An official website of the United States government
Highly resolved laboratory measurements of the airflow over wind-generated waves are examined using a novel wave growth diagnostic that quantifies the presence of Miles’ critical layer mechanism of wind-wave growth. The wave growth diagnostic is formulated based on a linear stability analysis, and results in growth rates that agree well with those found by a pressure reconstruction method as well as other, less direct, methods. This finding, combined with a close agreement between the airflow measurements and the predictions of linear stability (critical layer) theory, demonstrate that the Miles’ critical layer mechanism can cause significant wave growth in young (wave age $$c/u_* = 6.3$$ , where $$c$$ is the wave phase speed, and $$u_*$$ the friction velocity) wind-forced waves.
more »
« less
Observations of mean and wave orbital flows in the ocean’s upper centimetres
Sophisticated measurements of fluid velocity near to an undulating air–water boundary have traditionally been confined to the laboratory setting. Developments in camera technology and the opening of novel modes of analysis have allowed for sensitive measurements of the current profile in the ocean’s uppermost layer. Taking advantage of the Research Platform R/P FLIP as a ‘laboratory at sea’, here we present first-of-their-kind thermal and polarimetric camera-based observations of wave orbital velocities and mean shear flows in the upper centimetres of the ocean surface layer. Measurements reveal a well-defined logarithmic layer as seen in laboratory measurements and described by classical surface layer theory; however, substantial spread of observations is found at low levels of wind forcing, where the Stokes drift of swell may have a substantial impact on the near-surface current profile. A novel application of short time window Fourier transforms allows for the estimation of near-surface wave orbital velocity magnitudes. These are found to be in general agreement with the prescriptions of linear wave theory, although observations diverge from theory at high levels of wind forcing where the interface is subject to surface wave breaking. Finally, the surface gravity wave phase-coherent short wave growth is presented and discussed in the context of hydrodynamic wave and airflow modulation.
more »
« less
- Award ID(s):
- 1923935
- PAR ID:
- 10191188
- Date Published:
- Journal Name:
- Journal of Fluid Mechanics
- Volume:
- 887
- ISSN:
- 0022-1120
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract The wind shear stress at the ocean surface drives momentum exchange across the air-sea interface regulating atmospheric and oceanic phenomena. Theoretically, the mean wind stress acts in a reference frame moving with the ocean surface; however, the relative motion between the air and ocean surface layers is conventionally neglected in bulk transfer formulae. Recent developments improving air-sea momentum flux quantification advocate for explicitly defining the air-sea relative wind, especially in the regime of low wind forcing, where surface currents may approach a significant fraction of the total wind speed. Yet, in practice, this new approach is typically applied using opportunistic definitions of the near-surface current. Here, we build on this recent work and propose a general framework for the bulk air-sea momentum flux that directly accounts for vertical current shear and surface waves in quantifying the stress at the interface. Our approach partitions the stress at the interface into viscous skin and (wave) form drag components, each applied to their relevant surface advections, which are quantified using the inertial motions within the sub-surface log layer and the modulation of waves by currents predicted by linear theory, respectively. The efficacy of this approach is demonstrated using an extensive oceanic dataset from the Coastal Endurance Array (Ocean Observatories Initiative) offshore of Newport, Oregon (2017–2023) that includes co-located measurements of direct covariance wind stress, directional wave spectra, and current profiles. As expected, our framework does not alter the overall dependence of momentum flux on mean wind forcing, and we found the largest impacts at relatively low wind speeds. Below 3 m s$$^{-1}$$, accounting for sub-surface shear reduced form drag variation by 40–50% as compared to a current-agnostic approach; as compared to a shear-free current, i.e., slab ocean, a 35% reduction in form drag variation was found. At this wind forcing, neglecting the currents led to systematically overestimating the form stress by 20 to 50%—an effect that could not be captured by using the slab ocean approach. This framework builds on the existing understanding of wind-wave-current interaction, yielding a novel formulation that explicitly accounts for the role of current shear and surface waves in air-sea momentum flux. This work holds significant implications for air-sea coupled modeling in general conditions.more » « less
-
