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Reynolds Number Dependence of Unbounded Stratified Shear Turbulence: A New Framework for Comparing Ocean and Laboratory Scale Measurements
  • Daniel George MacDonald
Daniel George MacDonald
University of Massachusetts Dartmouth

Corresponding Author:[email protected]

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Advances in understanding of stratified-shear turbulence have been made over the last several decades through ocean measurements, which typically quantify net turbulent quantities, and through laboratory and direct numerical simulations (DNS), which have sufficient resolution to investigate the internal dynamics of individual instabilities. Stratified shear layer thicknesses in these environments can range from cms for laboratory and DNS studies to 100s of m in ocean environments, complicating extrapolation of results between environments. This study provides a direct comparison of field measurements from oceanic stratified shear environments with laboratory flows, demonstrating that non-dimensional turbulent quantities at ocean scales can fall several orders of magnitude below laboratory values for similar bulk Richardson numbers, , suggesting that scale plays a critical role. Here, the dependence of the non-dimensional turbulence intensity, expressed as , on a layer Reynolds number, , is evaluated via a ratio of the shear layer thickness, , to the Kolmogorov turbulence length scale,η. Using a mechanistically driven, empirical approach a parameterization for turbulence is defined in parameter space, and by extension, - parameter space. The mechanisms invoke a “building block” approach to initiation of stratified shear turbulence, which explains the presence of turbulence values exceeding the critical value of the gradient Richardson number, , and increases in at low . The results describe a new “turbulent geography” in the – plane that can build intuition about stratified shear turbulence and facilitate interpretation of ocean measurements in comparison to laboratory experiments and modeling.