|Title||Wave-coherent airflow and critical layers over ocean waves|
|Publication Type||Journal Article|
|Year of Publication||2013|
|Authors||Grare L, Lenain L., Melville W.K|
|Journal||Journal of Physical Oceanography|
An analysis of coherent measurements of winds and waves from data collected during the Office of Naval Research (ONR) High-Resolution air-sea interaction (HiRes) program, from the Floating Instrument Platform (R/P FLIP), off the coast of northern California in June 2010 is presented. A suite of wind and wave measuring systems was deployed to resolve the modulation of the marine atmospheric boundary layer by waves. Spectral analysis of the data provided the wave-induced components of the wind velocity for various wind-wave conditions. The power spectral density, the amplitude, and the phase (relative to the waves) of these wave-induced components are computed and bin averaged over spectral wave age c/U(z) or c/u(*), where c is the linear phase speed of the waves, U(z) is the mean wind speed measured at the height z of the anemometer, and u(*) is the friction velocity in the air. Results are qualitatively consistent with the critical layer theory of Miles. Across the critical height z(c), defined such that U(z(c)) = c, the wave-induced vertical and horizontal velocities change significantly in both amplitude and phase. The measured wave-induced momentum flux shows that, for growing waves, less than 10% of the momentum flux at z approximate to 10 m is supported by waves longer than approximately 15 m. For older sea states, these waves are able to generate upward wave-induced momentum flux opposed to the overall downward momentum flux. The measured amplitude of this upward wave-induced momentum flux was up to 20% of the value of the total wind stress when C-p/u(*) > 60, where C-p is the phase speed at the peak of the wave spectrum.