  Over the years, the directional distribution functions of wind-generated wave
field have been assumed to be unimodal. While details of various functional
forms differ, these directional models suggest that waves of all spectral
components propagate primarily in the wind direction. The beamwidth of the
directional distribution is narrowest near the spectral peak frequency, and
increases toward both higher and lower frequencies. Recent advances in global
positioning, laser ranging and computer technologies have made it possible to
acquire high-resolution 3D topography of ocean surface waves. Directional
spectral analysis of the ocean surface topography clearly shows that in a young
wave field, two dominant wave systems travel at oblique angles to the wind and
the ocean surface display a crosshatched pattern. One possible mechanism
generating this bimodal directional wave field is resonant propagation as
suggested by Phillips resonance theory of wind wave generation. For a more
mature wave field, wave components shorter than the peak wavelength also show
bimodal directional distributions symmetric to the dominant wave direction. The
latter bimodal directionality is produced by Hasselmann nonlinear wave-wave
interaction mechanism. The implications of these directional observations on
remote sensing (directional characteristics of ocean surface roughness) and
air-sea interaction studies (directional properties of mass, momentum and
energy transfers) are significant.
