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- Author or Editor: Gregory P. Chini x
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Abstract
Numerical simulations of the oceanic (atmospheric) boundary layer are complicated by the need to specify appropriate âoutflowâ or âradiationâ boundary conditions at the artificial lower (upper) boundary of the computational domain. If the boundary layer is stratified, particular care is necessary to insure that internal-gravity-wave disturbances generated within the domain are not artificially reflected by the computational boundary. A major advance was made almost 20 years ago by Klemp and Durran; their radiation condition relates the Fourier transformed pressure fluctuation to the Fourier transformed vertical-velocity perturbation along the artificial boundary. Because it is local in time, the Klemp and Durran (KD) condition is easily incorporated into a wide variety of numerical models for only a minor computational expense. Indeed, it has been widely used in the atmospheric and oceanic sciences communities. For simulations of dissipative systems, however, perturbation-flux conditions must also be specified at the artificial boundaryâthese are in addition to the KD condition (or some other constraint) on the normal velocity component at that boundary. This article considers the performance of the KD condition in conjunction with zero perturbation stress and zero perturbation buoyancy-flux conditions (âKDZâ conditions, collectively), because the latter are generally assumed to be appropriate for simulations of boundary layer phenomena. Analysis of the response of a weakly dissipative, uniformly stratified fluid to forcing concentrated at a given depth reveals two potentially serious drawbacks of the KDZ conditions. First, nonhydrostatic dynamics are not adequately treated by the KD condition, itself. Moreover, the imposition of zero perturbation-flux conditions causes artificial boundary layers to form along the outflow boundary. Although these boundary layers are passive, they are unlikely to be resolved in numerical simulations; thus, discretization of the KDZ conditions may cause further errors in the simulated internal-wave dynamics. A consistent set of boundary conditions for simulations of dissipative, stratified fluids is proposed.
Abstract
Numerical simulations of the oceanic (atmospheric) boundary layer are complicated by the need to specify appropriate âoutflowâ or âradiationâ boundary conditions at the artificial lower (upper) boundary of the computational domain. If the boundary layer is stratified, particular care is necessary to insure that internal-gravity-wave disturbances generated within the domain are not artificially reflected by the computational boundary. A major advance was made almost 20 years ago by Klemp and Durran; their radiation condition relates the Fourier transformed pressure fluctuation to the Fourier transformed vertical-velocity perturbation along the artificial boundary. Because it is local in time, the Klemp and Durran (KD) condition is easily incorporated into a wide variety of numerical models for only a minor computational expense. Indeed, it has been widely used in the atmospheric and oceanic sciences communities. For simulations of dissipative systems, however, perturbation-flux conditions must also be specified at the artificial boundaryâthese are in addition to the KD condition (or some other constraint) on the normal velocity component at that boundary. This article considers the performance of the KD condition in conjunction with zero perturbation stress and zero perturbation buoyancy-flux conditions (âKDZâ conditions, collectively), because the latter are generally assumed to be appropriate for simulations of boundary layer phenomena. Analysis of the response of a weakly dissipative, uniformly stratified fluid to forcing concentrated at a given depth reveals two potentially serious drawbacks of the KDZ conditions. First, nonhydrostatic dynamics are not adequately treated by the KD condition, itself. Moreover, the imposition of zero perturbation-flux conditions causes artificial boundary layers to form along the outflow boundary. Although these boundary layers are passive, they are unlikely to be resolved in numerical simulations; thus, discretization of the KDZ conditions may cause further errors in the simulated internal-wave dynamics. A consistent set of boundary conditions for simulations of dissipative, stratified fluids is proposed.
Abstract
Between 5% and 25% of the total momentum transferred between the atmosphere and ocean is transmitted via the growth of long surface gravity waves called âswell.â In this paper, we use large-eddy simulations to show that swell-transmitted momentum excites near-inertial waves and drives turbulent mixing that deepens a rotating, stratified, turbulent ocean surface boundary layer. We find that swell-transmitted currents are less effective at producing turbulence and mixing the boundary layer than currents driven by an effective surface stress. Overall, however, the differences between swell-driven and surface-stress-driven boundary layers are relatively minor. In consequence, our results corroborate assumptions made in Earth system models that neglect the vertical structure of swell-transmitted momentum fluxes and instead parameterize all airâsea momentum transfer processes with an effective surface stress.
