Recent advances in the study of wind waves

Abstract

The field of wind-generated ocean surface gravity waves is reviewed for the period covering the last fifteen years. Theories and observations relevant to understanding the physics of wind waves are discussed, as well as techniques for measuring and forecasting waves.It is found that although a great deal of recent progress has been made on certain aspects of the wind wave problem, there are still important aspects which are poorly understood. I n particular, the central problem of how the wind generates waves in the ocean has not yet been solved; the primary physical mechanism(s) by which the wind makes waves has not been found. When the wind blows how much energy and momentum goes into waves and how much goes into currents? At the present time it is not possible to give a very definite answer to this important question. However, wave generation theories are available which perhaps can be modified to give better agreement with observations than they do now. New wave measuring techniques developed recently and/or under development now may provide the badly needed field observations necessary for future advances in understanding wind waves.Very little is known about the dissipation of wind waves, either in the open ocean or near coastal boundaries. Relevant theories and observations are not sufficient for understanding wave dissipation at present. Relatively little work has been done in the general area of the interaction of wind waves with currents. Some preliminary theoretical work suggests that this interaction could be quite important in the particular problem of the propagation of wind waves into major ocean currents, but an experimental test of the theory is lacking.Present methods of wave forecasting incorporate more physics than the earlier empirical methods. However, these methods still require a set of 'engineering approximations' in order to produce decent forecasts. With proper tuning the numerical forecast models produce good estimates of the one-dimensional wave spectrum.

On the Non-Dimensionalisation, Scaling and Resulting Interpretation of the Classical Governing Equations for Water Waves

“…Experimental evidence shows that the length scales associated with an adjustment of the fluid velocity distribution due to laminar viscosity are long compared to typical wavelengths encountered in sea waves [10]. Also, for sea waves that are not near the breaking stage, the effects of friction due to turbulent mixing viscosity are also known to be negligible [2], so that we may reasonably neglect viscosity altogether. The effects of surface tension are negligible for wave lengths greater than a few centimeters [13,22], the Copyright c 2008 by A Constantin and R S Johnson typical wave length of a wind-generated wave at sea being about 150 m [2].…”

“…Also, for sea waves that are not near the breaking stage, the effects of friction due to turbulent mixing viscosity are also known to be negligible [2], so that we may reasonably neglect viscosity altogether. The effects of surface tension are negligible for wave lengths greater than a few centimeters [13,22], the Copyright c 2008 by A Constantin and R S Johnson typical wave length of a wind-generated wave at sea being about 150 m [2]. Therefore the balance between gravity and the inertia of the system is the major factor governing the evolution of sea waves from their initial profile.…”

Upwelling by Surface Gravity Waves

“…The two variables of the linear momentum, wave energy and frequency, were measured as a function of offshore distance. These variable could be used to estimate the upwelling velocity with the aid of a few assumptions, since the Stokes drift equals the wave energy divided by the phase velocity [5], and the phase velocity of deep water waves is inversely proportional to the frequency. That is a job for the future.…”

On the open sea propagation of water waves generated by a moving bed

“…As the effects of surface tension are negligible for wavelengths greater than a few centimetres [6,7], the major factor governing the wave motion is the balance between gravity and the inertia of the system. The corresponding kinematic boundary conditions,…”