Wave-current Interaction And Its Effects On Wave Modelling

Both ocean waves and currents are important components of the ocean dynamic environment,and the exchange of momentum and energy between them can be collectively referred to as wave-current interaction(WCI);such kind of interactions,which usually occur in nearshore areas and in typhoon,Kuroshio,and Gulf Stream affected sea states,can markedly influence the growth and dissipation of waves.The influence of WCIs on wave modelling includes: i)currents can affect wave propagation in forms of changing the velocity and direction of wave groups and leading to refraction;ii)gaining or losing energy during the interaction between water particle oscillation and share flow,i.e.,the gradient of current velocity,may reshape the simulated energy spectrum.Because the wave action density is conserved in WCIs,third-generation wave models,such as ECMWF-WAM,Wave Watch III,and SWAN prefer solving the wave action spectrum balance equation which is free from considering the effects of WCIs on wave energy spectra.And that is why the effects mentioned above are less discussed in wave modelling.In this study,under the theoretical framework of wave-current interaction source function,hereafter,the effects of WCIs on wave energy spectra modelling are elucidated through theoretical analysis and ideal experiments.It is concluded that the shapes of wave energy spectra can be modified by,and the modification can be affected by the magnitude of current velocity gradient,water depth,maturity of wind waves,and wave propagation direction in the flow field,but the current velocity and direction cannot change the spectral shape.With a current velocity gradient whose value can be found in the real-world oceans,the influence of on simulated significant wave height and spectral peak direction are quite small.However,considering long-distance wave propagation,the small modification on wave direction can still result in a dramatic change in the simulated sea state.In addition,based on solving the energy spectrum using a characteristic inlaid scheme,the influences of flow field with or without velocity gradient on wave modeling are also discussed.The main conclusions are as follows: the flow field without velocity gradient can break the conservation of the energy contained in the lower and higher frequencies around the spectral peak,and the frequency bands that gaining or losing energy can appear in different directions in the two-dimensional spectral space;furthermore,the directions in which spectral energy has changed are associated with the intersection angle between the directions of current and wave propagation.The spectral energy increasing and decreasing can be further enhanced or attenuated by both the effect of velocity gradient on wave propagation and the,resulting in different simulated wave heights,peak periods,and peak directions.Finally,four wave modelling cases under typhoon conditions with the consideration of WCIs are performed.It is indicated that the value of current velocity gradient can be up to,and gradient with the largest value can be found near the center of the typhoon.With the consideration of WCIs,the rate of modification on significant wave height in the simulated area can be up to 10%,and the modelled wave direction can deviate from the gradient-free results for more than 20 degrees in the scenario of long-distance propagation.Therefore,the WCIs can play an important role in the sea states with the occurrence of typhoons,even in the open oceans that are far from the coast.