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Please use this identifier to cite or link to this item: http://ntour.ntou.edu.tw:8080/ir/handle/987654321/11732

Title: 地形與內波對於雷達背向散射的影響(III)---子計畫---海洋表面波譜對雷達背向散射的效應(III)
Effects on Radar Backscatter by Ocean Surface Spectrum (III)
Authors: 董東璟;莊士賢;高家俊
Contributors: NTOU:Department of Marine Environmental Informatics
國立臺灣海洋大學:海洋環境資訊系
Keywords: 內波;表面波譜量測;波譜分析
Internal wave;surface spectrum measurement;spectral analysis
Date: 2009-08
Issue Date: 2011-06-28T08:44:05Z
Abstract: 摘要:衛載合成孔徑雷達(SAR)是目前常用來觀測海洋內波的工具之一。 衛星能夠偵測到水下的現象,尤其是內波波前的形狀、傳播、波長等, 是因為水下的現象造成流速分佈不均勻,在水體表面產生流速輻合與輻 散的現象所導致。水面流速輻合的地方是聚集表面波的地方,如內波發 生時的前波面是流速輻合區,造成小波聚集,表面粗糙度增加,雷達背 向散射強烈,在SAR 影像造成亮帶;反之,內波的後波面是流速輻散 區,在影像上形成暗帶。雷達電磁波和表面小波之間的作用關係?不同 尺度小波(公分等級與公尺等級)對雷達背向散射有何影響?是本整合型 計畫的研究目的。本子計畫負責的部分是表面波譜的量測與分析。 內波發生時海水表面的微尺度波波長大約介於幾公分到數公尺 長,這種尺度的表面波難以利用現有的測波技術量測,尤其目前內波研 究只能藉由研究船進行現場觀測,因此如何在研究船上量測水面流速輻 合/輻散區的波譜特徵是本計畫重要的工作之一。本計畫前兩年的研究 區域設定在基隆海域,第三年才到南海從事實際內波的觀測。由於基隆 港和基隆嶼之間的海檻地形受潮流的作用下出現類似內波發生時海表 面輻合與輻散的現象,輻合帶出現明顯的激浪,此現場每日發生,且位 置幾乎不變(實際內波的輻合帶會快速傳遞),是本課題研究最良好的地 點。本計畫前兩年工作改良了在實驗室造波水槽中量測波浪的電容式波 高計,使其可以在研究船上進行觀測,並改良至高頻的取樣速率(50Hz) 以能量測表面小波,經過連串的失敗與改進,截至目前為止,量測系統 已能妥善工作,並已取得超過10組的現場數據。 計畫第三年度除了將持續在基隆海檻上方進行現場量測試驗,繼續 改良量測系統,收集更多資料,提昇分析結果的可靠性之外,也預計在 取得船期的條件下,前往南海從事實際內波表面波譜的量測。分析方 面,將利用波譜分析方法,計算水體輻合/輻散區在時間上(發生之初、 最強烈時與衰落時)以及空間上(平浪區、激浪區與交界面)的平均波譜, 除傅立葉轉換與小波轉換外,非穩態可獲得瞬時資訊的HHT轉換也將被 應用在本計畫的分析工作中。本計畫將使用其他子計畫所得到的流速觀 測資料與模擬資料,探討非均勻流場作用在小尺度風浪波譜的效應,提 供瞭解流速輻合/輻散引起雷達背向散射機制的知識,此結果還需要其 他子計畫利用光學遙測分析結果以及衛星影像分析背向散射的結果一 同研討,以期完成本整合型計畫設定之研究目標。 本計畫完成後預期可以對於內波發生處,海水表面輻合/輻散區的 波譜特性有更多的掌握,根據波譜在時間與空間上的演變分析,以及同 步海流、光學影像以及雷達影像的分析,增加內波表面徵兆造成雷達背 向散射機制的瞭解。
abstract:Satellite images (such as SAR image) are often used for Internal Waves (IW) observation. Surface current distortion to induce convergence and divergence areas in the surface is the main mechanism for SAR to detect the location of IW. In the forward slope, the surface water converges as well as divergence happens in the backward slope when an IW passes. In the convergence surface, sea surface roughness is high therefore a white belt is visible in the SAR image. SAR emits EM wave in C band. The resonance ocean waves should be few centimeter. However, we see the wavelengths within the rough area when IW passes are not in the same scale of resonance waves. The mechanism of how SAR images IW surface is valid to be studied. It is the main purpose of this group project. In this sub‐project, the objective is to measurement and analysis of wave spectra in the rough surface (convergence area). The first target of this project is to design a device to measure the surface wave spectrum from a research vessel. In the first year of this project, a shipboard wave spectrum detection system (SWASD) using the capacitance transducer was developed. It was examined many times on research vessels to verify its capability for field experiments. More than 10 data sets have been acquired by the measurement system. This project is the second phase. The SWASD is going to be further improved to have better performance for further measurements. Most of the field experiments in the third phase project will be still carried out at the Keelung sill area because of the daily surface water convergence phenomenon happens due to interaction of strong tidal current and rapid change of bottom topography. In addition, this project has been applied for research vessel to go to SCS to measure the surface spectra of real internal wave. For the data analysis, the temporal variations of wave spectra during the period when convergence phenomenon happens (from start, grow up, strongest and decay) over Keelung sill will be studied. In addition, spatial variations will also be measured and analyzed at convergence and divergence areas respectively. In addition to Fourier and Wavelet transformations, HHT will be also applied to estimate the wave spectra because it has the capability of detecting instantaneous phenomena. By combination of the measurement and analysis results of wave spectrum at water surface convergence/divergence areas in this sub‐project and simultaneously measurements within other sub‐projects, it is helpful to understand the mechanism of SAR images sea surface convergence phenomenon. This is helpful to know more on the IW images of SAR.
Relation: NSC98-2611-M019-018
URI: http://ntour.ntou.edu.tw/ir/handle/987654321/11732
Appears in Collections:[海洋環境資訊系] 研究計畫

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