• Barry, M. E., , G. N. Ivey, , K. B. Winters, , and J. Imberger, 2001: Measurements of diapycnal diffusivities in stratified fluids. J. Fluid Mech., 442, 267291, doi:10.1017/S0022112001005080.

    • Search Google Scholar
    • Export Citation
  • Dillon, T. M., 1982: Vertical overturns: a comparison of Thorpe and Ozmidov length scales. J. Geophys. Res., 87, 96019613, doi:10.1029/JC087iC12p09601.

    • Search Google Scholar
    • Export Citation
  • Ellison, T. H., 1957: Turbulent transport of heat and momentum from an infinite rough plane. J. Fluid Mech., 2, 456466, doi:10.1017/S0022112057000269.

    • Search Google Scholar
    • Export Citation
  • Gargett, A. E., , T. R. Osborn, , and P. W. Nasmyth, 1984: Local isotropy and the decay of turbulence in a stratified fluid. J. Fluid Mech., 144, 231280, doi:10.1017/S0022112084001592.

    • Search Google Scholar
    • Export Citation
  • Garrett, C., , P. MacCready, , and P. Rhines, 1993: Boundary mixing and arrested Ekman layers: Rotating stratified flow near a sloping boundary. Annu. Rev. Fluid Mech., 25, 291323, doi:10.1146/annurev.fl.25.010193.001451.

    • Search Google Scholar
    • Export Citation
  • Geyer, W. R., , M. E. Scully, , and D. K. Ralston, 2008: Quantifying vertical mixing in estuaries. Environ. Fluid Mech., 8, 495509, doi:10.1007/s10652-008-9107-2.

    • Search Google Scholar
    • Export Citation
  • Gibson, C. H., 1986: Internal waves, fossil turbulence, and composite ocean microstructure spectra. J. Fluid Mech., 168, 89117, doi:10.1017/S0022112086000307.

    • Search Google Scholar
    • Export Citation
  • Gregg, M. C., 1999: Uncertainties and limitations in measuring ε and . J. Atmos. Oceanic Technol., 16, 14831490, doi:10.1175/1520-0426(1999)016<1483:UALIMA>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Inoue, R., , and W. D. Smyth, 2009: Efficiency of mixing forced by unsteady shear flow. J. Phys. Oceanogr., 39, 11501166, doi:10.1175/2008JPO3927.1.

    • Search Google Scholar
    • Export Citation
  • Ivey, G. N., , and J. Imberger, 1991: On the nature of turbulence in a stratified fluid. Part I: The energetics of mixing. J. Phys. Oceanogr., 21, 650658, doi:10.1175/1520-0485(1991)021<0650:OTNOTI>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Kaimal, J. C., , Y. Izumi, , J. C. Wyngaard, , and R. Cote, 1972: Spectral characteristics of surface-layer turbulence. Quart. J. Roy. Meteor. Soc., 98, 563589, doi:10.1002/qj.49709841707.

    • Search Google Scholar
    • Export Citation
  • Kay, D. J., , and D. A. Jay, 2003: Interfacial mixing in a highly stratified estuary. 1. Characteristics of mixing. J. Geophys. Res., 108, 3072, doi:10.1029/2000JC000252.

    • Search Google Scholar
    • Export Citation
  • Lavery, A. C., , W. R. Geyer, , and M. E. Scully, 2013: Broadband acoustic quantification of stratified turbulence. J. Acoust. Soc. Amer., 134, 4054, doi:10.1121/1.4807780.

    • Search Google Scholar
    • Export Citation
  • MacDonald, D. G., , and W. R. Geyer, 2004: Turbulent energy production and entrainment at a highly stratified estuarine front. J. Geophys. Res., 109, C05004, doi:10.1029/2003JC002094.

    • Search Google Scholar
    • Export Citation
  • Macoun, P., , and R. Lueck, 2004: Modelling the spatial response of the airfoil shear probe using different sized probes. J. Atmos. Oceanic Technol., 21, 284297, doi:10.1175/1520-0426(2004)021<0284:MTSROT>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Mashayek, A., , and W. R. Peltier, 2013: Shear-induced mixing in geophysical flows: Does the route to turbulence matter to its efficiency? J. Fluid Mech., 725, 216261, doi:10.1017/jfm.2013.176.

    • Search Google Scholar
    • Export Citation
  • Mashayek, A., , C. P. Caulfield, , and W. R. Peltier, 2013: Time-dependent, non-monotonic mixing in stratified turbulent shear flows: implications for oceanographic estimates of buoyancy flux. J. Fluid Mech., 736, 570593, doi:10.1017/jfm.2013.551.

    • Search Google Scholar
    • Export Citation
  • Moum, J. N., 1996: Efficiency of mixing the main thermocline. J. Geophys. Res., 101, 12 05712 069, doi:10.1029/96JC00508.

