A NOTE ON THE BIMODAL DIRECTIONAL SPREADING OF FETCH-LIMITED WIND-WAVES

Measurements of the directional spectra of fetch-limited wind waves ar e presented, The directional spreading functions for these spectra are unimodal and narrowest in the region of the spectral peak frequency. Consistent with previous measurements, the spreading broadens for freq uencies just above and below the spectral peak frequency. At frequenci es of approximately twice the peak frequency, however, the unimodal sp reading becomes bimodal, and more wave energy propagates at an angle t o the wind than in the wind direction. The bimodal sidelobes continue to separate with increasing frequency and become larger in magnitude. Results obtained with a numerical model with a full solution to the no nlinear terms indicate that the bimodal structure is maintained by dir ectional transfer of energy through nonlinear wave-wave interactions.