Numerical Studies of the Nonlinear Interaction Between Turbulent Air Flow and Sea Surface Waves, with Application to Ocean Surface Wave Turbulence

The generation of sea surface waves by winds is one of the fundamental air-sea interaction processes which affect the global climate. Although this topic has been studied extensively through both physical modeling and experimental measurements, understanding of the physics involved remains limited due to the nonlinear phenomena implicit in both air flow and sea surface wave evolution. Numerical simulations offer a means to improve understanding of the complex interactions of this problem, but only recently have computing resources improved to make sufficiently large scale simulations possible. Recent studies have begun to apply computational fluid dynamics methods to simulate turbulent air flow over sea waves, but to date the sea surface models applied have been based on waves of permanent form, such as Stokes' waves. The neglect of nonlinear processes in the sea surface evolution results in wave growth estimates which are constant in time, clearly incorrect if an equilibrium between the air and water fields is to be obtained. Nonlinear hydrodynamic models have also been investigated previously, but air-forcing effects are included in these models through empirical estimates only, without regard to the effect of the sea surface profile on wind forcing. Results from these hydrodynamic-only models dramatically underestimate short sea wave modulation effects observed in remotely sensed sea images at microwave frequencies. The proposed project develops a numerical model for turbulent air flow over nonlinearly evolving water surfaces which overcomes the limitations of previous studies. Efficient computational algorithms for modeling air flow, efficient hydrodynamic methods to evolve the sea surface boundary , and parallel computing techniques will be combined to make statistical analyses of realistic problems in both two and three dimensions possible. Scientific visualization methods will be applied to analyze results and to improve understanding of the nonlinear interactions between wind and waves that are important for sea surface growth and dissipation. Studies will be performed to determine the conditions under which " decoupling " the interacting complex systems into air-flow-only or hydrodynamic-only simulations is possible by including appropriate forcing terms; the form of these forcing terms, when applicable, will be compared with existing empirical and analytical models to clarify the important physical processes. Decoupled hydrodynamic-only simulations will be pursued to investigate wave-wave energy transfer effects in larger scale problems than those possible in the coupled air-water model, and resulting forms of the equilibrium sea surface spectrum will be obtained. Commonly applied approximate hydrodynamic models will also be …