scholarly journals The form and orientation of Langmuir cells for misaligned winds and waves

2012 ◽  
Vol 117 (C5) ◽  
pp. n/a-n/a ◽  
Author(s):  
L. P. Van Roekel ◽  
B. Fox-Kemper ◽  
P. P. Sullivan ◽  
P. E. Hamlington ◽  
S. R. Haney
Keyword(s):  
Langmuir Cells

Langmuir turbulence in the ocean

1997 ◽  
Vol 334 ◽  
pp. 1-30 ◽  
Author(s):  
JAMES C. McWILLIAMS ◽  
PETER P. SULLIVAN ◽  
CHIN-HOH MOENG
Keyword(s):  
Volume Transport ◽  
Stokes Drift ◽  
Equilibrium Solutions ◽  
Vertical Fluxes ◽  
Averaged Equations ◽  
Large Eddy ◽  
Convergence Zones ◽  
The Right ◽  
Langmuir Cells ◽  
Oceanic Currents

Solutions are analysed from large-eddy simulations of the phase-averaged equations for oceanic currents in the surface planetary boundary layer (PBL), where the averaging is over high-frequency surface gravity waves. These equations have additional terms proportional to the Lagrangian Stokes drift of the waves, including vortex and Coriolis forces and tracer advection. For the wind-driven PBL, the turbulent Langmuir number, Latur = (U∗/Us)1/2, measures the relative influences of directly wind-driven shear (with friction velocity U∗) and the Stokes drift Us. We focus on equilibrium solutions with steady, aligned wind and waves and a realistic Latur = 0.3. The mean current has an Eulerian volume transport to the right of the wind and against the Stokes drift. The turbulent vertical fluxes of momentum and tracers are enhanced by the presence of the Stokes drift, as are the turbulent kinetic energy and its dissipation and the skewness of vertical velocity. The dominant coherent structure in the turbulence is a Langmuir cell, which has its strongest vorticity aligned longitudinally (with the wind and waves) and intensified near the surface on the scale of the Stokes drift profile. Associated with this are down-wind surface convergence zones connected to interior circulations whose horizontal divergence axis is rotated about 45° to the right of the wind. The horizontal scale of the Langmuir cells expands with depth, and there are also intense motions on a scale finer than the dominant cells very near the surface. In a turbulent PBL, Langmuir cells have irregular patterns with finite correlation scales in space and time, and they undergo occasional mergers in the vicinity of Y-junctions between convergence zones.

Langmuir turbulence in shallow water. Part 1. Observations

2007 ◽  
Vol 576 ◽  
pp. 27-61 ◽  
Author(s):  
ANN E. GARGETT ◽  
JUDITH R. WELLS
Keyword(s):  
Shallow Water ◽  
Turbulent Flows ◽  
Deep Water ◽  
Vertical Velocity ◽  
Surface Stress ◽  
Large Eddy Simulations ◽  
Wave Groups ◽  
Large Eddy ◽  
Langmuir Circulations ◽  
Langmuir Cells

During extended deployment at an ocean observatory off the coast of New Jersey, a bottom-mounted five-beam acoustic Doppler current profiler measured large-scale velocity structures that we interpret as Langmuir circulations filling the entire water column. These circulations are the large-eddy structures of wind-wave-driven turbulent flows that occur episodically when a shallow water column experiences prolonged strong wind forcing. Many observational characteristics agree with former descriptions of Langmuir circulations in deep water. The three-dimensional velocity field reveals quasi-organized structures consisting of pairs of surface-intensified counter-rotating vortices, aligned approximately downwind. Maximum downward velocities are stronger than upward velocities, and the downwelling region of each cell, defined as a pair of vortices, is narrower than the upwelling region. Maximum downward vertical velocity occurs at or above mid-depth, and scales approximately with wind speed. The estimated crosswind scale of cells is roughly 3–6 times their vertical scale, set under these conditions by water depth. The long axis of the cells appears to lie at an angle ∼10°–20° to the right of the wind. A major difference from deep-water observations is strong near-bottom intensification of the downwind ‘jets’ found typically centred over downwelling regions. Accessible observational features such as cell morphology and profiles of mean velocities, turbulent velocity variances, and shear stress components are compared with the results of associated large-eddy simulations (reported in Part 2) of shallow water flows driven by surface stress and the Craik–Leibovich vortex forcing generally used to represent generation of Langmuir cells. A particularly sensitive diagnostic for identification of Langmuir circulations as the energy-containing eddies of the turbulent flow is the depth trajectory of invariants of the turbulent stress tensor, plotted in the Lumley ‘triangle’ corresponding to realizable turbulent flows. When Langmuir structures are present in the observations, the Lumley map is distinctly different from that of surface-stress-driven Couette flow, again in agreement with the large-eddy simulations (LES). Unlike the LES, observed velocity fields contain two distinct and significant scales of variability, documented by wavelet analysis of observational records of vertical velocity. Variability with periods of many minutes is that expected from Langmuir cells drifting past the instrument at the slowly time-varying crosswind velocity. Shorter period variability, of the order of 1–2 min, has roughly the observed periodicity of surface wave groups, suggesting a connection with the wave groups themselves and/or the wave breaking associated with them in high wind conditions.