  • Moum, J. N., , D. M. Farmer, , W. D. Smyth, , L. Armi, , and S. Vagle, 2003: Structure and generation of turbulence at interfaces strained by internal solitary waves propagating shoreward over the continental shelf. J. Phys. Oceanogr., 33, 20932112, doi:10.1175/1520-0485(2003)033<2093:SAGOTA>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Müller, P., , and C. Garrett, 2002: From stirring to mixing in a stratified ocean. Oceanography, 15, 1219, doi:10.5670/oceanog.2002.10.

    • Search Google Scholar
    • Export Citation
  • Munk, W. H., , and E. R. Anderson, 1948: Notes on the theory of the thermocline. J. Mar. Res., 3, 276295.

  • Nash, J. D., , and J. N. Moum, 2002: Microstructure estimates of turbulent salinity flux and the dissipation spectrum of salinity. J. Phys. Oceanogr., 32, 23122333, doi:10.1175/1520-0485(2002)032<2312:MEOTSF>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Oakey, N. S., 1982: Determination of the rate of dissipation of turbulent energy from simultaneous temperature and velocity shear microstructure measurements. J. Phys. Oceanogr., 12, 256271, doi:10.1175/1520-0485(1982)012<0256:DOTROD>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Osborn, T., 1980: Estimates of the local rate of vertical diffusion from dissipation measurements. J. Phys. Oceanogr., 10, 8389, doi:10.1175/1520-0485(1980)010<0083:EOTLRO>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Osborn, T., , and C. Cox, 1972: Oceanic fine structure. Geophys. Fluid Dyn., 3, 321345, doi:10.1080/03091927208236085.

  • Peters, H., 1997: Observations of stratified turbulent mixing in an estuary: Neap-to-spring variations during high river flow. Estuarine Coastal Shelf Sci., 45, 6988, doi:10.1006/ecss.1996.0180.

    • Search Google Scholar
    • Export Citation
  • Ruddick, B., , D. Walsh, , and N. Oakey, 1997: Variations in apparent mixing efficiency in the North Atlantic Central Water. J. Phys. Oceanogr., 27, 25892605, doi:10.1175/1520-0485(1997)027<2589:VIAMEI>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Saggio, A., , and J. Imberger, 2001: Mixing and turbulent fluxes in the metalimnion of a stratified lake. Limnol. Oceanogr., 46, 392409, doi:10.4319/lo.2001.46.2.0392.

    • Search Google Scholar
    • Export Citation
  • Schumann, U., , and T. Gerz, 1995: Turbulent mixing in stably stratified shear flows. J. Appl. Meteor., 34, 3348, doi:10.1175/1520-0450-34.1.33.

    • Search Google Scholar
    • Export Citation
  • Scully, M. E., , W. R. Geyer, , and J. H. Trowbridge, 2011: The influence of stratification and nonlocal turbulent production on estuarine turbulence: An assessment of turbulence closure with field observations. J. Phys. Oceanogr., 41, 166185, doi:10.1175/2010JPO4470.1.

    • Search Google Scholar
    • Export Citation
  • Seim, H. E., , and M. C. Gregg, 1994: Detailed observations of a naturally occurring shear instability. J. Geophys. Res., 99, 10 04910 073, doi:10.1029/94JC00168.

    • Search Google Scholar
    • Export Citation
  • Seim, H. E., , and M. C. Gregg, 1995: Energetics of a naturally occurring shear instability. J. Geophys. Res., 100, 49434958, doi:10.1029/94JC03199.

    • Search Google Scholar
    • Export Citation
  • Shaw, W. J., , J. H. Trowbridge, , and A. H. Williams III, 2001: Budgets of turbulent kinetic energy and scalar variance in the continental shelf bottom boundary layer. J. Geophys. Res., 106, 95519564, doi:10.1029/2000JC000240.

    • Search Google Scholar
    • Export Citation
  • Shih, L. H., , J. Koseff, , G. N. Ivey, , and J. H. Ferziger, 2005: Parameterization of turbulent fluxes and scales using homogeneous sheared stably stratified turbulence simulations. J. Fluid Mech., 525, 193214, doi:10.1017/S0022112004002587.

    • Search Google Scholar
    • Export Citation
  • Smyth, W. D., , J. N. Moum, , and D. R. Caldwell, 2001: The efficiency of mixing in turbulent patches: Inferences from direct simulations and microstructure observations. J. Phys. Oceanogr., 31, 19691992, doi:10.1175/1520-0485(2001)031<1969:TEOMIT>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Stacey, M. T., , S. G. Monismith, , and J. R. Burau, 1999: Observations of turbulence in a partially stratified estuary. J. Phys. Oceanogr., 29, 19501970, doi:10.1175/1520-0485(1999)029<1950:OOTIAP>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • St. Laurent, L. C., , J. M. Toole, , and R. W. Schmitt, 2001: Buoyancy forcing by turbulence above rough topography in the abyssal Brazil Basin. J. Phys. Oceanogr., 31, 34763495, doi:10.1175/1520-0485(2001)031<3476:BFBTAR>2.0.CO;2.

