LES of Stratified Marine Atmospheric Boundary Layer over Waves

We perform large-eddy simulation (LES) of stratified turbulent flows in the atmospheric boundary layer over broadband ocean surface waves with the wave phases resolved. The wind field and the transport of scalar quantities therein are computed by LES, and the nonlinear wave field is simulated using a high-order spectral (HOS) method. The wind and wave fields are dynamically coupled using a fractional step scheme. The subgrid-scale (SGS) stress tensor and the SGS scalar fluxes in LES are modeled using the Lagrangian averaged scale-dependent dynamic model. Due to the high Reynolds number, a surface-layer model is employed to impose proper sea-surface flux boundary conditions to the LES.

For the study of the effects of atmosphere stability on the transport of scalar quantities, such as temperature and humidity, and on the wind input to the surface waves, a variety of atmospheric stability conditions are investigated. To investigate the effects of surface waves on scalar transport and wind input, we consider both fetch limited and fully developed sea conditions. For fetch limited wind-seas, we use the spectrum obtained from the Joint North Sea Wave Observation Project (JONSWAP). For fully developed seas, wave fields satisfying the Pierson-Moskowitz (PM) spectrum are considered. To investigate the role played by swells in air-sea interaction, we also simulate cases of wave field with swells, with variations in the wavelength, amplitude, and propagation direction of the swells.

From simulations, we obtain wind and scalar quantities fields over various surface waves. The results show that the surface wave condition has significant influence on the air flow and scalar quantities, and stratification can greatly alter their mean and fluctuation intensity profiles. For example, in the stable boundary layer, the wind turbulence is greatly suppressed. Consequently, the wind input to the waves is reduced, so that the surface waves grow more slowly under wind action than in the neutral boundary layer condition. The detailed description of the wind, temperature, and moisture fields from LES can advance our understanding of marine atmospheric boundary layer.