Turbulent Kinetic Energy Input Research Articles

Influence of Stokes Drift Decay Scale on Langmuir Turbulence

AbstractAccurately scaling Langmuir turbulence (LT) in the ocean surface boundary layer (OSBL) is critical for improving ocean, weather, and climate models. The physical processes by which the structure of LT depends on surface waves’ Stokes drift decay length scale are examined. An idealized model for OSBL turbulent kinetic energy (TKE) provides a conceptual framework with three physical processes: TKE transport, dissipation, and production by the Craik–Leibovich (CL) vortex force (VF) associated with the Stokes drift shear. TKE profiles depend on OSBL depth h, surface roughness length z0, and wavenumber k through the nondimensional parameters kh and kz0. These parameters determine the rate and length scale for the dissipation of TKE produced by the CL-VF. For kz0 ≫ 1, TKE input by the CL-VF is governed by a surface flux with TKE rapidly decaying with depth. Only for kz0 < 1 can TKE penetrate deeper into the OSBL, with the TKE penetration depth controlled by kh. Turbulence-resolving large-eddy simulation results support this conceptual framework and indicate that the dominant Langmuir cell size scales with (kh)−1. Within the depth of dominant Langmuir cells, TKE dissipation is approximately balanced by CL-VF production. Shorter waves contribute less to deeper vertical velocity variance 〈w2〉 because the CL-VF is less effective in generating larger-scale LT. Depth-averaged 〈w2〉 scales with a modified Langmuir number Laϕ = (u*/usϕ)1/2, where u* denotes the water-side surface friction velocity and usϕ is a depth-integrated weighted Stokes drift shear or, equivalently, a spectrally filtered surface Stokes drift.

Flow field in a reservoir subject to pumped-storage operation – in situ measurement and numerical modeling

In pumped-storage reservoirs, turbulent kinetic energy input from plant operation can be considerably high. Flow velocities were measured by Acoustic Doppler Current Profilers near the intake/outlet structure to describe flow patterns induced by pumped-storage operation in an Alpine reservoir. Recorded data allowed reproducing 1D and 2D velocity profiles along the water column. The comparison between the main frequencies of the velocity signal and the discharge series from the plant reveals correlation between recorded flow patterns and pumped-storage operation. Numerical modeling enhanced the understanding of flow patterns developing near the intake/outlet structure. Both results reveal that water withdrawal by pumping only marginally affects flow patterns in front of the intake, whereas water injected during turbine mode leads to backflow areas and large-scale recirculation cells. Numerical modeling further revealed that steady flow patterns are developing only after some 2.5 h of continuous turbine operation.

Passive buoyant tracers in the ocean surface boundary layer: 2. Observations and simulations of microplastic marine debris

AbstractThis paper is the second of a two‐part series that investigates passive buoyant tracers in the ocean surface boundary layer (OSBL). The first part examines the influence of equilibrium wind‐waves on vertical tracer distributions, based on large eddy simulations (LESs) of the wave‐averaged Navier‐Stokes equation. Motivated by observations of buoyant microplastic marine debris (MPMD), this study applies the LES model and the parametric one‐dimensional column model from part one to examine the vertical distributions of MPMD. MPMD is widely distributed in vast regions of the subtropical gyres and has emerged as a major open ocean pollutant whose distribution is subject to upper ocean turbulence. The models capture shear‐driven turbulence, Langmuir turbulence (LT), and enhanced turbulent kinetic energy input due to breaking waves (BWs). Model results are only consistent with observations of MPMD profiles and the relationship between surface concentrations and wind speed if LT effects are included. Neither BW nor shear‐driven turbulence is capable of deeply submerging MPMD, suggesting that the observed vertical MPMD distributions are a characteristic signature of wave‐driven LT. Thus, this study demonstrates that LT substantially increases turbulent transport in the OSBL, resulting in deep submergence of buoyant tracers. The parametric model is applied to 11 years of observations in the North Atlantic and North Pacific subtropical gyres to show that surface measurements substantially underestimate MPMD concentrations by a factor of 3–13.

