A Comparison Of Various Numerical Wave Prediction Techniques

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Abstract
In recent years, the number of wave prediction models has proliferated. These models range from relatively simple parameterizations of significant wave height as a function of wind, duration and fetch to rather sophisticated solutions for the generation, propagation and dissipation of two dimensional wave spectra. It is sometimes suggested that any wave model will provide reasonable answers when properly applied, and that most of the deviations between measured waves and predicted waves can be explained by discrepancies between actual and estimated wind fields. Although almost certainly much of the error in wave prediction is related to problems in determining a wind field, this paper examines the specific question of whether or not there are differences among these models such that even if the wind field were perfectly specified, there would remain significant deviations among predicted waves. First, wave generation under uniform wind fields are compared using non-dimensional parameters. Then, the models are again compared under conditions of time varying, space varying wind fields and with irregular fetch boundaries. It is concluded that, in the open ocean with long duration, slowly varying weather system, most models produce rather similar results; however, near a coastal or in regions with rapidly varying weather systems rather marked differences can be expected from the use of different models. INTRODUCTION The need for wave data has increasingly led to the use of wave hindcast techniques to produce wave climates, and a number of major hindcast efforts are underway in the U. S. alone (Lazanoff, 1977; Ward et al., 1978; Vincent et al., 1978). Numerous techniques are available, ranging from significant wave techniques, in which wave parameters can be estimated from nomograms, to directional spectral models, which are usually run on largecore high-speed digital computers. Table I lists a sample of some of these techniques. An underlying assumption commonly made by practicing engineers is that each of the techniques will produce similar results when properly applied with correct wind input. It is the purpose of this paper to demonstrate that this is not always the case. Instead, various models can be shown to have theoretical differences which in climatological, as well as specific, applications might lead to significant discrepancies in estimates of sea state. Since all wave hindcasts begin with reconstruction of past wind fields from historical records a baseline error present in all wave estimates comes from inaccuracies in available meteorological data. Often it seems as though it is tacitly assumed by investigators that the wind error dominates the total error term in hindcast studies and hence that the absolute accuracy of the wave model is not all that important. A consequence of this might seem to be that, where available meteorological data is of high quality, a wave model of high quality should be used; but, where available meteorological data is of low quality (or sparse in time and space), a simple wave model will suffice.

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