The Research Of Wave Propagation In Ocean Environment With Mesoscale Phenomena

Mesoscale phenomena has been considered to be the main contribution of the temporal and spatial variation in the actual ocean environment. It directly results in the changing of the sound velocity profile structures, which can make an impact on wave propagation further. Mesoscale phenomena is popular in our continental ocean zones and around the Taiwan island. The research of wave propagation in the ocean environment with mesoscale phenomena and the typical results obtained can be gradually used in the development of acoustic equipment and tactics. So, the topic of this dissertation has important theoretical meanings and practical values.In this dissertation, the distribution and activity of mesoscale phenomena in our continental ocean zones, including mesoscale eddy, oceanfront and internal waves, have been concluded at first. Several models of sound velocity profile structures have been set up correspondently based on the parameter descriptions of mesoscale phenomena. Then, in response to the range-dependent and even three dimensional features of mesoscale phenonmena distribution, FOR3D, a model of parabolic equation approach has been adopted. As a 'pure' numerical method, it is especially suited for sound field calculation in complex ocean enviroment. At last, a series of ocean environments have been taken as examples for numerical analysis and comparison in order to illustrate the impact of mesoscale phenomena on wave propagation, including the impact of eddy and oceanfront on the surface duct and covergence zones effect in deep ocean, the prominently changing of propagation condition by front and cold/warm water in continental shelf region, and the temporal variation and spatial inhomogeneity of sound field induced by internal waves in shallow water.The main contents of this dissertation is listed as follows:(1) The distribution and activity of mesoscale phenomena in our continental ocean zones, including mesoscale eddy, oceanfront and internal waves, have been concluded and analyzed according to the literatures and field data. Mesoscale eddy is a kind of coherently rotating water mass. It has the 'closure' structure in horizontal plane. The size of mesoscale eddy has the order of tens to hundreds kilometers in diameter. Oceanfront is formed from the meet of distinct water masses. The main feature of oceanfront is the prominent temperature gradient in horizontal range. Oceanfront has the 'line' structure in horizontal plane. In the actual ocean environment, the width of oceanfront transition zone has the order of tens kilometers. Internal wave is a kind of interface-wave in the steadily stratified ocean. It is composed of linear and nonlinear internal wave. The linear internal wave exists mainly in deep ocean, while the nonlinear internal wave exists mainly in shallow water. In comparison with the eddy and oceanfront, internal wave is smaller in size, but has more prominent temporal and spatial variation.(2) Based on the parameter description of mesoscale phenomena, the impact on sound velocity profile structures have been analyzed, and several models have been set up correspondingly. The mesoscale eddy is simulated as a series of elliptic structures in horozontal plane at different depths. The parameters, which may be varied in depth, include the position of eddy center, the dimensions in horizontal, temperatures at the center and edge of eddy, and the depth area. The oceanfront structure is described with the width of front transition zone, the temperature gradient in range, and the depth area. In oceanfront zones, sound velocity profiles can be constructed by the linear interpolation of the velocity iso-lines. The internal wave is described with the displacement function of density or temperature iso-line. For the linear internal wave, the Garrett-Munk (GM) model has been successfully used in the past years. As the nonlinear internal wave, soliton wave is now received extensive researches. The typical parameters for soliton wave description include the characteristic width, amplitude, and the moving speed or position at different time.(3) The parabolic equation model FOR3D has been described in details, with some benchmark problems and standard numerical models being used for the comparison and validation of this code. FOR3D has advantages over the other parabolic equation models in two or three dimensional sound field calculation. It employs the finite difference technique to solve the control equation. FOR3D has the wide-angle calculation capability and its algorithm is unconditionally stable.(4) The impact of eddy and oceanfront on the surface duct and convergence zones effect in deep ocean has been checked. The special sound velocity profile in deep ocean provides two favorable conditions for long-range propagation: the surface duct and SOFAR channel. The range-dependent variation of sound velocity resulted by mesoscale eddy and oceanfront can cause the changing of convergence zone structure and the emergence or disappearance of surface duct. As the eddy and oceanfront often locate in the upper layers, they cast more influence on the surface duct than that on the convergence zone.(5 ) The prominent changing of propagation condition by front and cold/warm water in continental shelf region has been discussed. In shelf region, distinct water masses from the outer sea and inland are confronted with each other, two or more different propagation environments can be distinguished. The properties of wave propagation between these environments or in each are significantly different. In the mean time, the intrusion of the outer sea water can form special wave propagation conditions, such as internal channels. The variation of sound field structure in depth is an important contributor to the abnormal attenuation for receiver at fixed depth.(6) The temporal variation and spatial inhomogeneity of sound field induced by the internal waves in shallow water have been researched. In comparison with the eddy and oceanfront, internal wave is smaller in size, but has more prominent variation in time and space. It is the main contributor of wave propagation fluctuation. In the range of sonar operation, internal wave has more significant impact on wave propagation than the others. Through the perturbation of the thermoclines, internal waves influence the energy transfer between the upper and bottom layer, and make different the wave propagation on the same or across side of the thermoclines. As a typical nonlinear internal wave, soliton wave has been researched extensively in recent years. Among the soliton packets, the distinction of sound velocity can result in the horizontal channel effect, while along t7he moving direction of the solitons, wave progation may be blocked. The movement or temporal variation of internal waves results in spatial distinction and temporal variation of wave propagation, which is also referred as fluctuation. The fluctuation is closely related to the frequency, properties of internal wave, and the relative locations of source and receivers.