Passive buoyant tracers in the ocean surface boundary layer: 1. Influence of equilibrium wind‐waves on vertical distributions

AbstractThis paper is the first of a two part series that investigates passive buoyant tracers in the ocean surface boundary layer. The first part examines the influence of equilibrium wind‐waves on vertical tracer distributions, based on large eddy simulations (LES) of the wave‐averaged Navier‐Stokes equation. The second part applies the model to investigate observations of buoyant microplastic marine debris, which has emerged as a major ocean pollutant. The LES model captures both Langmuir turbulence (LT) and enhanced turbulent kinetic energy input due to breaking waves (BW) by imposing equilibrium wind‐wave statistics for a range of wind and wave conditions. Concentration profiles of LES agree well with analytic solutions obtained for an eddy diffusivity profile that is constant near the surface and transitions into the K‐Profile Parameterization (KPP) profile shape at greater depth. For a range of wind and wave conditions, the eddy diffusivity normalized by the product of water‐side friction velocity and mixed layer depth, h, mainly depends on a single nondimensional parameter, the peak wavelength (which is related to Stokes drift decay depth) normalized by h. For smaller wave ages, BW critically enhances near‐surface mixing, while LT effects are relatively small. For greater wave ages, both BW and LT contribute to elevated near‐surface mixing, and LT significantly increases turbulent transport at greater depth. We identify a range of realistic wind and wave conditions for which only Langmuir (and not BW or shear driven) turbulence is capable of deeply submerging buoyant tracers.

Effects of Stokes production on summer ocean shelf dynamics

A two-dimensional numerical model, which is configured on the basis of Princeton ocean model (POM), is used to study the effect of Stokes production (SP) of the turbulent kinetic energy on a density profile and Ekman transport in an idealized shelf region in summer. The energy input from SP is parameterized and included into the Mellor-Yamada turbulence closure submodel. Results reveal that the intensity of wind-driven upwelling fronts near the sea surface is weakened by the SP-associated turbulent kinetic energy input. The vertical eddy viscosity coefficient in the surface boundary layer is enhanced greatly owing to the impact of SP, which decreases the alongshore velocity and changes the distribution of upwelling. In addition, the SP-induced mixing easily suppresses the strong stratification and significantly increases the depth of the upper mixed layer (ML) under strong winds.

Numerical Simulation of Cloud–Clear Air Interfacial Mixing: Effects on Cloud Microphysics

This paper extends the previously published numerical study of Andrejczuk et al. on microscale cloud–clear air mixing. Herein, the primary interest is on microphysical transformations. First, a convergence study is performed—with well-resolved direct numerical simulation of the interfacial mixing in the limit—to optimize the design of a large series of simulations with varying physical parameters. The principal result is that all conclusions drawn from earlier low-resolution (Δx = 10−2 m) simulations are corroborated by the high-resolution (Δx = 0.25 × 10−2 m) calculations, including the development of turbulent kinetic energy (TKE) and the evolution of microphysical properties. This justifies the use of low resolution in a large set of sensitivity simulations, where microphysical transformations are investigated in response to variations of the initial volume fraction of cloudy air, TKE input, liquid water mixing ratio in cloudy filaments, relative humidity (RH) of clear air, and size of cloud droplets. The simulations demonstrate that regardless of the initial conditions the evolutions of the number of cloud droplets and the mean volume radius follow a universal path dictated by the TKE input, RH of clear air filaments, and the mean size of cloud droplets. The resulting evolution path only weakly depends on the progress of the homogenization. This is an important conclusion because it implies that a relatively simple rule can be developed for representing the droplet-spectrum evolution in cloud models that apply parameterized microphysics. For the low-TKE input, when most of the TKE is generated by droplet evaporation during mixing and homogenization, an inhomogeneous scenario is observed with approximately equal changes in the dimensionless droplet number and mean volume radius cubed. Consistent with elementary scale analysis, higher-TKE inputs, higher RH of cloud-free filaments, and larger cloud droplets enhance the homogeneity of mixing. These results are discussed in the context of observations of entrainment and mixing in natural clouds.

Enhanced mixing in narrows: A case study at the Mainau sill (Lake Constance)

