Directional Wavenumber Spectra of Ocean Surface Waves

It has been accepted as a truth that under a steady wind forcing condition, the wind generated waves travel in the direction of the wind vector. Based on field measurements, the directional beamwidth is narrowest near the spectral peak, and increases toward both higher and lower frequency components. Over the last several decades, such scenario is the basis for the design of the directional distribution function of any spectral model for engineering and scientific applications. We present results from a spectral analysis of 3D topography of random surface waves generated by a quasi-steady wind field. The directional spectra display clear bimodal pattern in the wavenumber region just above the spectral peak. The generation mechanism of the bimodal directional distribution in the short wave region is identified to be nonlinear wave-wave interaction. Quantitative comparisons of measured and simulated directional distribution functions are in very good agreement.