Non-equilibrium winds/waves: Impact of wave-propagation direction and wave age on hub-height winds and turbulence

Much of what is currently known regarding the coupling between the marine atmospheric boundary layer and the underlying surface gravity wave field relies on the assumption of wind-wave equilibrium. However, wind-wave equilibrium is generally not achieved in coastal zones where future U.S. offshore turbine deployments are likely to be located (Hanley et al., 2010). Winds and waves in coastal regions depart from wind-wave equilibrium due to: 1) diurnal forcing associated with the daily sea breeze, 2) a shallow water column which limits wave growth, 3) distant storms generating high-amplitude swell that arrives in coastal regions often dominating the local wave state, and 4) coupling with temperature fronts in the ocean.

A critical aspect of wind-wave coupling that has received only limited attention is the influence of swell propagation orientation relative to the wind direction (e.g., Li et al., 2000). Previous analysis of winds and turbulence over swell-dominated wavy surfaces largely ignore the frequent nonalignment of swell and wind. How the local wind responds to misalignment with the surface waves and how to incorporate this coupling in parameterizations used to relate surface-observations to the hub-height of offshore wind turbines is unclear.

In this presentation, we will discuss a set of turbulence- and wave-resolving simulations exploring the influence of wave age (the wave-propagation speed relative to the wind speed) and φ (the wave-propagation direction relative to the wind direction) on statistics associated with the overlying wind field and their coupling with the wavy surface.