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

已有可能以实际足够的精度进行常规的海浪数值预报。但这一成功在很大程度上要归功于基于现场观测和卫星遥感获得的大量数据的模型的参数化，这并不一定意味着成功地阐明了与海洋波谱演变有关的所有基本机制。该项目旨在通过与大型直接数值模拟的对比研究来检验现有波浪预报模型中包含的各种假设和假设的有效性。我们首先对没有风输入或因破裂而耗散的海浪场进行了一系列直接数值模拟，以检测由于分量波之间的非线性相互作用而引起的频谱变化，发现直接模拟给出的频谱变化率与波浪预报模型的预测结果很好地吻合。这显然支持了波浪预报模型的有效性。然而，在一个相当短期的确定性演化中，频谱的变化率在每个时刻都被观测到，这似乎给目前公认的波浪预报模型的非线性项的推导方法带来了一些疑问，需要进一步的研究。对于由于破裂引起的耗散，我们新开发了一个简单而有效的模型，可以在确定性的直接数值模拟中建立。当应用于具有JONSWAP或P-M谱的随机波场时，发现该模型产生的耗散谱与WAM模型所采用的标准模型非常吻合。将该模型应用于各种研究中，例如破碎对波高分布的影响，目前正在进行中。作为这项研究的副产品，我们还对反常波现象进行了研究，并获得了各种有趣的结果。

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.