Impact of Breaking Wave Form Drag on Near-Surface Turbulence and Drag Coefficient over Young Seas at High Winds

The effects of breaking waves on near-surface wind turbulence and the drag coefficient are investigated using a large eddy simulation. The impact of intermittent and transient wave breaking events (of a range of scales) is modeled as localized form drag, which generates airflow separation bubbles downstream. The simulations are performed for very young sea conditions under high winds, comparable to previous laboratory experiments in hurricane-strength winds. Our results of the drag coefficient level off in high winds and are consistent with the laboratory observations. In such conditions more than 90% of the total air-sea momentum flux is due to the form drag of breakers, that is, the contributions of the non-breaking wave form drag and the surface viscous stress are small. Detailed analyses show that the breaking form drag impedes the shear production of the turbulent kinetic energy (TKE) near the surface, but instead produces a large amount of small-scale wake turbulence by transferring energy from large-scale motion (such as the mean wind and gusts). This process shortcuts the inertial energy cascade and results in large viscous dissipation of the TKE. Since the drag coefficient reduces as the total TKE dissipation inside the wave boundary layer increases for a given wind stress, the enhanced wake turbulence generation by breakers effectively reduces the drag coefficient. Our results also suggest that the common parameterizations of the mean wind profile and the TKE dissipation inside the wave boundary layer, used in the previous Reynolds averaged Navier-Stokes models, may not be valid.