The generation of Langmuir circulations by an instability mechanism

1977 ◽  
Vol 81 (2) ◽  
pp. 209-223 ◽  
Author(s):  
A. D. D. Craik
Keyword(s):  
Continuous Wave ◽  
Current System ◽  
Plane Waves ◽  
Wave Spectrum ◽  
Stokes Drift ◽  
Instability Mechanism ◽  
Fluid Layers ◽  
The Mean ◽  
Langmuir Cells ◽  
Relationship Of

Equations governing the current system in the upper layers of oceans and lakes were derived by Craik & Leibovich (1976). These incorporate the dominant effects of both wind and waves. Solutions comprising the mean wind-driven current and a system of ‘Langmuir’ cells aligned parallel to the wind were found for cases in which the wave field consisted of just a pair of plane waves. However, it was not clear that such cellular motions would persist for the more realistic case of a continuous wave spectrum.The present paper shows that, in the latter case, infinitesimal spanwise periodic perturbations will grow on account of an instability mechanism. Mathematically, the instability is closely similar to the onset of thermal convection in horizontal fluid layers. Physically, the mechanism is governed by kinematical processes involving the mean (Eulerian) wind-driven current and the (Lagrangian) Stokes drift associated with the waves. The relationship of this mechanism to instability models of Garrett and Gammelsrød is clarified.

scholarly journals Reply to “Comment on ‘Using an ADCP to Estimate Turbulent Kinetic Energy Dissipation Rate in Sheltered Coastal Waters’”

2017 ◽  
Vol 34 (6) ◽  
pp. 1391-1392
Author(s):  
A. D. Greene ◽  
P. J. Hendricks ◽  
M. C. Gregg
Keyword(s):  
Coastal Waters ◽  
Dissipation Rate ◽  
Energy Dissipation Rate ◽  
Stratified Turbulence ◽  
Dominant Mechanism ◽  
Turbulent Dissipation ◽  
Velocity Estimate ◽  
Horizontal Length ◽  
Large Eddy ◽  
Langmuir Cells

AbstractThis note is a comment in response to Gargett, who argues that a large-eddy estimate of turbulent dissipation rate using a horizontal length scale with a vertical velocity estimate, as in Greene et al., is a dubious approximation if the energy-containing eddies are anisotropic. A simulation of Langmuir cells and associated turbulence is used to support Gargett’s conclusions. This rebuttal reviews the approaches taken by Greene et al. and cites several instances of flawed reasoning by Gargett. This includes using Langmuir simulations to support the primary conclusion of Gargett, which seems unconnected to Greene et al.’s data and ignores a vast body of work on simulating Kelvin–Helmholtz instabilities, widely considered to be the dominant mechanism producing stratified turbulence.

A pair of Langmuir cells in a laboratory tank (I) wind-only experiment

1992 ◽  
Vol 48 (1) ◽  
pp. 37-57 ◽  
Author(s):  
Shinjiro Mizuno ◽  
Zhan Cheng
Keyword(s):  
Laboratory Tank ◽  
Langmuir Cells

scholarly journals Optics and remote sensing of Bahamian carbonate sediment whitings and potential relationship to wind-driven Langmuir circulation

2009 ◽  
Vol 6 (3) ◽  
pp. 487-500 ◽  
Author(s):  
H. M. Dierssen ◽  
R. C. Zimmerman ◽  
D. J. Burdige
Keyword(s):  
Remote Sensing ◽  
Water Column ◽  
Natural Waters ◽  
Spatial Scales ◽  
Light Attenuation ◽  
Sediment Resuspension ◽  
Visible Spectrum ◽  
Index Of Refraction ◽  
Langmuir Circulation ◽  
Langmuir Cells

Abstract. Regions of milky white seas or "whitings" periodically occur to the west of Andros Island along the Great Bahama Bank where the bottom sediment consists of fine-grained aragonite mud. We present measurements of inherent optical properties within a sediment whiting patch and discuss the potential for monitoring the frequency, extent, and quantity of suspended matter from ocean colour satellite imagery. Sea spectral reflectance measured in situ and remotely from space revealed highly reflective waters elevated across the visible spectrum (i.e., "whitened") with a peak at 490 nm. Particulate backscattering was an order of magnitude higher than that measured at other stations throughout the region. The whiting also had one of the highest backscattering ratios measured in natural waters (0.05–0.06) consistent with water dominated by aragonite particles with a high index of refraction. Regular periodicity of 40 and 212 s evident in the light attenuation coefficient over the sampling period indicated patches of fluctuating turbidity on spatial scales that could be produced from regular rows of Langmuir cells penetrating the 5-m water column. We suggest that previously described mechanisms for sediment resuspension in whitings, such as tidal bursting and fish activity, are not fully consistent with these data and propose that wind-driven Langmuir cells reaching the full-depth of the water column may represent a plausible mechanism for sediment resuspension and subsequent whiting formation. Optics and remote sensing provide important tools for quantifying the linkages between physical and biogeochemical processes in these dynamic shallow water ecosystems.

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