利用X波段雷達研究海表面流場－以淡水河口為例

Abstract

本文利用X波段雷達影像序列，並藉由水波之分散關係運算海表面流場，此種演算方式是利用三維波數頻率能譜運算提取出的子區，再藉由各項參數的擷取與運算，找出現場流速的最近值。 以雷達影像序列觀測海表面流場時，依物理機制做調整是必要的，本文以能量權重駐波消除、能量閾值設定、諧波能譜還原做為調整手段，將流速實測值對照觀測樁流場資料，證實用雷達影像時序列估算流速能有不錯的效果，並以其運算用於淡水河口流場，可大範圍觀測表面流速以及其物理現象，亦可用於航行船舶即時量測現場流況。 淡水河口表面流場主要受到半日潮影響，流向呈現東北、西南方向，而雷達回波在淡水舌(River Plume)鋒面附近尤其明顯，淡水舌鋒面常現於漲退潮期間通過觀測區，此時觀測區內波浪週期約在7秒左右。由於波流幅合效應，使得雷達回波在淡水舌鋒面附近訊號特別強烈，此時流速估算會較一般時間更為精準。

This thesis addresses on methodologies of how to estimate the sea surface current field by using time series of marine Radar imageries; the skills developed are then applied to analyze the tidal-flow patterns associated with the tongue-like plume of Danshuei River estuary. The kernel of the methodology contains respectively the segmenting sub-areas from qualified raw Radar data, calculating corresponded 3-dimensional spectrum of Radar echo intensity, setting criteria for eliminating noises and interferences in the spectral domain, and finally transforming the higher harmonic components and least square fitting the spectral components to obtain the current components. In a situation with higher radio interferences (such as that from a nearby Radar), the method of standing-wave removal, has been used tentatively for reducing the interferences, shows delighted results for pre-processing the spectral components.

For validation purpose, currents estimated from Radar imageries by using the developed methodology have been compared with the synchronous measurements of water flows by virtue of a S4 current meter, which is 5m below water surface and was mounted on an observation pile (the latter is located about 3.284 km west of the Radar). Comparison results show reliable relationship between the estimations and the measurements. The Danshuei River estuary is influenced mainly by semi-diurnal tidal currents. A clear tongue-like bright zone (implying flow convergence and energetic scatters) has been observed occurring frequently in the Radar imageries. The tongue-like pattern, moves outward and inward of the estuary with the alternation of tidal flows, has close correlation to the behavior of the river plume. Due to wave-current interactions, the arrival of tongue-like pattern implies a higher intensity of horizontal convergence of surface flow, which then favors the intensification of surface scatters and therefore beneficial to the accuracy of current estimation by using Radar imageries. The flow patterns around the river plume of Danshuei River estuary, as well as some of behaviors of the latter have been reported in the context too.