Large-eddy simulations of marine boundary layers above a measured spectrum of phase-resolved nonlinear ocean waves

The coupling dynamics between atmospheric turbulence and surface gravity waves at scales ranging from millimeters to several hundred meters continues to remain an outstanding question in air-sea interaction. Both observational and computational approaches encounter severe impediments owing to the temporal and spatial broadband complexity of the coupling and the fundamental oscillatory motion of the air-water interface; an interface which is also intermittently fractured by breaking waves. To further our understanding of air-water coupling we have recently developed a large-eddy simulation (LES) model of the marine boundary layer with the ability to impose a measured phase-resolved two-dimensional time evolving wave spectrum at its lower boundary. In this presentation, we use measured wavefields collected over several hours from a conventional Xband ship radar with time sampling of 1.5 seconds and spatial resolution up to wavenumber k = 0.44 radians/meter. In order to match the spatial resolution of the LES, a high wavenumber tail is further added to the Xband wave measurements; the amplitude of the wave tail is picked to match high-resolution altimeter measurements collected from an airborne laser system. We briefly describe the technical advances required to injest these measured non-homogeneous surface waves in LES. As an illustration of the simulation capability, we consider two cases, aligned and slightly mis-algined winds and waves, that match the observed conditions from the high-resolution air-sea interaction (HiRes) field campaign carried out in June 2010 off the coast of California. HiRes was sponsored by the Office of Naval Research. Bulk conditions were near neutral stability with surface layer winds approachiing 15 m/s with significant wave heights between 3 and 3.5 m. Results from the fine mesh simulations, (1280 x 1280 x 512) gridpoints with spatial resolution of (2 x 2 x 0.5) m at the water surface, are described.