    • Search Google Scholar
    • Export Citation
  • Tennekes, H., , and J. Lumley, 1972: A First Course in Turbulence.MIT Press, 300 pp.

  • Thorpe, S. A., 1977: Turbulence and mixing in a Scottish Loch. Philos. Trans. Roy. Soc. London, A286, 125181, doi:10.1098/rsta.1977.0112.

    • Search Google Scholar
    • Export Citation
  • Thorpe, S. A., 2005: The Turbulent Ocean.Cambridge University Press, 439 pp.

  • Trowbridge, J. H., 1992: A simple description of the deepening and structure of a stably stratified flow driven by a surface stress. J. Geophys. Res., 97, 15 52915 543, doi:10.1029/92JC01512.

    • Search Google Scholar
    • Export Citation
  • Warner, J. C., , C. R. Sherwood, , H. G. Arango, , and R. P. Signell, 2005: Performance of four turbulence closure models implemented using a generic length scale method. Ocean Modell., 8, 81113, doi:10.1016/j.ocemod.2003.12.003.

    • Search Google Scholar
    • Export Citation
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Stratified Turbulence and Mixing Efficiency in a Salt Wedge Estuary

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  • 1 Department of Applied Ocean Physics and Engineering, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts
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Abstract

High-resolution observations of velocity, salinity, and turbulence quantities were collected in a salt wedge estuary to quantify the efficiency of stratified mixing in a high-energy environment. During the ebb tide, a midwater column layer of strong shear and stratification developed, exhibiting near-critical gradient Richardson numbers and turbulent kinetic energy (TKE) dissipation rates greater than 10−4 m2 s−3, based on inertial subrange spectra. Collocated estimates of scalar variance dissipation from microconductivity sensors were used to estimate buoyancy flux and the flux Richardson number Rif. The majority of the samples were outside the boundary layer, based on the ratio of Ozmidov and boundary length scales, and had a mean Rif = 0.23 ± 0.01 (dissipation flux coefficient Γ = 0.30 ± 0.02) and a median gradient Richardson number Rig = 0.25. The boundary-influenced subset of the data had decreased efficiency, with Rif = 0.17 ± 0.02 (Γ = 0.20 ± 0.03) and median Rig = 0.16. The relationship between Rif and Rig was consistent with a turbulent Prandtl number of 1. Acoustic backscatter imagery revealed coherent braids in the mixing layer during the early ebb and a transition to more homogeneous turbulence in the midebb. A temporal trend in efficiency was also visible, with higher efficiency in the early ebb and lower efficiency in the late ebb when the bottom boundary layer had greater influence on the flow. These findings show that mixing efficiency of turbulence in a continuously forced, energetic, free shear layer can be significantly greater than the broadly cited upper bound from Osborn of 0.15–0.17.

Current affiliation: San Francisco Estuary Institute, Richmond, California.

Corresponding author address: R. C. Holleman, San Francisco Estuary Institute, 4911 Central Ave., Richmond, CA 94804. E-mail: rustyh@sfei.org

Abstract

High-resolution observations of velocity, salinity, and turbulence quantities were collected in a salt wedge estuary to quantify the efficiency of stratified mixing in a high-energy environment. During the ebb tide, a midwater column layer of strong shear and stratification developed, exhibiting near-critical gradient Richardson numbers and turbulent kinetic energy (TKE) dissipation rates greater than 10−4 m2 s−3, based on inertial subrange spectra. Collocated estimates of scalar variance dissipation from microconductivity sensors were used to estimate buoyancy flux and the flux Richardson number Rif. The majority of the samples were outside the boundary layer, based on the ratio of Ozmidov and boundary length scales, and had a mean Rif = 0.23 ± 0.01 (dissipation flux coefficient Γ = 0.30 ± 0.02) and a median gradient Richardson number Rig = 0.25. The boundary-influenced subset of the data had decreased efficiency, with Rif = 0.17 ± 0.02 (Γ = 0.20 ± 0.03) and median Rig = 0.16. The relationship between Rif and Rig was consistent with a turbulent Prandtl number of 1. Acoustic backscatter imagery revealed coherent braids in the mixing layer during the early ebb and a transition to more homogeneous turbulence in the midebb. A temporal trend in efficiency was also visible, with higher efficiency in the early ebb and lower efficiency in the late ebb when the bottom boundary layer had greater influence on the flow. These findings show that mixing efficiency of turbulence in a continuously forced, energetic, free shear layer can be significantly greater than the broadly cited upper bound from Osborn of 0.15–0.17.

Current affiliation: San Francisco Estuary Institute, Richmond, California.

Corresponding author address: R. C. Holleman, San Francisco Estuary Institute, 4911 Central Ave., Richmond, CA 94804. E-mail: rustyh@sfei.org
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