| 摘要: | 海岸地區為海-氣-陸之交界，此區域陸源物質(包括沉積物、營養鹽與汙染物等)、熱量和動量傳遞及交互作用劇烈。陸源物質在海岸及河口傳輸主要受潮流、風驅流與密度流的控制，風驅流主要受風吹海面所引起，由此可知風為影響海岸地區陸源物質傳輸與擴散主要機制之一，風場特性會直接反應於海面上，海洋與大氣之間熱量傳遞也受表面風影響，海氣交互作用與氣候變遷息息相關，了解表面風場對海岸地區的影響，對海岸工程施工(如海洋平台、海洋能源利用等)有所裨益。從實際層面上，再生能源的推動成為各國開發的重點。近年來離岸式風力電廠成為風能發電發展的新趨勢，台灣土地面積狹小，能發展成陸上風力電廠地區有限，故發展離岸式風力電廠為台灣開發再生能源的重要政策之一。每日風能的週期性振盪會影響發電效率，Capacity Factor取決於風速大小，風速越大其值越高，Capacity Factor越高代表發電效率越好且發電成本越低，了解台灣周邊海域表面風場的分布，有利預估潛在風能與年發電量，本研究希望藉由現有觀測風場資料，探討表面風場於近岸海洋地區之特性，以助於離岸風能發電。由前人研究得知於表面摩擦力較低的高空中與海洋表面，風場有顯著的日週期與半日週期現象，在陸地上因陸地表面粗糙度大，僅存在日週期現象。海陸風與大氣潮皆為引起日週期與半日週期風場現象之成因，台灣海岸，海陸交界上，表面風場之日週期特性，實在值得探討。因此，本研究目的針對台灣周邊海域離岸風能發電之應用，描述海岸表面風速每日週期性振盪強度與特性，及其於季節上與空間上的變化特性，進一步探討主導台灣周邊海域日週期與半日週期風場之成因與機制。經研究結果顯示，台灣周邊海域表面風存在日週期與半日週期現象，日週期與半日週期風速變化於季節上無顯著差異，於空間上大致區分為西北岸、西南岸與東岸；以瞬時Rotary spectrum考慮風速橢圓特性，台灣海域不論東、西岸日週期振盪振幅約為半日週期之2-3倍，日週期與半日週期振盪強度以低緯度海岸較高，其次為東岸，西岸最小；日週期風速橢圓長軸方向統計結果，新竹與永安站特性相似，大鵬灣與小琉球相似，其餘之外各測站均呈現差異甚大，可能與地形及表面摩擦力差異有關；依據全日週期風速橢圓長軸方位之統計特性，定性上大氣潮對於日週期特性不可忽略，定量上尚有待進一步研究。Coastal zone is the interface among the ocean, atmosphere and the land, where, the flux of terrestrial (i.e. sediment, nutrients and pollutants etc.), heat and momentum transfer take. The transportation and dispersion of terrestrial material in coastal waters and estuaries are mainly controlled by tidal current, wind-driven current and density flow, among them, the wind-driven current that incurred by the wind stress is the dominate mechanism. As the characteristics of wind variation will bring direct impacts to the ocean surface, understanding the oscillation of surface wind fields on the coastal waters are essential to the realization of nature environments as well as the application such as the coastal engineering and marine energy utilization).From the viewpoint of practical application, renewable energy conversion had become an increasing source in recent years. Due to the restriction of limited land area of, Taiwan island, the utilization of off-shore wind energy had gained the importance. Temporal oscillation of wind speed in various scale will affect the Capacity Factor(CF), for which, small CF will result in high cost/benefit ratio. The aim of this study is to use existing observation wind data, explore the characteristics of the temporal oscillation of coastal wind fields around Taiwan waters, with the hope to assist the off-shore wind power generation.Previous studies had shown that the surface wind exhibit significant diurnal and semidiurnal periodicity, due to several mechanisms such as atmospheric tide and land-sea breeze. Hence, the aim of this article is twofold. First, in order to fulfill the needs from off-shore wind farms application around Taiwan, the statistical descriptions of the coastal wind speed oscillation and associated characteristics of seasonal and spatial variations are discussed. Second, the causes and mechanisms of the diurnal and semidiurnal periodicity wind around Taiwan waters are investigated using short-time rotary spectral analysis.The results show that surface wind features diurnal oscillation around Taiwan waters. There was a significant seasonal difference of the intensity of diurnal and semidiurnal wind. The categorization of the coastal zone with respect to surface wind can be divided into several regions, i.e. the northwestern coast, southwest coast and east coast. Consider the wind ellipses features, the diurnal oscillation amplitude is 2-3 times higher than semidiurnal cycle, the low-latitude coastal has higher oscillation intensity, followed for the east coast, west coast is smallest. The statistics of the direction of major axis of the diurnal wind ellipses, Hsichu and Yung-an feature similar characteristic, Dapeng Bay and Xiao Liuqiu also have same characteristic, the other station showed descrepencies, which might owing to the differences in terrain and surface friction. As a whole, land-sea breeze is the dominating factor to the diurnal wind and the effects from atmosphere tide cannot be ignored. |
