Do wind-generated waves under steady forcing propagate primarily in the downwind direction?

TL;DR

This study challenges the long-held assumption that wind-generated ocean waves predominantly propagate downwind, presenting photographic evidence of crosswind and multi-directional wave patterns that vary with fetch and location.

Contribution

The paper provides empirical photographic evidence showing that wind-generated waves often propagate in directions other than the wind, revealing a fundamental flaw in current wave propagation assumptions.

Findings

Waves display crosswind and multi-directional patterns in photographs.

Wave propagation directions vary with fetch and location.

The assumption of downwind propagation is often invalid in observed wave fields.

Abstract

Measuring the directional distribution of ocean waves is a difficult task in ocean wave research. For many decades, wind-generated waves are assumed to propagate primarily in the wind direction. The concept is applied either implicitly or explicitly in the formulation of the wind input function and the design of the wave directional distribution function. Photographs of the ocean surface wave fields under steady wind forcing, however, display very different directional properties. In the near shore region, the surface undulations are dominantly crosswind. As the fetch increases, the geometry of surface waves is crosshatched, indicating two dominant wave trains crossing each other. This pattern of wave propagation in off-wind directions is repeatedly photographed in different locations under different offshore (toward) wind events. The wavelengths of these airborne photographs are in the deca-meter length scale. Shorter waves with wavelengths ranging from a few centimeters to a few decimeters also show crosshatched patterns, based on pictures taken from land and ship. In the crosshatched cases, most pictures show that the wind vector bisecting the diamond shape of the surface wave geometry. The observed wave geometries and their fetch evolution expose a major flaw in our fundamental assumption (implicitly or explicitly) that "wind-generated waves propagate primarily in the wind direction." The assumption has been taken for granted for many decades in applications including basic research of wave dynamics and the design of directional wave spectral model for calculations of the ocean surface roughness and the force, energy, and momentum of a wave field.