Surface wave effects on submesoscale fronts and filaments

2018 ◽  
Vol 843 ◽  
pp. 479-517 ◽  
Author(s):  
James C. McWilliams
Keyword(s):  
Surface Wave ◽  
Turbulent Mixing ◽  
Wind Wave ◽  
Surface Wind ◽  
Stokes Drift ◽  
Momentum Balance ◽  
Surface Wind Stress ◽  
Time Scale Separation ◽  
Complementary Effect ◽  
Wave Effects

A diagnostic analysis is made for the ageostrophic secondary circulation, buoyancy flux and frontogenetic tendency (SCFT) in upper-ocean submesoscale fronts and dense filaments under the combined influences of boundary-layer turbulent mixing, surface wind stress and surface gravity waves. The analysis is based on a momentum-balance approximation that neglects ageostrophic acceleration, and the surface wave effects are represented with a wave-averaged asymptotic theory based on the time scale separation between wave and current evolution. The wave’s Stokes-drift velocity $\boldsymbol{u}_{st}$ induces SCFT effects that are dominant in strong swell with weak turbulent mixing, and they combine with Ekman and turbulent thermal wind influences in more general situations near wind–wave equilibrium. The complementary effect of the submesoscale currents on the waves is weak for longer waves near the wind–wave or swell spectrum peak, but it is strong for shorter waves.

scholarly journals Turbulence-Induced Anti-Stokes Flow and the Resulting Limitations of Large-Eddy Simulation

2018 ◽  
Vol 48 (1) ◽  
pp. 117-122 ◽  
Author(s):  
Brodie Pearson
Keyword(s):  
Stokes Flow ◽  
Energy Production ◽  
Surface Wind ◽  
Stokes Drift ◽  
Near Surface ◽  
Eddy Simulation ◽  
Surface Wind Stress ◽  
Planetary Rotation ◽  
Large Eddy ◽  
Anti Stokes

AbstractThis study shows that the presence of Stokes drift us in the turbulent upper ocean induces a near-surface Eulerian current that opposes the Stokes drift. This current is distinct from previously studied anti-Stokes currents because it does not rely on the presence of planetary rotation or mean lateral gradients. Instead, the anti-Stokes flow arises from an interaction between the Stokes drift and turbulence. The new anti-Stokes flow is antiparallel to us near the ocean surface, is parallel to us at depth, and integrates to zero over the depth of the boundary layer. The presence of Stokes drift in large-eddy simulations (LES) is shown to induce artificial energy production caused by a combination of the new anti-Stokes flow and LES numerics. As a result, care must be taken when designing and interpreting simulations of realistic wave forcing, particularly as rotation becomes weak and/or us becomes perpendicular to the surface wind stress. The mechanism of the artificial energy production is demonstrated for a generalized LES subgrid scheme.

Submesoscale surface fronts and filaments: secondary circulation, buoyancy flux, and frontogenesis

2017 ◽  
Vol 823 ◽  
pp. 391-432 ◽  
Author(s):  
James C. McWilliams
Keyword(s):  
Surface Wind ◽  
Circulation Pattern ◽  
Solution Procedure ◽  
Secondary Circulation ◽  
Buoyancy Flux ◽  
Momentum Balance ◽  
Rossby Number ◽  
Surface Wind Stress ◽  
Wide Range ◽  
Ageostrophic Circulation

Problems are posed and solved for upper-ocean submesoscale density fronts and filaments in the presence of surface wind stress and the associated boundary-layer turbulent mixing, their associated geostrophic and secondary circulations and their instantaneous buoyancy fluxes and frontogenetic evolutionary tendencies in both velocity and buoyancy gradients. The analysis is diagnostic rather than prognostic, and it is based on a momentum-balanced approximation that assumes the ageostrophic acceleration is negligible, although the Rossby number is finite and ageostrophic advection is included, justified by the quasi-steady, coherent-structure flow configurations of fronts and filaments. Across a wide range of wind and buoyancy-gradient parameters, the ageostrophic secondary circulation for a front is a single overturning cell with downwelling on the dense side, hence with a positive (restratifying) vertical buoyancy flux. For a dense filament the circulation is a double cell with central downwelling and again positive vertical buoyancy flux. The primary explanation for these secondary-circulation cells is a ‘turbulent thermal wind’ linear momentum balance. These circulation patterns, and their associated frontogenetic tendencies in both the velocity and buoyancy gradients, are qualitatively similar to those due to the ‘classical’ mechanism of strain-induced frontogenesis. For linear solutions, the secondary circulation and frontogenesis are essentially independent of wind direction, but in nonlinear solutions ageostrophic advection provides a strong intensification of the peak vertical velocity, while generally preserving the ageostrophic circulation pattern, when the Rossby number is order one and the wind orientation relative to the frontal axis is favourable. At large Rossby number the solution procedure fails to converge, with an implication of a failure of existence of wholly balanced circulations.

