Wind-Induced Changes to Surface Gravity Wave Shape in Deep to Intermediate Water

TL;DR

This paper develops a theoretical framework to understand how wind influences the shape of ocean waves, focusing on phase and harmonic changes, which are crucial for various marine and coastal applications.

Contribution

It introduces a new theory linking wind-induced surface pressure profiles to wave shape parameters, filling a gap in understanding wave shape changes due to wind.

Findings

Wave shape parameters depend on pressure profile, magnitude, phase, and water depth.

Predictions of harmonic phase align qualitatively with laboratory data.

Wave asymmetry correlates with nondimensional growth rate.

Abstract

Wave shape (i.e. skewness or asymmetry) plays an important role in beach morphology evolution, remote sensing, and ship safety. Wind's influence on ocean waves has been extensively studied theoretically in the context of growth, but most theories are phase averaged and cannot predict wave shape. Most laboratory and numerical studies similarly focus on wave growth. A few laboratory experiments have demonstrated that wind can change wave shape, and two-phase numerical simulations have also noted wind-induced wave shape changes. However, wind's effect on wave shape is poorly understood, and no theory for it exists. For weakly nonlinear waves, wave shape parameters are the phase of the harmonic relative to the primary frequency (or harmonic phase HP, zero for a Stokes wave) and relative amplitude of the harmonic to the primary. Here, surface pressure profiles (denoted Jeffreys, Miles, and Generalized Miles) are prescribed based on wind-wave generation theories. Theoretical solutions are derived for quasi-periodic progressive waves and the wind-induced changes to HP, relative harmonic amplitude, as well as already known phase speed changes and growth rates. The wave shape parameters depend upon the chosen surface pressure profile, pressure magnitude and phase relative to the wave profile, and the nondimensional depth. Wave asymmetry is linked to the nondimensional growth rate. Atmospheric large eddy simulations constrain pressure profile parameters. HP predictions are qualitatively consistent with laboratory observations. This theory, together with the observables of HP and relative harmonic amplitude, can provide insight into the actual wave surface pressure profile.