Turbulent airflow at young sea states with frequent wave breaking events: large eddy simulation

A neutrally stratified turbulent airflow over a very young sea surface at a high-wind condition is investigated using a large eddy simulation. In such a condition, the relationship between local instantaneous surface-wind and surface stress is highly variable and anisotropic owing to the intermittency and anisotropic shape and orientation of the breaking waves. In order to model these characteristics, a bottom surface stress parametrization for the sea surface having individual breakers is proposed. The proposed parameterization is compared to the commonly used bottom boundary stress parametrization for three-dimensional isotropic roughness. Over both the three-dimensional roughness and the very young sea surface, the main large-scale turbulence structures near the bottom boundary are quasi-streamwise vortices. The three-dimensional roughness weakens the swirling motions of these vortices by spanwise form drag. In contrast, the young sea surface exerts little spanwise form drag and develops more intense vortices which result in more intense turbulence and mixing. These vortices decrease the vertical shear in the roughness sublayer and thereby increase the roughness length in the overlying logarithmic layer. Thus, the enhancement of the air-sea momentum flux (drag coefficient or roughness length) due to breaking waves is not only caused by the form drag over individual breakers but also by the enhanced turbulent mixing in the roughness sublayer. Contrary to an assumption in most existing wave-boundary-layer models, our results suggest that the wave effect may extend significantly higher, as much as 10 to 20 times the breaking wave height.