A note on the bimodal directional spreading of fetch‐limited wind waves

Measurements of the directional spectra of fetch‐limited wind waves are 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 frequencies just above and below the spectral peak frequency. At frequencies of approximately twice the peak frequency, however, the unimodal spreading becomes bimodal, and more wave energy propagates at an angle to 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 nonlinear terms indicate that the bimodal structure is maintained by directional transfer of energy through nonlinear wave‐wave interactions.