yeon s. chang, xiaobiao xu, tamay m. Özgökmen, eric p. chassignet, hartmut peters, paul f. fischer...
TRANSCRIPT
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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 DivisionArgonne 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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Nek5000 HYCOM
Configuration of experiments and initial conditions
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Distribution of salinity surface, Nek5000
3-D
2-Daveraged in span-wise
T=9350s
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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
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HYCOM, before tuning
mx 1000 mx 20
KPP scmK /50 2
max : LES studies of upper tropical ocean (e.g., Large, 1998)
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TP
0.1AC
HYCOM, before tuning
mx 1000 mx 20
: Lab. Exp. by Ellison and Turner(1959), Turner(1986)
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)(
)()()(
0
tl
thth
dX
dhtE
scmK /50,KPP
tuningBefore2
max
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0.1,TP
tuningBefore
AC
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scmK /2500,KPP
ngAfter tuni2
max
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15.0,TP
ngAfter tuni
AC
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After tuning
mx 20
KPP scmK /2500 2
max
mx 20
TP 15.0AC
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After tuning
mx 1000
KPP scmK /2500 2
max
mx 1000
TP 15.0AC
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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 mixing2. Kmax should vary with the strength of the forcing, and a particular value of Kmax cannot hold in bottom gravity current 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)
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Test of adjustment to forcing by employing different low-slopes
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scmK /50,KPP
tuningBefore2
max
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y F b
b
L tyX
x
hz
zFy
dxdydztzyxStzyxuhxXL
tSF0
),'(
000
),,,(),,,(1
)(
11)(
KPP
Salt Flux:
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KPP ng,After tuni
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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, 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.