Refraction of a Gaussian Seaway

TL;DR

This paper develops a theory combining Gaussian sea models and wave refraction by currents, showing that refraction significantly increases the probability of freak wave formation, especially in the distribution tail.

Contribution

It introduces a new analytical framework that accounts for non-uniform sampling due to refraction, enhancing predictions of freak wave probabilities in ocean waves.

Findings

Refraction causes persistent local energy variations in the sea.

Increased freak wave probability linked to the 'freak index' gamma.

Distribution tail effects are significant even at modest gamma values.

Abstract

Refraction of a Longuet-Higgins Gaussian sea by random ocean currents creates persistent local variations in average energy and wave action. These variations take the form of lumps or streaks, and they explicitly survive dispersion over wavelength and incoming wave propagation direction. Thus, the uniform sampling assumed in the venerable Longuet-Higgins theory does not apply following refraction by random currents. Proper handling of the non-uniform sampling results in greatly increased probability of freak wave formation. The present theory represents a synthesis of Longuet-Higgins Gaussian seas and the refraction model of White and Fornberg, which considered the effect of currents on a plane wave incident seaway. Using the linearized equations for deep ocean waves, we obtain quantitative predictions for the increased probability of freak wave formation when the refractive effects are taken into account. The crest height or wave height distribution depends primarily on the ``freak index", gamma, which measures the strength of refraction relative to the angular spread of the incoming sea. Dramatic effects are obtained in the tail of this distribution even for the modest values of the freak index that are expected to occur commonly in nature. Extensive comparisons are made between the analytical description and numerical simulations.