Large Eddy Simulation of the Ocean Mixed Layer: The Effects of Wave Breaking and Langmuir Circulation (original) (raw)
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Observations and numerical simulations of large-eddy circulation in the ocean surface mixed layer
Geophysical Research Letters, 2014
Two near-surface dye releases were mapped on scales of minutes to hours temporally, meters to order 1 km horizontally, and 1-20 m vertically using a scanning, depth-resolving airborne lidar. In both cases, dye evolved into a series of rolls with their major axes approximately aligned with the wind and/or near-surface current. In both cases, roll spacing was also of order 5-10 times the mixed layer depth, considerably larger than the 1-2 aspect ratio expected for Langmuir cells. Numerical large-eddy simulations under similar forcing showed similar features, even without Stokes drift forcing. In one case, inertial shear driven by light winds induced large aspect ratio large-eddy circulation. In the second, a preexisting lateral mixed layer density gradient provided the dominant forcing. In both cases, the growth of the large-eddy structures and the strength of the resulting dispersion were highly dependent on the type of forcing.
Disruption of the bottom log layer in large-eddy simulations of full-depth Langmuir circulation
Journal of Fluid Mechanics, 2012
We report on disruption of the log layer in the resolved bottom boundary layer in large-eddy simulations (LES) of full-depth Langmuir circulation (LC) in a wind-driven shear current in neutrally-stratified shallow water. LC consists of parallel counterrotating vortices that are aligned roughly in the direction of the wind and are generated by the interaction of the wind-driven shear with the Stokes drift velocity induced by surface gravity waves. The disruption is analysed in terms of mean velocity, budgets of turbulent kinetic energy (TKE) and budgets of TKE components. For example, in terms of mean velocity, the mixing due to LC induces a large wake region eroding the classical log-law profile within the range 90 < x + 3 < 200. The dependence of this disruption on wind and wave forcing conditions is investigated. Results indicate that the amount of disruption is primarily determined by the wavelength of the surface waves generating LC. These results have important implications for turbulence parameterizations for Reynolds-averaged Navier-Stokes simulations of the coastal ocean.
Three-dimensional Langmuir circulations and enhanced turbulence in upper mixed ocean layers
Field and laboratory data confirm the presence of longitudinal billows in fluid flow under wind-wavy surfaces. In the ocean these vortices (called Langmuir cells) act by mixing nutrients and other biological material, and thus their role cannot be neglected in vertical transfer modelling. In this work non-dimensional mean velocity field equations are formulated with Craik & Leibovich theory including interaction terms between surface wave Stokes drift and mean current. A first order turbulence closure model (k,) is used to model the Reynolds stress tensor. The model is formulated in non-dimensional grounds, and numerical experiments are performed using a finit-volume technique. In the first set of simulations, model outputs are compared to measurements obtained at three different laboratory wind-water facilities (Cheung & Street, 1988; Thais & Magnaudet, 1996). Results suggest that the presence of secondary motions is necessary for explaining the observed channel flows. A second gro...
Journal of Physical Oceanography, 2009
A Large Eddy Simulation (LES) model is used to investigate upper-ocean response to a fall storm in the open ocean of the North Pacific Ocean. The storm is characterized by rapid increases in wind speed and surface heat loss but relatively steady wave field. The LES model shows that surface convergence zones or windrows organize into line patterns aligned with the wind direction, evolving from nearly parallel lines to irregular structures featuring Y-junctions as the wind speed increases. The downwelling-to-upwelling velocity ratio ranges between 1.2 and 1.6, indicating a moderate level of asymmetry between the downwelling and upwelling plumes in Langmuir circulation. During the storm, the turbulent Langmuir number La t increases from 0.2 to 0.5 while the vertical turbulence intensity decreases from (1.4 to 0.7) where u 2 w σ 2 * u * is the friction velocity. The order of turbulence intensities in three directions switches from crosswind ≈ vertical > downwind directions to downwind > crosswind > vertical directions. This suggests a transition from Langmuir to shear turbulence as the storm progresses. The Hoennikker number Ho remains below 0.1 and the strong evaporative heat loss does not contribute much to turbulence generation in the ocean mixed layer. The LES results are compared with in-situ and acoustic measurements collected during the storm. Patterns of model-predicted nearsurface downwelling zones are in good agreement with horizontal distributions of bubble clouds revealed in sidescan sonar images. Striking similarity is also found in temperature
A regime diagram for classifying turbulent large eddies in the upper ocean
Deep Sea Research Part I: Oceanographic Research Papers, 2005
A large eddy simulation (LES) model is used to examine how buoyancy-driven thermal convection, wind-driven shear turbulence and wind/wave-driven Langmuir circulation compete to generate turbulence in the ocean surface mixed layer. The turbulent Langmuir number La t , a ratio of friction velocity to surface Stokes drift velocity, and the Hoenikker number Ho, a ratio of buoyancy forcing to wave forcing, are two controlling dimensionless parameters. We explore low-order turbulence statistics in the La t and Ho parameter space for a wide range of atmospheric forcing conditions and construct a regime diagram to differentiate buoyancy-, shear-and wave-driven turbulence. All three types of turbulent flows are anisotropic but show different orderings of turbulence intensities: vertical 4 (downwind, crosswind) in convective turbulence; downwind4crosswind4vertical in shear turbulence; crosswind E vertical4 downwind in Langmuir turbulence. These orderings of turbulence intensities can be explained by examining the turbulence energy production in three directions. Buoyancy production in the vertical direction dominates turbulence generation in convective turbulence, whereas shear production in the downwind direction dominates turbulence generation in shear-driven turbulence. In Langmuir turbulence, however, Stokes production due to surface waves generates turbulence energy in both crosswind and vertical directions. Turbulence in the wind-driven upper ocean shows a transition from shear to Langmuir turbulence as La t decreases. A fully-developed sea state corresponds to La t E0.3 and lies within the Langmuir regime. Vertical turbulence intensity in Langmuir turbulence is about two times larger than that in shear turbulence and falls into the range observed in the upper ocean. Hence the wind-driven upper ocean will be dominated by Langmuir turbulence under typical sea state conditions. Transition from Langmuir to convective turbulence occurs around Ho ¼ Oð1Þ; which is much greater than Ho ¼ Oð0:01Þ obtained using typical heat fluxes and wind speeds.
