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 in hurricane-strength winds are investigated using large eddy simulations. The impact of intermittent and transient wave breaking events (of a range of scales) is modeled as localized form drag. In the first chapter, the breaker drag appears in the bottom boundary stress parameterization, and its impact on the eddies that are much taller than the breaking wave heights is studied. It is found that the main tall eddies over young sea surfaces are surface-attached quasi-streamwise vortices similar to the eddies over other types of roughness. However, these vortices are more vigorous over the young seas compared to isotropic rough surfaces such as a flat grass field. This is because form drag at the young seas is anisotropic owing to the directionality of the breaking waves. The anisotropic breaker drag exerts little spanwise drag, which opposes and dissipates the swirling motions of the quasi-streamwise vortices. The more intense vortices result in greater mixing in the near-surface region extending up to 10 to 20 times the breaking wave height. In the second chapter, our focus is on the airflow well inside the wave boundary layer, where the airflow is strongly influenced by individual wakes of breaking waves. The airflow very close to the water surface is resolved, and the breaker drag now appears in the computational domain interior. Such breaker drag 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 for the drag coefficient level off in high winds and are consistent with the laboratory observations. In such conditions more than 90 percent 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 analysis shows that the breaker form drag impedes the shear production of the turbulent kinetic energy (TKE) near the surface and, instead, produces a large amount of small-scale wake turbulence by transferring energy from large-scale motions (such as mean wind and gusts). This process shortcuts the inertial energy cascade and results in large TKE dissipation (integrated over the surface layer) normalized by friction velocity cubed. Consequently, the large production of wake turbulence by breakers in high winds results in the small drag coefficient obtained in this study. Our results also suggest that common parameterizations for the mean wind profile and the TKE dissipation inside the wave boundary layer, used in previous Reynolds averaged Navier-Stokes models, may not be valid.^
"Impact of breaking wave form drag on near-surface turbulence and drag coefficient over young seas at high winds"
Dissertations and Master's Theses (Campus Access).