Effect of breaking waves on air-sea momentum flux

Investigators:
Nobuhiro Suzuki, Tetsu Hara

This project is funded by the National Science Foundation

In collaboration with: Peter Sullivan (NCAR)

Major goals:

Breaking waves support a significant fraction of air-sea momentum flux under high wind and sea state conditions. Since energy and momentum transfer to each breaking wave event occurs intermittently and is localized and intense, it fundamentally alters the turbulence field inside the atmospheric boundary layer. Consequently, air-sea fluxes of momentum, kinetic energy, and heat are significantly modified depending on the distribution of surface breaking waves. Because of the intermittent nature of breaking events, traditional boundary layer models based on Reynolds averaged equations are not sufficient for investigating the breaking wave effects on the boundary layer turbulence. High resolution large eddy simulation (LES) that explicitly resolves breaking wave impacts is needed.

The main objectives of this project are to develop a new LES methodology for the atmospheric boundary layer that accounts for both roughness (subgrid-scale breaking and nonbreaking waves) and dominant (resolved) wave breaking, and to investigate how the boundary layer turbulence and the air-sea momentum flux are affected by different distributions of surface breaking waves. The new model is constructed by combining the existing theoretical wave boundary layer model and the atmospheric boundary layer LES developed by the PI and his collaborators. The wave boundary layer model provides the spectrum of waves and the statistics of breaking waves, and is used to develop a hierarchy of LES tau-rules (relationships between wind speed and wind stress at the lowest LES gridpoint) that vary in space and time according to intermittent subgrid-scale breaking wave events. The wave boundary layer model also provides the statistics of resolved-scale breaking waves so that the momentum loss due to intermittent breaking events is explicitly accounted for within the LES domain. This combination of the state-of-the-art LES and theoretical modeling is the key to successful modeling of boundary layer turbulence over realistic seas. This study produces simulated wind (turbulence) fields over realistic seas with a range of breaking wave distributions, which can be used for various applications. This project provides an essential framework for future investigation of sea spray generation, evolution and their impact on air-sea heat flux under high wind conditions.

Significant results:

A neutrally stratified turbulent air flow over a very young sea surface at a high-wind condition was investigated using large eddy simulations. In such a state, the dominant drag at the sea surface occurs over breaking waves, and the relationship between the dominant drag and local instantaneous surface-wind is highly stochastic and anisotropic. To model such a relationship, a bottom boundary stress parametrization was proposed for the very young sea surface resolving individual breakers. This parametrization was compared to the commonly used parametrization for isotropic surfaces. Over both the young sea and isotropic surfaces, the main, wall-attached, large-scale, turbulence structures were quasi-streamwise vortices. Over the young sea surface, these vortices were more intense and the near-surface mean velocity gradient was smaller. The isotropic surface weakens the swirling motions of the vortices by spanwise drag. In contrast, the young sea surface exerts little spanwise drag and develops more intense vortices which result in greater turbulence and mixing. The vigorous turbulence decreases the mean velocity gradient in the roughness sublayer and thereby increases the roughness length in the logarithmic layer. Thus, the enhancement of the air-sea momentum flux efficiency (drag coefficient or roughness length) due to breaking waves is not only caused by the large streamwise form drag over individual breakers but also by the small spanwise drag. Furthermore, contrary to an assumption used in most existing wave-boundary-layer models, our results suggest that the wave effect may extend as high as 10 to 20 times the breaking wave height.

The effects of breaking waves on near-surface wind turbulence and drag coefficient are investigated using large eddy simulation. The impact of intermittent and transient wave breaking events (over 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 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.

Large-eddy simulation (LES) is used to investigate how dominant breaking waves at the ocean under hurricane-force winds affect the drag and near-surface airflow turbulence. The LES explicitly resolves the wake turbulence produced by dominant-scale breakers. Effects of unresolved roughness such as short breakers, nonbreaking waves, and sea foam are modeled as the subgrid-scale drag. Compared to the laboratory conditions previously studied using the same method, dominant-scale breakers in open ocean conditions are less frequent and the subgrid-scale drag is more significant. Nevertheless, dominant-scale breakers are more fully exposed to high winds and produce more intense wakes individually. As a result, they support a large portion of the total drag and significantly influence the turbulence at many ocean conditions that are likely to occur. The intense wake turbulence is characterized with flow separation, upward bursts of wind, and upward flux of the turbulent kinetic energy (TKE), all of which may influence sea spray dispersion. Similarly to the findings in the laboratory conditions, high production of wake turbulence shortcuts the inertial energy cascade, causes high TKE dissipation, and contributes to reduction of the drag coefficient. Our results also indicate that, if the drag coefficient decreases with increasing wind at very high winds as some recent observations suggest, then the unresolved roughness must also reduce.

Publications:

Suzuki, N., T. Hara, and P. P. Sullivan, 2014: Impact of Dominant Breaking Waves on Air-Sea Momentum Exchange and Boundary Layer Turbulence at High Winds. J. Phys. Oceanogr., 44, 1195-1212.

Suzuki, N., T. Hara, and P. P. Sullivan, 2013: Impact of Breaking Wave Form Drag on Near-Surface Turbulence and Drag Coefficient over Young Seas at High Winds. J. Phys. Oceanogr., J. Phys. Oceanogr., 43, 324–343.

Suzuki, N., T. Hara, and P. P. Sullivan, 2011: Turbulent airflow at young sea states with frequent wave breaking events: large eddy simulation. J. Atoms. Sci., 68, 1290-1305.