Wave-Coherent Airflow and Critical Layers over Ocean Waves

Abstract

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 zc, defined such that U(zc) = 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 ≈ 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 Cp/u* > 60, where Cp is the phase speed at the peak of the wave spectrum.

Structure of the Airflow above Surface Waves

“…The first, focused on turbulent flux measurements and efforts to parameterize drag as a function of wind speed (e.g., Edson et al 2013), were motivated by a need to rapidly improve operational forecasting and climate modeling (see, e.g., the COARE momentum flux model; Webster and Lukas 1992). Others, concerned with the fundamental physics of wave generation and growth mechanisms, were able to relate their flux measurements to the phase of the waves, thereby estimating wave-coherent fluxes, which provided evidence of Miles's critical layer mechanism for a certain range of wave ages (Hristov et al 2003;Grare et al 2013a). However, these field studies, challenged by the technical difficulties involved with the study of small-scale dynamics in the open ocean near a highly dynamic interface, were limited to fixed-height vertical profile measurements some distance above the level of the highest wave crest.…”

Airflow measurements at a wavy air–water interface using PIV and LIF

“…In the engineering sector, coupled interfacial gas-liquid dynamics are of interest because they influence the efficiency of chemical reactors, boilers, turbines (Turney and Banerjee 2008;Hewitt 2013). In the 1 3 161 Page 2 of 20 and Fairall 1998;Janssen 1999;Sullivan and McWilliams 2002;Sullivan et al 2008;Yang and Shen 2010;Suzuki et al 2013;Grare et al 2013a;Hara and Sullivan 2015). Airflow separation for example, is the kinematic air-side equivalent to wave breaking in the water (Banner and Melville 1976;Gent and Taylor 1977), a process which is known to generate turbulence and enhance dissipation (Rapp and Melville 1990;Melville et al 2002;Drazen and Melville 2009).…”

“…(Lumley and Terray 1983;Thais and Magnaudet 1996;Hristov et al 1998;Veron et al 2008;Grare et al 2013a). This is in part due to the technical difficulty associated with making measurements close to the air water interface in the laboratory or the field, and the difficulty in modeling the complex non-linear interactions between the turbulence and the surface waves.…”

“…Later, Veron et al (2007) were able to estimate viscous stresses in the air above wind-generated waves by PIV. Grare (2009) and Grare et al (2013b) obtained single point airflow velocity measurements above laboratory wind waves, by repeatedly plunging a hot wire anemometer into the water for each experimental condition. Using this innovative method, they measured vertical airflow velocity profiles with 1 velocity measurement every 50 μm, and computed viscous and form drags above wind waves (Grare et al 2013b).…”

“…Using this innovative method, they measured vertical airflow velocity profiles with 1 velocity measurement every 50 μm, and computed viscous and form drags above wind waves (Grare et al 2013b). However, due to the one-dimensionality of their measurement system, wave phase detection was only indirectly achieved using a single point wave gauge (Grare 2009); additionally the instantaneous spatial along-wave structure of the airflow was not resolved. Recently, Buckley and Veron (2016), using the combined PIV-LIF experimental system described herein, were able to characterize, as a function of wave age, the instantaneous fine-scale structure of the airflow above wind and mechanically generated waves, in low to moderate wind speeds.…”