Transient Separation-Like Airflow over Wind Waves and Its Impact on Air-sea Momentum Flux

Accurate predictions of momentum exchanges between ocean and atmosphere are essential to understanding various air-sea coupled processes. It is usually assumed that when airflow separates at the wave crest, it enhances the form drag (pressure force on the wave) and increases the air-sea momentum flux and the drag coefficient. It is also assumed that airflow can separate only when waves below are breaking. However, recent laboratory observations suggest that separation like airflow patterns are ubiquitous and may occur over steep but non-breaking waves. In this study we investigate the airflow over a finite amplitude non-breaking wave train using a three-dimensional large eddy simulation. First, instantaneous velocity, vorticity, and pressure fields are examined. The airflow sometimes exhibits separation like flow patterns, identified with thin high vorticity layers detached from the surface and separation bubbles below. At other times the airflow remains attached to the surface. The pressure acting on the windward slope of the wave varies significantly depending on the flow pattern. Next, conditional phase averaging of the airflow is performed depending on the pressure force (pressure multiplied by the wave slope) acting on each wave. It is found that smaller pressure force is associated with separated airflow, while larger pressure force is associated with attached airflow. When incoming wind speed above the wave crest is faster, the flow remains attached, whereas slower incoming wind tends to enhance detachment of the airflow. These observations question some of the traditional assumptions of airflow separations and their impact on air-sea momentum flux.