The Depth-Dependent Current and Wave Interaction Equations: A Revision

2008 ◽  
Vol 38 (11) ◽  
pp. 2587-2596 ◽  
Author(s):  
George L. Mellor
Keyword(s):  
Ocean Circulation ◽  
Wave Energy ◽  
Energy Equation ◽  
Surface Wind ◽  
Three Dimensional ◽  
Wave Interaction ◽  
Stokes Drift ◽  
Mean Velocity ◽  
Surface Wind Stress ◽  
The Mean

Abstract This is a revision of a previous paper dealing with three-dimensional wave-current interactions. It is shown that the continuity and momentum equations in the absence of surface waves can include waves after the addition of three-dimensional radiation stress terms, a fairly simple alteration for numerical ocean circulation models. The velocity that varies on time and space scales, which are large compared to inverse wave frequency and wavenumber, is denoted by ûα and, by convention, is called the “current.” The Stokes drift is labeled uSα and the mean velocity is Uα ≡ ûα + uSα. When vertically integrated, the results here are in agreement with past literature. Surface wind stress is empirical, but transfer of the stress into the water column is a function derived in this paper. The wave energy equation is derived, and terms such as the advective wave velocity are weighted vertical integrals of the mean velocity. The wave action equation is not an appropriate substitute for the wave energy equation when the mean velocity is depth dependent.

scholarly journals Whitecap and Wind Stress Observations by Microwave Radiometers: Global Coverage and Extreme Conditions

2019 ◽  
Vol 49 (9) ◽  
pp. 2291-2307 ◽  
Author(s):  
Paul A. Hwang ◽  
Nicolas Reul ◽  
Thomas Meissner ◽  
Simon H. Yueh
Keyword(s):  
Surface Wave ◽  
Wind Speed ◽  
Wind Stress ◽  
Thermal Emission ◽  
Microwave Radiometer ◽  
Wave Spectrum ◽  
Surface Wind ◽  
Unit Surface Area ◽  
Whitecap Coverage ◽  
Surface Wind Stress

AbstractWhitecaps manifest surface wave breaking that impacts many ocean processes, of which surface wind stress is the driving force. For close to a half century of quantitative whitecap reporting, only a small number of observations are obtained under conditions with wind speed exceeding 25 m s−1. Whitecap contribution is a critical component of ocean surface microwave thermal emission. In the forward solution of microwave thermal emission, the input forcing parameter is wind speed, which is used to generate the modeled surface wind stress, surface wave spectrum, and whitecap coverage necessary for the subsequent electromagnetic (EM) computation. In this respect, microwave radiometer data can be used to evaluate various formulations of the drag coefficient, whitecap coverage, and surface wave spectrum. In reverse, whitecap coverage and surface wind stress can be retrieved from microwave radiometer data by employing precalculated solutions of an analytical microwave thermal emission model that yields good agreement with field measurements. There are many published microwave radiometer datasets covering a wide range of frequency, incidence angle, and both vertical and horizontal polarizations, with maximum wind speed exceeding 90 m s−1. These datasets provide information of whitecap coverage and surface wind stress from global oceans and in extreme wind conditions. Breaking wave energy dissipation rate per unit surface area can be estimated also by making use of its linear relationship with whitecap coverage derived from earlier studies.

scholarly journals Langmuir Turbulence in Coastal Zones: Structure and Length Scales

2018 ◽  
Vol 48 (5) ◽  
pp. 1089-1115 ◽  
Author(s):  
Kalyan Shrestha ◽  
William Anderson ◽  
Joseph Kuehl
Keyword(s):  
Boundary Layer ◽  
Surface Wind ◽  
Stokes Drift ◽  
Small Scale ◽  
Langmuir Turbulence ◽  
Coastal Zones ◽  
Length Scales ◽  
Environmental Forcing ◽  
Surface Wind Stress ◽  
Langmuir Cells

AbstractLangmuir turbulence is a boundary layer oceanographic phenomenon of the upper layer that is relevant to mixing and vertical transport capacity. It is a manifestation of imposed aerodynamic stresses and the aggregate horizontal velocity profile due to orbital wave motion (the so-called Stokes profile), resulting in streamwise-elongated, counterrotating cells. The majority of previous research on Langmuir turbulence has focused on the open ocean. Here, we investigate the characteristics of coastal Langmuir turbulence by solving the grid-filtered Craik–Leibovich equations where the distinction between open and coastal conditions is a product of additional bottom boundary layer shear. Studies are elucidated by visualizing Langmuir cell vortices using isosurfaces of Q. We show that different environmental forcing conditions control the length scales of coastal Langmuir cells. We have identified regimes where increasing the Stokes drift velocity and decreasing surface wind stress both act to change the horizontal size of coastal Langmuir cells. Furthermore, wavenumber is also responsible in setting the horizontal extent Ls of Langmuir cells. Along with that, wavenumber that is linked to the Stokes depth δs controls the vertical extent of small-scale vortices embedded within the upwelling limb, while the downwelling limb occupies the depth of the water column H for any coastal surface wave forcing (i.e., and ). Additional simulations are included to demonstrate insensitivity to the grid resolution and aspect ratio.

