Estimation of the bispectra and phase distribution of storm sea states with abnormal waves

This work deals with the estimation of the bispectrum of wind waves during severe storms containing abnormal or freak waves. It presents the basic definitions of higher-order spectra and of the bispectra in particular and further suggests how to interpret some of the results to identify non-linearity in the wave time series. Different estimation methods are used and compared so as to identify the differences in the estimated bispectra that results from the estimation procedure and the ones that result from the physics of the sea states. It is found that as a result of the second-order self-coupling the phase distribution of the wind wave during the severe storms differs from the uniform one and is well approximated by the distribution proposed by Tayfun and Lo [1989. Envelope, phase and narrow-band models of sea waves. Journal of Waterway Port Coast and Ocean Engineering 115(5), 594–613.].