gravity current mixing parameterization and calibration of hycom
DESCRIPTION
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. - PowerPoint PPT PresentationTRANSCRIPT
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 DivisionArgonne National Laboratory
Gravity current mixing parameterization and calibration of
HYCOM
Objectives
1. To explore how common mixing parameterizations, particularly KPP and TP, perform in HYCOM 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.
Outline
1. Numerical test of gravity currents over idealized sloped basin using a OGCM, HYCOM
2. Comparison with 3-D nonhydrostatic model (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
Nek5000 HYCOM
Configuration of experiments and initial conditions
Salinity surface : Nek5000
2-Daveraged over y-dir.
T=9350s
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
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
7.0
,11*
32
max
c
cshear
Ri
Ri
RiminKK
8.0 if
51
1.008.0
Ri
Ri
RiC
U
wA
E
TP (Hallberg, 2000): developed for overflows based on Ellison and Turner(1959)
KPP (Large et al., 1994, 99): shear-induced, multi-purpose
HYCOM, before tuning
mx 1000 mx 20
KPP scmK /50 2
max : LES studies of upper tropical ocean (e.g., Large, 1998)
TP
0.1AC
HYCOM, before tuning
mx 1000 mx 20
: Lab. Exp. by Ellison and Turner(1959), Turner(1986)
)(
)()()(
0
tl
thth
dX
dhtE
scmK /50,KPP
tuningBefore2
max
0.1,TP
tuningBefore
AC
scmK /2500,KPP
ngAfter tuni2
max
15.0,TP
ngAfter tuni
AC
After tuning
mx 20
KPP scmK /2500 2
max
mx 20
TP 15.0AC
After tuning
mx 1000
KPP scmK /2500 2
max
mx 1000
TP 15.0AC
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:1. Kmax should vary with the strength of the forcing, and one specific value of Kmax cannot be generally applied.2. Eg.: Mediterranean outflow with KPP sink deeper due to weak mixing
Maximum turbulence forcing
Peters et al. (1988)
TP:1. 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 inoriginal Turner scheme by
shear Ri in Hallberg(2000)
Test of adjustment to forcing by employing different low-slopes
scmK /50,KPP
tuningBefore2
max
KPP ng,After tuni
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, 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.
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.
y F b
b
L tyX
x
hz
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tSF0
),'(
000
),,,(),,,(1
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KPP
Salt Flux:
TP