Sensitivity and Resolution Convergence Studies of North Atlantic Model Circulation
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Transcript of Sensitivity and Resolution Convergence Studies of North Atlantic Model Circulation
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Sensitivity and Resolution Convergence Studies of
North Atlantic Model Circulation
Matthew Hecht, Los Alamos
Frank Bryan, NCAR
Rick Smith, Los Alamos
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SMBH 2000
• In 2000 paper SMBH, saw pretty good sub-tropical gyre and Gulf Stream system
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• Gulf Stream separated from the coast at Cape Hatteras– if somewhat
too zonal
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• Azores Current matched obs well– far better than in earlier
0.28º global POP simulation
0.1º NA
0.28º global
At 32º W
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North Atlantic Current turned North around the Grand Banks, into the Northwest Corner (sea surface height shown).
0.1º NA 0.28º global
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• Resolution was a big part of the story– grid spacing,
varying with latitude, shown, with first Rossby radius
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Resolution is not the whole storyMaltrud and McClean, 2004 (MM): Most majorjets quite realistic in 0.1ºglobal versionof POP,but Gulf Streamdid not separateat Hatteras.
Also see experience of Boning et al, 1/12º.
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Do Gulf Stream/North
Atlantic Current biases
matter?
SST
SSTBias
North-West AtlanticCorner /LabradorSea
Sea Surface Temperaturesand ErrorRelative to Observations, from Community Climate System Model
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• Working towards high res climate simulation, with eddy-resolving ocean– With NCAR, CRIEPI (Japan), NPS– Appears possible to greatly reduce NA SST
biases, with a combination of informed choices and luck, so…
• Would like to understand how best to configure ocean model
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• So, SMBH had good GS system, MM not as satisfactory. Likely factors:– Source waters (NA overflow, Ant. Bottom Water, etc.)
– Grid (mercator NA regional -vs- “displaced pole” (Hudson’s Bay) global
– Viscous parameters• Had not been set cleanly in SMBH, coef’s had been changed
twice during integration
• Here we concentrate mostly on sensitivity to viscous param’s, w/ some experimentation w/ horizontal grid resolution as well.
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Model• POP free surface z-coord hydrostatic primitive
equation code– “Full cell” topography (partial cell option also exists)
– Mercator grids (x= y, so y also scales w/ cos(y))• Mostly 0.1º res, but also 0.2 º, 0.4 º
– 40 Levels• 10 m in upper ocn, 250 m in deep, 5500 m max
– 20º S to 73º N• Including Gulf of Mexico, Western Mediterranean
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Forcing
• Barnier monthly heat flux climatology– with penetrative solar
• E/P from 30 day (over 10 m) restoring to Levitus monthly climatology
• Restore SST to -2º under diagnosed ice
• Daily ECMWF winds, 1986--2000
• Restoring at lateral boundaries– North of NA Sill, S Atlantic, Sicily Channel
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Mixing
• Pacanowski and Philander Ri-number-based vertical mixing– Have used KPP in other 0.1º simulations
• Biharmonic horizontal mixing of tracers and momentum– Have used anisotropic viscosity, GM,
anisotropic GM in other simulations
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Scaling of biharmonic coef’s
• As standard practice we scale horizontal mixing coef’s with (area)3/2 so as to maintain constant grid-Reynolds (-Peclet) number: Grid-Re = U (area/area0)3/2/, for biharmonic form
• Scaled within simulation, as grid cell area varies, and between simulation, by default
• In these experiments we also include an adjustable factor C so that = C 0 (area/area0)3/2
0=-2.7x1010m4/s, area0=(11.2 km)2 (equatorial grid cell area)
• C=1 for “standard” set of simulations
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• 15 year integration gives reasonable equilibration of KE– Time series of three 0.1º cases shown
– Averages over last 3 years shown for other cases
C=1/4
C=1/2
C=1SMBH
0.2º, C=1/8
C=80.2º, C=1
0.4º, C=1
same dissipative parameters
same dissipative parameters
Basin Mean Kinetic Energyg(
cm/s
)^2
5040
100
2000199619921988
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• Munk width of 0.1º case at C=1 is just under 2 grid lengths.
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C=1, 0.4º
C=1, 0.2º
C=1, 0.1º C=8, 0.1º
C=1/8, 0.2º
• Boundary eddy in non-separating case
• Northern recirc gyre extends to ~Hatteras in C=1, 0.1º case
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• Recirculation gyres really spin up with decreasing viscosity
• Does northern recirculation gyre cause Stream to go too zonal off Hatteras in low viscosity (C=1/4) case?
C=1/4, 0.1º
C=1/2, 0.1º
C=1, 0.1º
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Gulf Stream pathsfrom intersection of 12°C isotherm and 400m, 1998-2000
• By this point in time (years 13-15) the more viscous (C=1) case has a more realistic separation (obs in green, model mean, 1 and extreme envelope in blue)
C=1, 0.1º
C=1/2, 0.1º
C=1/4, 0.1º
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1990-1992 1998-2000
C=1
C=1/4
Separation sets up earlier in less viscous case.
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SSH, C=1/2 case
QuickTime™ and aPhoto decompressor
are needed to see this picture.
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• Northwest Corner is better with lower viscosity, in SSH var.
C=1
C=1/2
C=1/4
TOPEX
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• How different the density (thermal) fronts are! Yet:
• Many of the features of the flow are similar here, and deeper -- the E/W portions of the flow as it winds northward.
• More viscous case tends to lose much of the NAC out eastern boundary relatively early.
C=1
C=1/4
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• Deeper penetration of Current in less viscous case
• Ozgokmen (97) found jet needed to be highly inertial with low eddy activity to separate and cross f/H, in process study.– We don’t see relation
between low eddy activity and separation at Hatteras
– but maybe high eddy activity for reattachment at Grand Banks
Eddy Kinetic Energies, 55º W
C=1
C=1/4
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1998 1999
2000
Obs from Pickart and Smethie. Model with C=1/2.
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• Magnitude of NAC somewhat weak– Structure is reasonable
– WBC too strong?
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• Peak velocities in NAC match pretty well– but see how 10 cm/s isotach is at 1600 m in model, 3500 m in obs– Particularly weak in recirculated “Lab Sea”, “upper-Lab Sea” waters– Too much transport of South/Westward-flowing boundary water in
this least-viscous case.
C=1/4
+32.5-20.0
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• DWBC looked good off Abaco in SMBH– Now we’re looking in more detail upstream of Hatteras, where
stream and bathymetry are both complex, comparison less satisfactory
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Unexplored tangent: low mixing in idealized overflow at ~0.1º
Legg, Hallbergand Girton,OMOD, in press.
• NW Corner not as realistic as Gulf Stream, Azores Current. May be worth looking more at:– model source waters,
mixing in overflow, inclusion of passive tracers?
– Nevertheless, SST biases reduced over low res models
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Conclusions• With biharmonic horizontal dissipation in POP at
0.1º, best compromise between GS separation and NW Corner, Azores Current is C=1/2 case.
• Some evidence for convergence at undesirably high level of dissipation.– As expected, not converged at more desirable level of
dissipation
• Has led to investigations of anisotropic horizontal viscosity, GM, partial bottom cells, smoothing of topography