On the heterogeneity of stratified-shear turbulence: Observations from a near-field river plume

MacDonald, Daniel G.; Carlson, J. D.; Goodman, Louis

doi:10.1002/2013jc008891

Cited by 22 publications

(20 citation statements)

References 45 publications

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“…Though the ratio can vary as a Kelvin‐Helmholtz billow ages (Smyth, ) and observational estimates include large uncertainties (Ferron et al, ; Mater et al, ), the ratio has been supported by numerous open‐ocean (Wesson & Gregg, ) and lake (Dillon, ) measurements. In estuaries and plumes, which better reflect the conditions observed in this study, the ratio has also been observed to hold (Peters, ; Orton, ), with comparisons between other methods yielding values within the same order of magnitude (MacDonald & Horner‐Devine, ; MacDonald et al, ). A comparison of these overturn scales in the Deep Cove plume, however, shows that the expected linear relationship did not hold here (Figure ).…”

Section: Discussionsupporting

confidence: 80%

“…A comparison of methods to derive TKE dissipation rates from VMP profiles where (a) unsorted temperature (dashed) and buoyancy frequency squared ( N 2 ) profile (solid), (b) the instantaneous Thorpe scale ( L T ), and (c) direct microstructure‐derived ϵ (solid) and Thorpe scale‐derived ϵ T (dotted). The shaded region around each profile represents one standard deviation from the distribution mean, calculated using the method described in MacDonald et al (). The mean is calculated from nine VMP profiles taken approximately 1 km from the tailrace discharge site, downstream of DCA.…”

Section: Resultsmentioning

confidence: 99%

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Turbulent Scales Observed in a River Plume Entering a Fjord

Stevens

O'Callaghan

2019

JGR Oceans

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Observations of turbulent mixing are quantified in the near-field region (first several kilometers) of a controlled river flow entering Doubtful Sound, New Zealand. Measurement techniques include direct turbulence measurements using microstructure profilers in both Eulerian and quasi-Lagrangian perspectives and turbulent overturn analysis. Over 500 profiles of vertical microstructure were collected within the inner fjord. With high flow speeds of the surface plume over 2 m/s and some of the strongest stratification (N 2 ∼ 10 −1 s −2 ) recorded in the coastal ocean, turbulent overturning (Thorpe, L T ) scales are used to indirectly infer turbulence dissipation rates ( ) and compared with direct measurements in a quasi-idealized laboratory setting. There exists an inverse relationship between directly measured estimates of and distance from the plume discharge point. Peak values of >10−2 W/kg are observed within 500 m from the discharge point, among the highest turbulence dissipation rates recorded in oceanic shear flows. Thorpe scales range from below 1 cm in the pycnocline to a few meters in the weakly stratified ambient water below the plume. Stratification within the surface layer constrains the vertical dimension, limiting L T . Therefore, L T is an unreliable estimator of L 0 , where L O is the Ozmidov scale and is used to infer . Care must thus be taken when applying this method of overturn analysis in an environment where stratification or physical boundaries limit the vertical extent of overturns.Direct turbulence measurements in estuaries and river plumes have been used to examine stratified-shear flow instabilities as a mechanism of mixing (Geyer et al.

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Section: Discussionsupporting

confidence: 80%

Section: Resultsmentioning

confidence: 99%

Turbulent Scales Observed in a River Plume Entering a Fjord

Stevens

O'Callaghan

2019

JGR Oceans

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“…Especially in geophysical scenarios, this is a common scenario, with examples including river plumes (e.g. MacDonald, Carlson & Goodman 2013), the deepening of the upper-ocean wind-mixed layer (e.g. Pollard, Rhines & Thompson 1972) and the very intricate case of cloud-top entrainment (e.g.…”

