Wave-coherent airflow and critical layers over ocean waves

TitleWave-coherent airflow and critical layers over ocean waves
Publication TypeJournal Article
Year of Publication2013
AuthorsGrare L, Lenain L., Melville W.K
JournalJournal of Physical Oceanography
Date Published2013/10
ISBN Number0022-3670
Accession NumberWOS:000325534400008

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.

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