Session 17A.2 On the wave induced wind in the marine atmospheric boundary layer

Friday, 13 June 2008: 10:45 AM
Aula Magna Vänster (Aula Magna)
Alvaro Semedo, Uppsala University, Uppsala, Sweden; and Ø. Sætra and A. Rutgersson

Presentation PDF (127.5 kB)

Recent field observations and large-eddy simulations have shown that the impact of fast running waves (swell) on the marine atmospheric boundary layer (MABL) might be stronger than previously assumed. In low to moderate wave following winds, swell propagates faster than the mean wind, resulting on increasing values of the wave age ( ). The total stress (momentum flux) above the sea surface will therefore have two major components: the turbulent shear stress, and the wave-induced stress, directed downward and upward, respectively. For sufficiently high wave age values, the wave-induced component becomes increasingly dominant, and the total momentum flux will be directed into the atmosphere. Recent field measurements have shown that this upward momentum transfer from the ocean into the atmosphere has a considerable impact on the surface layer flow dynamics and on the turbulence structure of the MABL.The vertical wind profile will no longer exhibit a logarithmic shape, since an acceleration of the air flow near the surface will take place, generating a low level wave-driven wind maximum (a wind jet).The picture that emerges from this feedback process is of momentum being transferred from the wind into the ocean. Part of this momentum will be used to drive ocean currents, but some will be responsible by the wave generation process along storm tracks. As waves propagate away from their generation area as swell, some of the momentum will be returned to the atmosphere in the form of wave-driven winds. A model that reproduces quantitatively and qualitatively the wave following atmospheric flow and the wave generated wind maximum, as seen from measurements, is proposed. The model has no previous assumptions or restrictions on the turbulence structure of the surface layer of the MABL. New parameterizations for the wave-induced stress at the surface (expressed as a function of the wave damping ratio and wave slope) and the variation of the wave-induced stress with height are included in the model.
- Indicates paper has been withdrawn from meeting
- Indicates an Award Winner