維風速利用渦流相關法分析而得；粗糙長度的估算是利用風剖面法與波譜估算法分析而得；海面平均坡度的估算是將底碇式都卜勒聲學流速剖面儀量測得水位時間序列經傅立葉轉換和頻散關係式得到波數域頻譜，再由波數域頻譜推導坡度譜，由坡度譜積分得海面平均坡度。海面平均坡度與完整波數域頻譜高頻尾端形狀有很大的關係，為了了解近岸海域波數域頻譜高頻帶特性，本研究依據前人於深海觀測之譜形假設六種不同波數域頻譜高頻尾端形狀，並以不同波數門檻為分界，區分高頻海面平均坡度和低頻海面平均坡度。其中，低頻海面平均坡度與波浪特性相關；高頻海面平均坡度與風速特性相關。本文以S波段雷達絕對回波強度作為依據，決定完整波數域頻譜形狀高頻尾端特性。最後探討此完整波數域頻譜所計算出之高頻海面平均坡度與風速、低頻海面平均坡度與波齡之相關性，並計算此地區的波浪狀態對海面粗糙度之貢獻程度。本研究結果顯示，完整波數域頻譜尾端斜率以k-4遞減較合乎雷達觀測特性。積分波數分界約為42.3?g/U102時所計算出之高頻海面平均坡度與風速比對結果較吻合Cox and Munk(1954)提出之光滑表面(slick surface)下風速與海面平均坡度之線性結果。低頻海面平均坡度在波齡(Cp/U10)約等於0.8時會呈遞減現象，代表波齡較大的波浪波長較大、波高較小、波浪尖銳度下降使得海面平均坡度變小，海面粗糙程度較低，與前人研究例如：Donelan(1990)提出波齡與無因次化粗糙長度之結果相近，海面粗糙度與波齡的關係為隨著波齡變大而遞增至一臨界值後開始遞減。由此結果所得本實驗地區波浪對海面粗糙度的貢獻程度約佔三分之二。 Physical surface roughness represents the actual elevation variations in various scales at the air-sea interface. It is affected by the gravity waves and the aerodynamic factors in the atmospheric boundary layer. It is one of the most crucial factors that determine the momentum, heat and water vapor exchange between the air and sea. Due to the shoaling of gravity waves in the coastal ocean, sea surface roughness features significant deviation compared with those observed in deep seas. The understanding of the characteristics of surface roughness in coastal ocean is of great importance since it has much influence on the climate change, water cycle, carbon cycle, wind driven current/storm surge predictions and the assessment and operation of offshore wind energy conversion. So far, parameters of Drag coefficient, Cd and Aerodynamic Roughness length Z0 are used to infer the sea surface roughness. However, they do not fully reflect the contribution of gravity waves to the surface roughness. In order to realize the characteristics of the sea surface roughness in the coastal ocean, the Mean Square Slope, MSS of surface elevation was adopted as the index to describe the sea surface roughness. In-situ observations were carried out at National Central University Coastal Observatory, TaiCOAST station located at the western coast of Taiwan. Synchronized observations of atmospheric boundary layer air-sea flux, gravity waves and current profiles, shore-based microwave radars were implemented during the period of northeast monsoon, i.e. from January 14, 2011 to January 31, 2011. The Drag coefficient is estimated using eddy-covariance method. The Roughness length is estimated by using the wind profile method and the spectral method. MSS is estimated from the surface elevation recorded by the three bottom-mounted Acoustic Doppler Current Profilers (ADCP). The surface tracking mode was implemented to obtain high resolution water elevation from the acoustic transducers. The wavenumber spectra were then calculated from the frequency spectra using the linear wave dispersion relationship. The slope spectrum can be derived and the MSS can be estimated by taking the integral of the slope spectrum. The magnitude of MSS is highly influenced by the spectral tail of full-range wavenumber spectrum. For non-saturated limited depth waves, it is not yet clear about the tail shape in high wavenumber range. In present study, six hypothetic spectral shapes were tested. Moreover, the criteria that distinguishes the spectral bands that governed by three wave interactions or quadruplet wave interactions, was determined. By using the criteria, the MSS that obtained from the lower-frequency slope spectra are associated with gravity waves; whereas those from high-frequency are associated with the turbulence. In present study, we use the absolute S-band radar backscatter intensity as reference to determine the tail property at the high wavenumber bands as well as the above-mentioned criteria. The result showed that the tail of full-range wavenumber spectrum is proportional to k-4. The best-fitted wavenumber cut-off wavenumber is about 42.3?g/U102 . Futhermore, we discuss the dependency of the high-frequency MSS to the wind speed, and the low-frequency MSS to the wave age. The results show consistency with previous studies by Cox and Munk (1954). Moreover, when the wave age approximately equal to 0.8, the low-frequency MSS decreases. It is similar to the previous studies by Donelan(1990). Finally, an estimation of the contribution from the gravity waves to the surface roughness is about two-thirds at coastal oceans.
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