Yeon S' Chang, Xiaobiao Xu, Tamay M' zgkmen,

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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

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• 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.

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• 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

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Configuration of experiments and initial
conditions
Nek5000
HYCOM
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Distribution of salinity surface,
Nek5000
3-D
2-D averaged in span-wise
T9350s
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KPP (Large et al., 1994, 99) shear-induced,
multi-purpose
TP (Hallberg, 2000) developed for overflows
based on Ellison and Turner(1959)
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HYCOM, before tuning
LES studies of upper tropical ocean (e.g.,
Large, 1998)
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HYCOM, before tuning
TP
Lab. Exp. by Ellison and Turner(1959),
Turner(1986)
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After tuning
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After tuning
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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
• 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
• ( gt10)
• 3. Replacement of bulk Ri in
• original Turner scheme by
• shear Ri in Hallberg(2000)

Maximum turbulence forcing
Peters et al. (1988)
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Test of adjustment to forcing by employing
different low-slopes
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Salt Flux
KPP
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TP
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• 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 theres 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 ?U, TP avoids
  hard limit for peak
• diffusivity, and the implied
  diffusivity is dependent both
• on Ri and on the forcing via ?U.
• 3. Further experiments with stratified flows
  are necessary.