Lundi 18 mars à 11h00 ; salle des séminaires IRPHE
Abstract: The exchanges of momentum, energy, and mass across the air-sea interface are controlled by smallscale turbulent processes that take place within the first millimeters to meters above and below the highly dynamic, wavy ocean surface. In fact, a majority of the energy input from the atmosphere into the ocean arguably originates from wind blowing over the wavy ocean surface.
Despite the importance of the processes of surface wave generation and dissipation, there are still fundamental gaps in our ability to conduct both process modelling, and observational studies of these processes operating near the air-sea interface. This is partly due to the technical challenges involved in the measurement of the small-scale motions very close to the rapidly moving ocean surface.
We present laboratory and in situ measurements of the submillimeter-scale motions in the coupled airwater boundary layers, above and below surface gravity waves. A high resolution, large field of view Particle Image Velocimetry (PIV) system was specially developed for the measurements, capable of capturing 2D velocity fields in the turbulent airflow directly above and below the wavy surface. The system was deployed from in situ offshore research platforms as well as in the laboratory (wind-wave tank).
Using these high resolution observations, we will discuss the impact of waves on the structure of the wind stress within the wave boundary layer and particularly its partitioning between viscous, form drag, wave-coherent and turbulent contributions. In spite of significant modulations of the surface viscous stress along the waves, especially in the presence of intense sheltering events (airflow separation), our results suggest a very limited contribution of viscous stress to wind-wave growth, along with a surprisingly strong agreement of the mean along-wave airflow dynamics with linear theory.
Marc Buckley / Institute of Coastal Research, Helmholtz-Zentrum Hereon, Germany