Vertical profiles of the wave-coherent airflow over ocean waves

Current wind-wave numerical models are largely based on a statistical description of the surface waves and the marine atmospheric boundary layer (MABL) and do not resolve the phase of the waves nor the modulation of the wind by the waves. However, the new generation of LES models provides wave-resolved dynamics, kinematics and the associated wave-coherent air-flow (Sullivan et al., 2014). It is therefore important to provide experimental descriptions of the wave field and the structure of the MABL to test the validity of the numerical simulations. We present an analysis of coherent wind and wave data collected from R/P FLIP off the coast of Southern California in November 2013. The data were collected to better understand the modulation of the MABL by the wave field. The wave-coherent airflow was measured by an array of five sonic anemometers distributed on a vertical telescopic mast extending up to 13.5m above the ocean surface, mounted at the end of R/P FLIP's port boom. The position of the sensors was modified throughout the experiment to collect measurements as close as possible to the surface, as low as 2.5 m above the mean sea level.

Amplitude and phase of the wave-induced components of the wind relative to the waves for various wind-wave conditions at each height were computed spectrally. Results show that the wave-induced fluctuations depend on both the critical height z_c and the normalized height kz. The critical height z_c is defined as the height where the mean wind speed U(z_c) equals the phase speed of the waves c. The normalized height kz is the height of the anemometers z normalized by the wavenumber k of the waves with k = g/c².

In the neighborhood of the critical height, results are qualitatively consistent with the critical layer theory of Miles (1957). Across the critical height, the wave-induced vertical and horizontal velocities change significantly in both amplitude and phase. Below the critical layers, a suitable normalization of the wave-induced fluctuations collapses the data from all the anemometers, on a curve which follows an exponential decay with the normalized height kz.

References:

Miles J. W. (1957). On the generation of surface waves by shear flows. J. Fluid. Mech., Vol 3, 185-204.

Sullivan et al. (2014). Large eddy simulation of marine atmospheric boundary layers above a spectrum of moving waves. J. Atmos. Sci., Advance online publication. doi: 10.1175/JAS-D-14-0095.1