Dynamic surface drag modeling of wind over ocean waves*

The small-scale dynamics near the wavy air-sea interface directly influence the surface wind stress and thus regulate the transfers of momentum and scalars between the atmosphere and ocean, which play an integral role in various geophysical and engineering applications. However, resolving the surface stress above ocean waves remains challenging for both experimental and numerical studies due to the broad range of scales involved. The multiscale height distribution of moving surface waves and the corresponding generation of drag forces at varying small-large scales, in particular, poses unique challenges for large eddy simulations (LESs). Here, to evaluate the surface drag, we developed a wall-layer model for wind stress using a surface-gradient-based drag (SGD) model. Model parameters, including the drag coefficient and roughness parameter, are evaluated using a dynamic methodology based on scale-invariance and self-consistency arguments of the surface drag. The main assumption is that the total drag force is independent of the filter scale and resolution. Results show that the proposed combined SGD and dynamic modeling approach can capture the effects of subgrid-scale waves on wind turbulence without ad hoc prescription of model parameters and resolving the wavy surface.