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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 Gravity current mixing parameterization and calibration of HYCOM

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Objectives 1.To explore how common mixing parameterizations, particularly KPP and TP, perform using an idealized setting and high- resolution nonhydrostatic solution 2.To quantify the differences and limitations of the two schemes, understanding why and how these parameterizations can be modified to produce consistent results.

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Outline 1.Numerical test of gravity currents over idealized sloped basin using a OGCM, HYCOM 2.Comparison with 3-D LES (Nek5000) in terms of Entrainment, E(t) 3.Tuning the vertical mixing parameters of KPP and TP 4.Adjustment of parameterization over varying slopes 5.Also testing it as a function of the grid resolution

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Nek5000HYCOM Configuration of experiments and initial conditions

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

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

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

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

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After tuning KPPTP

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After tuning KPPTP

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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: 1.KPP-modeled Mediterranean outflow sinks deeper: insufficient mixing 2.K max should vary with the strength of the forcing, and a particular value of K max cannot hold in bottom gravity current mixing Maximum turbulence forcing Peters et al. (1988) TP: 1.Papadakis et al.(2003) : applied TP every 144 th 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)

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Test of adjustment to forcing by employing different low-slopes

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KPP Salt Flux:

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TP

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Conclusion 1.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. 2.But there’s substantial difference between KPP and TP KPP: the amplitude of mixing term is quite dependent on its peak diffusivity, K max, but this given constant cannot respond to the variation of ambient forcing, TP: by relating W E 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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