Section: Introductionmentioning

confidence: 99%

The turbulent/non-turbulent interface in an inclined dense gravity current

et al. 2015

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We present an experimental investigation of entrainment and the dynamics near the turbulent/non-turbulent interface in a dense gravity current. The main goal of the study is to investigate changes in the interfacial physics due to the presence of stratification and to examine their impact on the entrainment rate. To this end, three-dimensional data sets of the density and the velocity fields are obtained through a combined scanning particle tracking velocimetry/laser-induced fluorescence approach for two different stratification levels with inflow Richardson numbers of Ri 0 = 0.23 and Ri 0 = 0.46, respectively, at a Reynolds number around Re 0 = 3700. An analysis conditioned on the instantaneous position of the turbulent/non-turbulent interface as defined by a threshold on enstrophy reveals an interfacial region that is in many aspects independent of the initial level of stratification. This is reflected most prominently in matching peaks of the gradient Richardson number Ri g ≈ 0.1 located approximately 10η from the position of the interface inside the turbulent region, where η = (ν 3 / ) 1/4 is the Kolmogorov scale, and ν and denote the kinematic viscosity and the rate of turbulent dissipation, respectively. A possible explanation for this finding is offered in terms of a cyclic evolution in the interaction of stratification and shear involving the buildup of density and velocity gradients through inviscid amplification and their subsequent depletion through molecular effects and pressure. In accordance with the close agreement of the interfacial properties for the two cases, no significant differences were found for the local entrainment velocity, v n (defined as the propagation velocity of an enstrophy isosurface relative to the fluid), at different initial stratification levels. Moreover, we find that the baroclinic torque does not contribute significantly to the local entrainment velocity. Comparing results for the surface area of the convoluted interface to estimates from fractal scaling theory, we identify differences in the interface geometry as the major factor in the reduction of the entrainment rate due to density stratification.

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“…Therefore, this limitation should be addressed in future studies by conducting intensive field surveys to investigate the inertial and K‐H instability processes generated around tidal plumes. A combination of “tow‐yo” (e.g., MacDonald et al, ) CTD/ADCP casts, and low‐altitude remote sensing, such as the balloon photography used this study, could be a powerful tool for evaluating the Rayleigh criterion (equation ) and Richardson number (equation ) in an actual situation. However, we will still encounter the difficulty arising from the fact that the spatiotemporal scales of these processes are 1 order of magnitude smaller than the field observations obtained in the coastal waters.…”

Section: Discussionmentioning

confidence: 99%

Tidally Induced Instability Processes Suppressing River Plume Spread in a Nonrotating and Nonhydrostatic Regime

Iwanaka

Isobe

2018

JGR Oceans

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Field surveys near a river mouth and numerical model experiments were conducted to investigate how fine structures generated in a tidally influenced river plume (tidal plume) affect plume behavior. The estuarine front, which was accompanied by a meander with a wavelength of several tens of meters, was visualized based on accumulated foam and debris visible in aerial photographs taken by a ship‐towed balloon equipped with a digital camera. The conductivity‐temperature‐depth sensor casts suggested the bottom of the river plume with a thickness of <5 m undulated because of the development of small eddies with horizontal lengths of <100 m. A nonhydrostatic numerical model was able to reproduce the observed fine‐scale disturbances in the tidal plume. The river plume without tidal currents expanded offshoreward like a balloon, while the tidal plume was confined near the river mouth. It was found that the tidal plume was dynamically equilibrated between the pressure gradient term and the advection term. The latter was composed mainly of contributions from the fine‐scale disturbances, which act as friction because of the momentum exchange between the plume and ambient saline water. The horizontal and vertical components of the disturbances were generated by inertial and Kelvin‐Helmholtz instability processes, respectively. It is considered that a combination of the river plume and tidal currents enhances the current shear favorable for such instabilities to occur.

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On the heterogeneity of stratified-shear turbulence: Observations from a near-field river plume

Cited by 22 publications

References 45 publications

Turbulent Scales Observed in a River Plume Entering a Fjord

Turbulent Scales Observed in a River Plume Entering a Fjord

The turbulent/non-turbulent interface in an inclined dense gravity current

Tidally Induced Instability Processes Suppressing River Plume Spread in a Nonrotating and Nonhydrostatic Regime

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