Vertical profiles of the wave-coherent airflow over ocean surface 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 wave-coherent airflow was measured by an array of five sonic anemometers, ranging from 2.5m up to 13.5m above the ocean surface, distributed on a vertical telescopic mast mounted at the end of R/P FLIP’s port boom.

Results show that, below the critical height z_c where the wind speed U(z_c) equals the phase speed of the waves c, the normalization of the wave-induced fluctuations by the amplitude of the wave orbital velocities collapses the data from all the anemometers on a curve which follows an exponential decay with the normalized height kz.

This experiment also highlighted discrepancies between data measured by Campbell CSAT3 and GILL R3-50 sonic anemometers. The differences between the anemometers depend strongly on the wind direction. The relative error of the mean wind speed can reach 4%, while the relative error of the friction velocity can reach 20% (i.e. 44% for the momentum flux). Several experiments conducted in various environmental conditions confirm these results.