Previous work has identified bottom currents as a significant source of turbulence in stratified lakes. Sills may therefore be a major factor determining overall turbulent diapycnal (vertical) exchange in lakes with multi-basin hypolimnia.¶In order to investigate the contribution of the Mainau sill (separating Upper Lake Constance from Lake Uberlingen) to the overall diapycnal mixing in Lake Constance, a series of temperature microstructure profiles was taken at Mainau Island in October 1993. From these profiles, using Batchelor's method, the rate of dissipation of turbulent kinetic energy was determined and related in an energy balance to the turbulent kinetic energy input from the wind, the energy content of internal seiches and the energy dissipation of bottom currents at the sill. Further, the vertical diffusivities were calculated using the dissipation method.¶The analysis shows that ≈ 5.5% of the wind energy flux was found in to the water column (below 2 m depth) and that energy dissipation was 8 times higher in the shear zone of the thermocline (≈ 8.4 ˙ 10-3 W m-2) than in the bottom boundary layer (≈ 1.1 ˙ 10-3 W m-2). Dissipation above the Mainau sill (≈ 9.5 ˙ 10-3 W m-2) exceeded the basin-wide average dissipation of internal seiche energy (≈ 0.3 ˙ 10-3 W m-2) by a factor of 34. However, since the areal extent of the sill is small (≈ 1% of the lake area), the sill contributes only about 40% to the basin-wide dissipation. Also vertical diffusivities within the thermocline were consistently enhanced by approximately the same amount over the sill. The synthesis of the observations implies that the sill plays a disproportionately large, but not dominant, role for small-scale diapycnal mixing in the hypolimnion of Lake Constance.

Modeling Wave-Enhanced Turbulence in the Ocean Surface Layer

Abstract Until recently, measurements below the ocean surface have tended to confirm “law of the wall” behavior, in which the velocity profile is logarithmic, and energy dissipation decays inversely with depth. Recent measurements, however, show a sublayer, within meters of the surface, in which turbulence is enhanced by the action of surface waves. In this layer, dissipation appears to decay with inverse depth raised to a power estimated between 3 and 4.6. The present study shows that a conventional model, employing a “level 2½” turbulence closure scheme predicts near-surface dissipation decaying as inverse depth to the power 3.4. The model shows agreement in detail with measured profiles of dissipation. This is despite the fact that empirical constants in the model are determined for situations very different from this near-surface application. The action of breaking waves is modeled by a turbulent kinetic energy input at the surface. In the wave-enhanced layer, the downward flux of turbulent kinetic en...

Interdecadal variability of the Pacific Ocean: model response to observed heat flux and wind stress anomalies

Variability of the Pacific Ocean is examined in numerical simulations with an ocean general circulation model forced by observed anomalies of surface heat flux, wind stress and turbulent kinetic energy (TKE) over the period 1970-88. The model captures the 1976-77 winter time climate shift in sea surface temperature, as well as its monthly, seasonal and longer term variability as evidenced in regional time series and empirical orthogonal function analyses. Examination of the surface mixed-layer heat budget reveals that the 1976-77 shift was caused by a unique concurrance of sustained heat flux input anomalies and very strong horizontal advection anomalies during a multi-month period preceding the shift in both the central Pacific region (where cooling occurred) and the California coastal region (where warming occurred). In the central Pacific, the warm conditions preceding and the cold conditions following the shift tend to be maintained by anomalous vertical mixing due to increases in the atmospheric momentum flux (TKE input) into the mixed layer (which deepens in the model after the shift) from the early 1970s to the late 1970s and 1980s. Since the ocean model does not contain feedback to the atmosphere and it succeeds in capturing the major features of the 1976-77 shift, it appears that the midlatitude part of the shift was driven by the atmosphere, although effects of midlatitude ocean-atmosphere feedback are still possible. The surface mixed-layer heat budget also reveals that, in the central Pacific, the effects of heat flux input and vertical mixing anomalies are comparable in amplitude while horizontal advection anomalies are roughly half that size. In the California coastal region, in contrast, where wind variability is much weaker than in the central Pacific, horizontal advection and vertical mixing effects on the mixed layer heat budget are only one-quarter the size of typical heat flux input anomalies.

The H-alpha velocity field of the Omega nebula (M 17)

view Abstract Citations (25) References (59) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The H alpha Velocity Field of the Omega Nebula (M17) Joncas, G. ; Roy, J. -R. Abstract Using a Fabry-Perot camera, 9054 H-alpha radial velocities were measured across the H II region M 17. The mean v(LSR) = 18.6 + or - 0.1 km/s, which is 1.4 km/s blueshifted with respect to the mean velocity of the associated molecular cloud M 17 SW. The dispersion of the velocity histogram is slightly skewed to the blue. A gradient in radial velocity is observed from west to east, with the most blueshifted velocities appearing closest to the molecular cloud. The velocity field can be explained as a flow of ionized gas approximatively in the plane of the sky. There is a relation between random velocity and size, with velocity fluctuations increasing as some power of size; the power index depends on the sampling method of the velocity field. An interpretation of the relation is suggested in terms of turbulence, and a brief discussion of mechanisms for turbulent kinetic energy input is presented. Publication: The Astrophysical Journal Pub Date: August 1986 DOI: 10.1086/164450 Bibcode: 1986ApJ...307..649J Keywords: H Alpha Line; Molecular Clouds; Nebulae; Radial Velocity; Velocity Distribution; Astronomical Maps; Gas Dynamics; H Ii Regions; Histograms; Ionized Gases; Astrophysics; NEBULAE: INDIVIDUAL MESSIER NUMBER: M17; RADIAL VELOCITIES full text sources ADS | data products SIMBAD (1)

