Gravity current mixing parameterization and calibration of HYCOM

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 in HYCOM 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 nonhydrostatic model (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. Özgökmen, T.M., P.F. Fischer, J. Duan and T. Iliescu, 2004: Three dimensional turbulent bottom density currents from a high-order non-hydrostatic spectral element model. J. Phys. Oceanogr., 34/9 2006-2026 Salinity surface : Nek5000 2-D averaged over y-dir. T=9350s

6. Özgökmen, T.M., P.F. Fischer, J. Duan and T. Iliescu, 2004: Entraiment in bottom gravity currents over complex topography from three- dimensional nonhydrostatic simulation. Geophys. Res. Letters, 31 , L13212, doi:10.1029/2004GL020186

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

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

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

14. KPP TP After tuning

15. KPP TP After tuning

16. Why is the significant modification necessary to adjust the entrainments ? • Turbulence parameterization is also dependent on flow forcing • as well as dependent on the Ri. • - This holds for TP but not for KPP. • KPP: • Kmax should vary with the strength of the forcing, and one specific value • of Kmax cannot be generally applied. • Eg.: Mediterranean outflow with KPP sink deeper due to weak 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)

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

21. 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. • Further experiments with stratified flows are necessary. • Reference : Chang, Y.S., X. Xu, T.M. Özgökmen, E.P. Chassignet, • H. Peters and P.F. Fisher, 2005: Comparison of gravity current mixing parameterization and calibration using a high-resolution 3D nonhydrostatic spectral element model. Ocen Modeling, in Press.

22. Salt Flux: KPP

23. TP