Large-eddy simulation of marine atmospheric boundary layers above a spectrum of moving waves

Momentum and scalar transport in the marine atmospheric boundary layer (MABL) is driven by a turbulent mix of winds, buoyancy, and surface gravity waves. To investigate the interaction between these processes, a large-eddy simulation (LES) model is developed with the capability to impose a broadband spectrum of time-varying finite-amplitude surface waves at its lower boundary. The LES model adopts a Boussinesq flow model and integrates the governing equations on a time-varying, surface-fitted, nonorthogonal mesh using cell-centered variables with special attention paid to the solution of the pressure Poisson equation near the wavy boundary. Weakly unstable MABLs are simulated with geostrophic winds increasing from 5 to 25 m s−1 and wave age varying from swell-dominated to wind-wave equilibrium. The simulations illustrate cross-scale coupling as wave-impacted near-surface turbulence transitions into shear-convective rolls with increasing distance from the water. In a regime with swell, low winds, and weak heating, wave-induced vertical velocity and pressure signals are readily observed well above the standard reference height ζa = 10 m. At wind-wave equilibrium, the small-scale wave-induced signals are detectable only near the water surface. Below ζa, a nearly-constant-flux layer is observed where the momentum flux carried by turbulence, form stress, and subgrid-scale motions shifts with varying wave age and distance above the water. The spectral content of the surface form stress is wave-age dependent, especially at low wavenumbers. The LES wind profiles deviate from Monin–Obukhov similarity theory in nonequilibrium wind-wave conditions, and entrainment is greatly enhanced by shear-induced engulfment events.