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Abstract In very stable boundary layers (VSBL), a “cocktail” of submeso motions routinely result in elevated mean wind speed maxima above the ground, acting as a new source of turbulence generation. This new source of turbulent kinetic energy enhances turbulent mixing and causes mean wind profile distortion (WPD). As a results, this transient distortion in the wind profile adjusts the classical log‐law. Addressing how WPD‐induced turbulence regulates flow structures, turbulent fluxes, and transitions in stability regimes across layers remains a challenge. Eddy covariance data measured at four levels on a 62‐m tower are employed to address these questions. It is shown that the WPD initiates large turbulent eddies that penetrate downward, leading to enhanced vertical mixing and comparable turbulent transport efficiencies across layers. As a consequence, turbulence intensity and fluxes are increased. As the WPD is intensified, turbulent fluxes and turbulent flux transport caused by large eddies are also enhanced, leading to a transition from very stable to weakly stable regimes. Due to the influence of WPD‐induced large eddies, the large‐eddy turbulent Prandtl number does not deviate appreciably from unity and the partitioning between turbulent kinetic and potential energies is linearly related to the gradient Richardson number.
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On Energy and Turbulent Mixing in the Thermocline
Abstract A new method for computing the rate at which turbulent mixing builds potential energy in the ocean is described. The traditional approach has focused on the rate of change of the background potential energy associated with an adiabatically leveled state. We argue that when examining mixing events, so‐called “Thorpe” sorting yields a useful and local measure of diabatically generated potential energy and exhibits some advantages relative to adiabatic leveling. Among these, the open question about the leveling domain is avoided, the fate of kinetic energy during a mixing event is clearly defined, and the computational load associated with the leveling is relieved. The resultant kinetic energy equation leads to a natural definition of mixing efficiency and turbulent diffusivity in terms of sign definite viscous and diffusive contributions. Applications to 2‐D Kelvin Helmholtz instability demonstrate the utility of the procedure. We find an integrated efficiency of ≈ 0.15 for a Prandtl number of 1, and of ≈ 0.08 for a Prandtl number of 10. The larger is comparable to the classical value of 0.2 used frequently by the mixing community and smaller than that found in some recent simulations.
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- Award ID(s):
- 1829856
- PAR ID:
- 10460357
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Advances in Modeling Earth Systems
- Volume:
- 11
- Issue:
- 3
- ISSN:
- 1942-2466
- Page Range / eLocation ID:
- p. 578-596
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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