Airflow separation above wind waves

The influence of waves on the flux of momentum is perhaps still the largest uncertainty in determining precise bulk air-sea momentum fluxes for a variety of conditions. Yet, an accurate knowledge of the drag of the ocean on the atmospheric flow is, for example, crucial to the prediction of high wind speed conditions such as those which occur in tropical storms and hurricanes, and also for the modeling of long term global climate trends. In recent studies, it was found that it is the small wind waves that support much of the stress at the interface between the ocean and the atmosphere. However, the role of the airflow separation above the waves is not well understood. This is due to the fact that there is, to date, virtually no direct observations of airflow separation above wind waves, let alone quantitative stress measurements. This, in turn, is due to the difficulty of making measurements within a wave height of the mean water level.

We present direct laboratory measurements of the velocity in the airflow above wind generated waves. These data were obtained using optical techniques and thus we were able to measure the velocity within the viscous sublayer above the wavy surface. We observe direct evidence of the separation of this viscous sublayer past the crest of the wind waves. The separation leads to the transport of vorticity away from the surface and the generation of significant mixing. Concurrently, we present the first direct estimate of the surface viscous tangential stress in the airflow. The intermittent separation of the viscous sublayer past the crest of the wind waves leads to dramatic along-wave variability in the surface viscous tangential stress. These results hold for wind speeds that would normally be considered low to moderate. Furthermore, the statistics of separation events correlate with surface wave slope parameters, indicating that, while linear and non-separated models might only be adequate for the very initial stages of the wave generation process and in extreme low wind speed conditions, parameterizations of the mean effects of airflow separation on the total air-sea stress based on surface wave spectra products might be successful approaches.