Hydrothermal Analysis of Lakes and Reservoirs

The impact of Hunter Rouse on the application of fluid mechanics to hydraulic engineering practice in the United States is reviewed. The development of physical and mathematical techniques for predicting the hydrothermal structure of lakes and reservoirs during the past fifteen years is discussed. Laboratory models have played a key role in the development and testing of early mathematical models based on the vertical diffusion concept. The sensitivity of these models to variations in turbulent diffusivity, internal heat absorption and vertical advection due to inflows and outflows is demonstrated. A wind-mixing model is proposed as an alternative to specifying temporal and spatial variations in the vertical diffusivity. The wind-mixing algorithm converts turbulent kinetic energy input by wind shear to potential energy through entrainment of hypolimnetic water across the thermocline. An iterative technique is used to achieve a heat balance based on meteorological inputs averaged over hourly or daily time steps. The temperature predictions of the numrical model are compared with field data for a number of lakes and reservoirs.

Sea Surface Temperature Response to Variations in Atmospheric Wind Forcing

Abstract Sea surface temperature changes occurring during atmospheric forcing events defined in terms of wind speed are examined with the long time series of surface data at Ocean Weather Ships P, V and N in the North Pacific Ocean. The 3 h forcing is expressed in terms of u*3, where u* is the atmospheric friction velocity, and various heat flux components. It is shown that the distributions of atmospheric forcing during the warming season are rather similar to those of the cooling season, although the intensity of the forcing is much less in the warming season. The nonlinearity in the u*3 distribution is very similar in both seasons. That is, a few of the most intense synoptic events provide a majority of the turbulent kinetic energy input to the upper layers of the ocean. During the majority of time, the winds are light and there is inadequate turbulent kinetic energy to maintain a deep layer against the stabilizing effects of near-surface heating. It is found that sea temperature increases during the w...

A one-dimensional model of the seasonal thermocline II.The general theory and its consequences

A theory of the layer formation due to surface processes is presented, which is more general than that used in the preceding paper I. Convection due to heating at depth and cooling at the surface is included, as well as the mechanical stirring due to wind action. The theory is applicable to arbitrary forms of heating, including intermittent or continuous processes, and could be used to investigate diurnal as well as seasonal effects. A detailed application is made to the case treated approximately in I, for which a solution is now obtained in analytic form. The results obtained allow a quantitative, as well as qualitative, comparison with the ocean. It is found that reasonable layer depths are predicted using measured heating rates, and a value of the turbulent kinetic energy input to the water deduced from the mean surface stress. The effects of heating at depth can be comparable with wind stirring, even when the temperature of the upper layer is increasing. During the winter, convection due to surface cooling dominates the processes which deepen the layer. DOI: 10.1111/j.2153-3490.1967.tb01462.x

A one-dimensional model of the seasonal thermocline: II. The general theory and its consequences

A theory of the layer formation due to surface processes is presented, which is more general than that used in the preceding paper I. Convection due to heating at depth and cooling at the surface is included, as well as the mechanical stirring due to wind action. The theory is applicable to arbitrary forms of heating, including intermittent or continuous processes, and could be used to investigate diurnal as well as seasonal effects. A detailed application is made to the case treated approximately in I, for which a solution is now obtained in analytic form. The results obtained allow a quantitative, as well as qualitative, comparison with the ocean. It is found that reasonable layer depths are predicted using measured heating rates, and a value of the turbulent kinetic energy input to the water deduced from the mean surface stress. The effects of heating at depth can be comparable with wind stirring, even when the temperature of the upper layer is increasing. During the winter, convection due to surface cooling dominates the processes which deepen the layer. DOI: 10.1111/j.2153-3490.1967.tb01462.x