Abstract Surface waves grow through a mechanism in which atmospheric pressure is offset in phase from the wavy surface. A pattern of low atmospheric pressure over upward wave orbital motions (leeward side) and high pressure over downward wave orbital motions (windward side) travels with the water wave, leading to a pumping of kinetic energy from the atmospheric boundary layer into the waves. This pressure pattern persists above the air–water interface, modifying the turbulent kinetic energy in the atmospheric wave-affected boundary layer. Here, we present field measurements of wave-coherent atmospheric pressure and velocity to elucidate the transfer of energy from the atmospheric turbulence budget into waves through wave-coherent atmospheric pressure work. Measurements show that the phase between wave-coherent pressure and velocity is shifted slightly above 90° when wind speed exceeds the wave phase speed, allowing for a downward energy flux via pressure work. Although previous studies have reported wave-coherent pressure, to the authors’ knowledge, these are the first reported field measurements of wave-coherent pressure work. Measured pressure work cospectra are consistent with an existing model for atmospheric pressure work. The implications for these measurements and their importance to the turbulent kinetic energy budget are discussed. Significance StatementSurface waves grow through a pattern of atmospheric pressure that travels with the water wave, acting as a pump against the water surface. The pressure pumping, sometimes called pressure work, or the piston pressure, results in a transfer of kinetic energy from the air to the water that makes waves grow larger. To conserve energy, it is thought that the pressure work on the surface must extract energy from the mean wind profile or wind turbulence that sets the shape of the wind speed with height. In this paper, we present direct measurements of pressure work in the atmosphere above surface waves. We show that the energy extracted by atmospheric pressure work fits existing models for how waves grow and a simple model for how waves reduce energy in the turbulent kinetic energy budget. To our knowledge, these are the first reported field measurements of wave-coherent pressure work.more » « less
-
In recent years, the global transition towards green energy, driven by environmental concerns and increasing electricity demands, has remarkably reshaped the energy landscape. The transformative potential of marine wind energy is particularly critical in securing a sustainable energy future. To achieve this objective, it is essential to have an accurate understanding of wind dynamics and their interactions with ocean waves for the proper design and operation of offshore wind turbines (OWTs). The accuracy of met-ocean models depends critically on their ability to correctly capture sea-surface drag over the multiscale ocean surface—a quantity typically not directly resolved in numerical models and challenging to acquire using either field or laboratory measurements. Although skin friction drag contributes considerably to the total wind stress, especially at moderate wind speeds, it is notoriously challenging to predict using physics-based approaches. The current work introduces a novel approach based on a convolutional neural network (CNN) model to predict the spatial distributions of skin friction drag over wind-generated surface waves using wave profiles, local wave slopes, local wave phases, and the scaled wind speed. The CNN model is trained using a set of high-resolution laboratory measurements of air-side velocity fields and their respective surface viscous stresses obtained over a range of wind-wave conditions. The results demonstrate the capability of our model to accurately estimate both the instantaneous and area-aggregate viscous stresses for unseen wind-wave regimes. The proposed CNN-based wall-layer model offers a viable pathway for estimating the local and averaged skin friction drag in met-ocean simulations.more » « less
-
Abstract Measurements collected by a Remote Environmental Monitoring Units (REMUS) 600 autonomous underwater vehicle (AUV) off the coast of southern California demonstrate large-scale coherent wave-driven vortices, consistent with Langmuir turbulence (LT), and played a dominant role in structuring turbulent dissipation within the oceanic surface boundary layer. During a 10-h period with sustained wind speeds of 10 m s−1, Langmuir circulations were limited to the upper third of the surface mixed layer by persistent stratification within the water column. The ensemble-averaged circulation, calculated using conditional averaging of acoustic Doppler dual current profile (AD2CP) velocity profiles using elevated backscattering intensity associated with subsurface bubble clouds, indicates that LT vortex pairs were characterized by an energetic downwelling zone flanked by broader, weaker upwelling regions with vertical velocity magnitudes similar to previous numerical studies of LT. Horizontally distributed microstructure estimates of turbulent kinetic energy dissipation rates were lognormally distributed near the surface in the wave mixing layer with the majority of values falling between wall layer scaling and wave transport layer scaling. Partitioning dissipation rates between downwelling centers and ambient conditions suggests that LT may play a dominant role in elevating dissipation rates in the ocean surface boundary layer (OSBL) by redistributing wave-breaking turbulence.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1017/jfm.2019.1019