A global perspective on Langmuir turbulence in the ocean surface boundary layer
Geophysical Research Letters, 2012
1] The turbulent mixing in thin ocean surface boundary layers (OSBL), which occupy the upper 100 m or so of the ocean, control the exchange of heat and trace gases between the atmosphere and ocean. Here we show that current parameterizations of this turbulent mixing lead to systematic and substantial errors in the depth of the OSBL in global climate models, which then leads to biases in sea surface temperature. One reason, we argue, is that current parameterizations are missing key surface-wave processes that force Langmuir turbulence that deepens the OSBL more rapidly than steady wind forcing. Scaling arguments are presented to identify two dimensionless parameters that measure the importance of wave forcing against wind forcing, and against buoyancy forcing. A global perspective on the occurrence of waveforced turbulence is developed using re-analysis data to compute these parameters globally. The diagnostic study developed here suggests that turbulent energy available for mixing the OSBL is under-estimated without forcing by surface waves. Wave-forcing and hence Langmuir turbulence could be important over wide areas of the ocean and in all seasons in the Southern Ocean. We conclude that surfacewave-forced Langmuir turbulence is an important process in the OSBL that requires parameterization.
Influence of Langmuir Circulation on the Deepening of the Wind-Mixed Layer
Journal of Physical Oceanography, 2011
Analysis of large eddy simulation data reveals that Langmuir circulation (LC) induces a significant enhancement of the mixed layer deepening, only if the mixed layer depth (MLD) h is shallow and the buoyancy jump across it ΔB is small, when simulations are initiated by applying the wind stress to a motionless mixed layer with stratification. The difference in the entrainment rate between the cases with and without LC decreases with hΔB/υL2, where υL is the velocity scale of LC. The ratio of the mixing length scale l between the cases with and without LC is close to 1 for larger Rt [=(Nl0/q)2; Rt > ∼1], but it increases to above 10 with the decrease of Rt, where N is the Brunt–Väisälä frequency and q and l0 are the velocity and length scales of turbulence in the homogeneous layer. It is also found that, in the presence of LC, the effect of stratification on vertical mixing should be parameterized in terms of Rt instead of Ri (=(N/S)2), because velocity shear S is no longer a domin...
Turbulence structure in the upper ocean: a comparative study of observations and modeling
Ocean Dynamics, 2014
Observations of turbulent dissipation rates measured by two independent instruments are compared with numerical model runs to investigate the injection of turbulence generated by sea surface gravity waves. The nearsurface observations are made by a moored autonomous instrument, fixed at approximately 8 m below the sea surface. The instrument is equipped with shear probes, a highresolution pressure sensor, and an inertial motion package to measure time series of dissipation rate and nondirectional surface wave energy spectrum. A free-falling profiler is used additionally to collect vertical microstructure profiles in the upper ocean. For the model simulations, we use a one-dimensional mixed layer model based on a k-ε type second moment turbulence closure, which is modified to include the effects of wave breaking and Langmuir cells. The dissipation rates obtained using the modified k-ε model are elevated near the sea surface and in the upper water column, consistent with the measurements, mainly as a result of wave breaking at the surface, and energy drawn from wave field to the mean flow by Stokes drift. The agreement between observed and simulated turbulent quantities is fairly good, especially when the Stokes production is taken into account.
Comparison of the simulated upper-ocean vertical structure using 1-dimensional mixed-layer models
Ocean Science Discussions, 2016
Atmospheric fluxes influence the momentum and scalar properties in the upper-cean. Buoyancy fluxes result in a diurnal variability in the sea-surface temperature (SST), whereas the wind stress forms near-inertial currents in the mixed layer (ML). In this study, we investigate the contrasts between the simulated SST and the vertical structure of the temperature and shear by three different mixing models: the PWP bulk mixed-layer model, the KPP non-local boundary layer model and the κ−ϵ local mixing model. We choose two upper-ocean datasets for our studies, namely the SWAPP (1990) and the MLML (1991). The SWAPP dataset shows the presence of strong near-inertial shear below the ML and negligible near-inertial shear within the ML. The MLML dataset shows a negligible rise in the SST during the first 22 day mixing phase, which is followed by a steep rise by 6 °C during the subsequent 75 day restratification phase. <br><br> Comparison with the SWAPP d...