Applicability of Normal Modes Projection using Finite-Depth Current Measurement on Tidal Analysis

• Main Authors: Bo-Song Wang, 王柏松
• Other Authors: 唐存勇
• Format: Others
• Language: zh-TW
• Published: 2010
• Online Access: http://ndltd.ncl.edu.tw/handle/12134020950717994570

碩士 === 國立臺灣大學 === 海洋研究所 === 98 === Tidal currents are ubiquitous motions in the ocean. The vertical structure of tidal currents can be regarded as the manifestation of infinite vertical modes. Theoretically, the vertical structure of tidal currents can be identified as the combination of barotropic modes and baroclinic modes. The barotropic mode is assumed that the current magnitude is homogeneous in full depth. The baroclinic mode is assumed that the current magnitude is function of depth. The current measurement usually covers partly water column. Consequently, the full-depth measurement of tidal current is usually unavailable. To complete the measurement lack, the full-depth tidal currents could be obtained using the vertical tidal current structures inferred from the full-depth density profile. The tidal current measured in partly water column can be projected to full-depth measurement according to the vertical tidal current structure. The method is well known as normal mode projection and is extensively applied. However, the performance of the method was rarely identified. In this thesis, using the simulation data, the impact to the method due to (1) different measurement of depth range, (2) different vertical modal structure and (3) the measurement error was analyzed. The analysis revealed that the larger the measurement depth range was, the less the error was produced. If the simulated data is limited in the upper column, the normal mode projection revealed better performance when the barotropic mode and 1st baroclinic mode were only considered in the modal structure. That is because the limited measurement could not resolve the higher mode structure. If one extra measurement below the upper column measurement was added, the higher mode structure can be well-resolved and consequently the error was lowered. The best result presented when the extra measurement was located at the lowest nodal point of the highest mode. The random error of measurement led to the error of normal mode method as well. This kind of error is more conspicuous when the higher mode was used. It is because the order of measurement error was generally close to the magnitude of higher mode structure. The field measurement taken only upper column were used to validate our results from simulation data. The normal mode method shown better performance when the barotropic mode and 1st baroclinic mode were considered. The result coincided with the prediction from the simulation test.