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Gravity current mixing parameterization and calibration of HYCOM

Gravity current mixing parameterization and calibration of HYCOM. Yeon S. Chang, Xiaobiao Xu, Tamay M. Ö zgökmen, Eric P. Chassignet, Hartmut Peters, Paul F. Fischer 1 MPO/RSMAS University of Miami 1 Mathematics and Computer Science Division Argonne National Laboratory.

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Gravity current mixing parameterization and calibration of HYCOM

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  1. Gravity current mixing parameterization and calibration of HYCOM Yeon S. Chang, Xiaobiao Xu, Tamay M. Özgökmen, Eric P. Chassignet, Hartmut Peters, Paul F. Fischer 1 MPO/RSMAS University of Miami 1 Mathematics and Computer Science Division Argonne National Laboratory

  2. Objectives • To explore how common mixing parameterizations, particularly KPP and TP, perform using an idealized setting and high-resolution nonhydrostatic solution • To quantify the differences and limitations of the two schemes, understanding why and how these parameterizations can be modified to produce consistent results.

  3. Outline • Numerical test of gravity currents over idealized sloped basin using a OGCM, HYCOM • Comparison with 3-D LES (Nek5000) in terms of Entrainment, E(t) • Tuning the vertical mixing parameters of KPP and TP • Adjustment of parameterization over varying slopes • Also testing it as a function of the grid resolution

  4. Configuration of experiments and initial conditions Nek5000 HYCOM

  5. Distribution of salinity surface, Nek5000 3-D 2-D averaged in span-wise T=9350s

  6. KPP (Large et al., 1994, 99) : shear-induced, multi-purpose TP (Hallberg, 2000) : developed for overflows based on Ellison and Turner(1959)

  7. KPP HYCOM, before tuning : LES studies of upper tropical ocean (e.g., Large, 1998)

  8. HYCOM, before tuning TP : Lab. Exp. by Ellison and Turner(1959), Turner(1986)

  9. KPP TP After tuning

  10. KPP TP After tuning

  11. Why significant modification is necessary to adjust the entrainment curves ? - Turbulence parameterization should include a dependence on the forcing as well as a dependence on the Ri ; this holds for TP but not for KPP. • KPP: • KPP-modeled Mediterranean outflow sinks deeper: insufficient mixing • Kmax should vary with the strength of the forcing, and a particular value • of Kmax cannot hold in bottom gravity current mixing • TP: • Papadakis et al.(2003) : • applied TP every 144th steps • 2. Turner (1986): small tank • (0.1x2 m), large slopes • ( >10°) • 3. Replacement of bulk Ri in • original Turner scheme by • shear Ri in Hallberg(2000) Maximum turbulence forcing Peters et al. (1988)

  12. Test of adjustment to forcing by employing different low-slopes

  13. Salt Flux: KPP

  14. TP

  15. Conclusion • With appropriate tuning of parameters, both KPP and TP can • be well matched with the nonhydrostatic 3-D solution, and • the results are fairly independent of the horizontal grid • resolution. • But there’s substantial difference between KPP and TP • KPP: the amplitude of mixing term is quite dependent on its • peak diffusivity, Kmax, but this given constant cannot • respond to the variation of ambient forcing, • TP: by relating WE to ΔU, TP avoids hard limit for peak • diffusivity, and the implied diffusivity is dependent both • on Ri and on the forcing via ΔU. • 3. Further experiments with stratified flows are necessary.

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