Abstract
Between 5% and 25% of the total momentum transferred between the atmosphere and ocean is transmitted via the growth of long surface gravity waves called âswell.â In this paper, we use large-eddy simulations to show that swell-transmitted momentum excites near-inertial waves and drives turbulent mixing that deepens a rotating, stratified, turbulent ocean surface boundary layer. We find that swell-transmitted currents are less effective at producing turbulence and mixing the boundary layer than currents driven by an effective surface stress. Overall, however, the differences between swell-driven and surface-stress-driven boundary layers are relatively minor. In consequence, our results corroborate assumptions made in Earth system models that neglect the vertical structure of swell-transmitted momentum fluxes and instead parameterize all airâsea momentum transfer processes with an effective surface stress.
Abstract
The interactions between boundary layer turbulence, including Langmuir turbulence, and submesoscale processes in the oceanic mixed layer are described using large-eddy simulations of the spindown of a temperature front in the presence of submesoscale eddies, winds, and waves. The simulations solve the surface-wave-averaged Boussinesq equations with Stokes drift wave forcing at a resolution that is sufficiently fine to capture small-scale Langmuir turbulence. A simulation without Stokes drift forcing is also performed for comparison. Spatial and spectral properties of temperature, velocity, and vorticity fields are described, and these fields are scale decomposed in order to examine multiscale fluxes of momentum and buoyancy. Buoyancy flux results indicate that Langmuir turbulence counters the restratifying effects of submesoscale eddies, leading to small-scale vertical transport and mixing that is 4 times greater than in the simulations without Stokes drift forcing. The observed fluxes are also shown to be in good agreement with results from an asymptotic analysis of the surface-wave-averaged, or CraikâLeibovich, equations. Regions of potential instability in the flow are identified using Richardson and Rossby numbers, and it is found that mixed gravitational/symmetric instabilities are nearly twice as prevalent when Langmuir turbulence is present, in contrast to simulations without Stokes drift forcing, which are dominated by symmetric instabilities. Mixed layer depth calculations based on potential vorticity and temperature show that the mixed layer is up to 2 times deeper in the presence of Langmuir turbulence. Differences between measures of the mixed layer depth based on potential vorticity and temperature are smaller in the simulations with Stokes drift forcing, indicating a reduced incidence of symmetric instabilities in the presence of Langmuir turbulence.
Abstract
The interactions between boundary layer turbulence, including Langmuir turbulence, and submesoscale processes in the oceanic mixed layer are described using large-eddy simulations of the spindown of a temperature front in the presence of submesoscale eddies, winds, and waves. The simulations solve the surface-wave-averaged Boussinesq equations with Stokes drift wave forcing at a resolution that is sufficiently fine to capture small-scale Langmuir turbulence. A simulation without Stokes drift forcing is also performed for comparison. Spatial and spectral properties of temperature, velocity, and vorticity fields are described, and these fields are scale decomposed in order to examine multiscale fluxes of momentum and buoyancy. Buoyancy flux results indicate that Langmuir turbulence counters the restratifying effects of submesoscale eddies, leading to small-scale vertical transport and mixing that is 4 times greater than in the simulations without Stokes drift forcing. The observed fluxes are also shown to be in good agreement with results from an asymptotic analysis of the surface-wave-averaged, or CraikâLeibovich, equations. Regions of potential instability in the flow are identified using Richardson and Rossby numbers, and it is found that mixed gravitational/symmetric instabilities are nearly twice as prevalent when Langmuir turbulence is present, in contrast to simulations without Stokes drift forcing, which are dominated by symmetric instabilities. Mixed layer depth calculations based on potential vorticity and temperature show that the mixed layer is up to 2 times deeper in the presence of Langmuir turbulence. Differences between measures of the mixed layer depth based on potential vorticity and temperature are smaller in the simulations with Stokes drift forcing, indicating a reduced incidence of symmetric instabilities in the presence of Langmuir turbulence.
