Dependence of bimodality of ocean waves on estimating method

摘要

Estimates of directional spreading function of fetch-limited wind wave and swell are made from measurements observed with spatial gauge array at a wave tank by Bayesian directional method (BDM) and maximum likelihood method (MLM) respectively. Numerical simulation shows that BDM is superior to MLM in stability and accuracy, especially in the estimation of bimodal spreading. It is shown that bimodality of wind waves at high frequency range reported by Young et al. (1995) and Ewans (1998) is an artifact of analysis technique. On the other hand, Both MLM and BDM demonstrate bimodal spreading of swell at frequencies lower than spectral peak frequency, and can be regarded as the existence in reality. The bimodality is speculated on the attribution to the resonance between air flow and wind wave, and the nonlinear wave-wave interaction is so weak that cannot transfer energy among different directions effectively.