Estimation of Directional Spectra and Applications to the Study of Wave Dissipation

  • 錢 樺

Student thesis: Doctoral Thesis

Abstract

Wave dissipation is one of the least understood phenomena among the mechanisms involved in wave evolution. In deep water, the dissipation of surface waves can be regarded as results of interactions of the waves with upper-ocean surface turbulence. This turbulence can be recognized to be partly caused by wind stress resulting in whitecapping, and other sources of turbulences. In present study, accurate estimations of the directional spectra from field observations are used as diagnostic tools of investigating the wave dissipation. In order to ensure the obtaining of valid, accurate and high-resolution directional spectra, assessments of the up-to-date directional spectrum estimators are carried out. Analysis software based on the relevant theoretical foundations and testing procedures are established to evaluate and compare the performances, characteristics and biases of difference methods. As a result, the EMEM is considered to be the best choice considering the high-resolution potential, noise-robust capability and instrumentation effects. Moreover, the recommendations of the utilization of directional spectra estimators and the arrangement of instrumentations are proposed.

With the attempt to use directional spectral parameters to estimate the total wave energy dissipation rate, concepts from Phillips (1985) equilibrium range theory are adopted. The directional spreading functions in Phillips’ theory have been replaced by the directional spreading obtained by the measurement from wave gauges array during the winter monsoons. The dissipation estimates by this approach are compared to the observations by Felizardo and Melville (1994). The results show similar dependence of the dissipation rate to the wind speed, which demonstrates the possibility to replace the directional spectral parameters instead of the external wind forcing parameters in the estimation of wave dissipation. Since the wave dissipation processes are dynamically related to the wave field itself, the use of wave spectral parameters is reasonable. Moreover, with the increasing In Situ measurements available, the accurate determination of directional spectra will help to improve the understanding of wave dissipation characteristics.

To further investigate the minor mechanisms associated with dissipation, a directional spreading parameter is proposed in present study to indicate the strength of swell dissipation rate. The conceptual idea is that since the swell, which propagates in the celerity faster than the wind, gains no more energy input from the wind, the dissipation of the each spectral components are coupled and stabilized by quadruplet nonlinear interactions, which redistribute the remain energy in both frequency and direction domain. Consequently, a parameter, which can be derived from the relationship of the normalized directional spreading to the non-dimensional frequency, is deduced based on the instinct properties of quadruplet nonlinear interaction. It is proposed to be utilized as an indicator to the strength of the energy dissipation rate. Qualitative computations by using DIA scheme are carries out to verify the idea. The application of the indicator was then applied to the cases of swell decay using field observations. The results demonstrate that wave steepness is connected to its dissipation rate. The steeper the swell, the stronger rate of the dissipation can be yielded. The qualitative phenomenon of upward momentum transfer, which is predicted by models, but not herebefore observed, is identified.
Date of Award2003
Original languageEnglish
SupervisorZsu-Hsin Chuang (Supervisor)

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