Development of frequency-dependent ocean wave directional distributions

• We present experimental and numerical investigation of ocean wave directional spread.
• A frequency dependent directional spread develops from no frequency dependence.
• Generation of frequency dependent directional spread in wave basin is not necessary.
• Many of the directional spectra from the Ekofisk field have a bimodal character.
• The bimodality is consistent for all four methods including the Burg maximum entropy.

The paper discusses the development of a frequency dependent directional spread from an initial condition of frequency-independence. The study applies basin directional measurements from the Maritime Research Institute Netherlands (MARIN), simulated data from a nonlinear wave equation, and field measurements from the Ekofisk field. The basin experiments and numerical simulations are initialized with a JONSWAP spectrum with frequency-independent directional distributions. In both cases we observe the development of a strong frequency-dependence of the directional spread. The numerical simulations suggest that static nonlinear contributions to the surface elevation partially explain the behavior below the spectral peak in accordance with [1]. There are also dynamic nonlinear contributions on both sides of the spectral peak.