Surface Wave Effects on High-Frequency Currents over a Shelf Edge Bank

2013 ◽  
Vol 43 (8) ◽  
pp. 1627-1647 ◽  
Author(s):  
H. W. Wijesekera ◽  
D. W. Wang ◽  
W. J. Teague ◽  
E. Jarosz ◽  
W. E. Rogers ◽  
...  
Keyword(s):  
Surface Wave ◽  
Kinetic Energy ◽  
Mixed Layer ◽  
High Frequency ◽  
Pressure Sensors ◽  
Stokes Drift ◽  
Langmuir Circulation ◽  
Temperature Conductivity ◽  
Wave Focusing ◽  
Wave Effects

Abstract Several acoustic Doppler current profilers and vertical strings of temperature, conductivity, and pressure sensors, deployed on and around the East Flower Garden Bank (EFGB), were used to examine surface wave effects on high-frequency flows over the bank and to quantify spatial and temporal characteristic of these high-frequency flows. The EFGB, about 5-km wide and 10-km long, is located about 180-km southeast of Galveston, Texas, and consists of steep slopes on southern and eastern sides that rise from water depths over 100 m to within 20 m of the surface. Three-dimensional flows with frequencies ranging from 0.2 to 2 cycles per hour (cph) were observed in the mixed layer when wind speed and Stokes drift at the surface were large. These motions were stronger over the bank than outside the perimeter. The squared vertical velocity w2 was strongest near the surface and decayed exponentially with depth, and the e-folding length of w2 was 2 times larger than that of Stokes drift. The 2-h-averaged w2 in the mixed layer, scaled by the squared friction velocity, was largest when the turbulent Langmuir number was less than unity and the mixed layer was shallow. It is suggested that Langmuir circulation is responsible for the generation of vertical flows in the mixed layer, and that the increase in kinetic energy is due to an enhancement of Stokes drift by wave focusing. The lack of agreement with open-ocean Langmuir scaling arguments is likely due to the enhanced kinetic energy by wave focusing.

Why Do Modeled and Observed Surface Wind Stress Climatologies Differ in the Trade Wind Regions?

2018 ◽  
Vol 31 (2) ◽  
pp. 491-513 ◽  
Author(s):  
Isla R. Simpson ◽  
Julio T. Bacmeister ◽  
Irina Sandu ◽  
Mark J. Rodwell
Keyword(s):  
Wind Stress ◽  
Surface Wind ◽  
Zonal Flow ◽  
Surface Flow ◽  
Global Climate Models ◽  
Momentum Balance ◽  
Trade Wind ◽  
Near Surface ◽  
Low Level ◽  
Surface Wind Stress

Global climate models (GCMs) exhibit stronger mean easterly zonal surface wind stress and near-surface winds in the Northern Hemisphere (NH) trade winds than observationally constrained reanalyses or other observational products. A comparison, between models and reanalyses, of the processes that contribute to the zonal-mean, vertically integrated balance of momentum reveals that this wind stress discrepancy cannot be explained by either the resolved dynamics or parameterized tendencies that are common to each. Rather, a substantial residual exists in the time-mean momentum balance of the reanalyses, pointing toward a role for the analysis increments. Indeed, they are found to systematically weaken the NH near-surface easterlies in winter, thereby reducing the diagnosed surface wind stress. Similar effects are found in the Southern Hemisphere, and further analysis of the spatial structure and seasonality of these increments demonstrates that they act to weaken the near-surface flow over much of the low-latitude oceans in both summer and winter. This suggests an erroneous/missing process in GCMs that constitutes a missing drag on the low-level zonal flow over oceans. This indicates either a misrepresentation of the drag between the surface and the atmosphere or a missing internal atmospheric process that amounts to an additional drag on the low-level zonal flow. If the former is true, then observation-based surface stress products, which rely on similar drag formulations to GCMs, may be underestimating the strength of the easterly surface wind stress.

FEATURES OF WIND FIELD OVER THE SEA SURFACE IN THE COASTAL AREA BASED ON SAR OBSERVATIONS

2017 ◽  
Author(s):  
Anna Monzikova ◽  
Anna Monzikova ◽  
Vladimir Kudryavtsev Vladimir ◽  
Vladimir Kudryavtsev Vladimir ◽  
Alexander Myasoedov ◽  
...  
Keyword(s):  
Wind Stress ◽  
Surface Wind ◽  
Quantitative Description ◽  
Energy Resource ◽  
Resource Assessment ◽  
Internal Boundary Layer ◽  
Offshore Wind ◽  
Internal Boundary ◽  
Sar Images ◽  
Surface Wind Stress

“Wind-shadowing” effects in the Gulf of Finland coastal zone are analyzed using high resolution Envisat Synthetic Aperture Radar (SAR) measurements and model simulations. These effects are related to the internal boundary layer (IBL) development due to abrupt change the surface roughness at the sea-land boundary. Inside the "shadow" areas the airflow accelerates and the surface wind stress increases with the fetch. Such features can be revealed in SAR images as dark areas adjacent to the coastal line. Quantitative description of these effects is important for offshore wind energy resource assessment. It is found that the surface wind stress scaled by its equilibrium value (far from the coast) is universal functions of the dimensionless fetch Xf/G. Wind stress reaches an equilibrium value at the distance Xf/G of about 0.4.

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