Study on the validity of wave prediction models by direct numerical simulations

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It has become possible to perform routinely numerical wave forecast with practically sufficient accuracy. But much of this success owes the parameterizations of the models based on huge amount of data acquired by in situ observations and satellite remote-sensing, and it does not necessarily mean the success of elucidating all the basic mechanisms that are relevant to the evolution of the ocean wave spectrum. This project aims at investigating the validity of various assumptions and hypothesis included in the present-day wave forecasting models through comparative study with large-scale direct numerical simulations.We first performed a series of direct numerical simulations of the wave fields without input from wind or dissipation due to breaking to detect the change of spectrum due to nonlinear interactions among component waves, and found that the rate of change of the spectrum which the direct simulations gives agrees quite well with the prediction by wave forecast models. This apparently supports the validity of the wave forecast models. However, the fact that the rate of the change of the spectrum was observed at every instant of time in a rather short-term deterministic evolution seems to throw some doubt on the way of derivation of the nonlinear term of the wave forecast models which are currently accepted, and further investigation should be necessary.With regard to the dissipation due to breaking, we have newly developed a simple but efficient model which can be built in the deterministic direct numerical simulations. When applied to random wave fields with JONSWAP or P-M spectrum, the model was found to generate a dissipation spectrum which agrees fairly well with the standard model employed by the WAM model. Application of this model to various studies, such as the effect of breaking on the wave-height distributions is now underway. As a byproduct of this study, we also investigated the phenomenon of freak waves and obtained various interesting results.