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Transcript of Nansen Environmental and Remote Sensing Center Modifications of the MICOM version used in the Bergen...
Nansen Environmental andRemote Sensing Center
Modifications of the MICOM version used in the Bergen Climate Model
Mats Bentsen and Helge Drange
Nansen Environmental and Remote Sensing CenterBjerknes Centre for Climate ResearchNansen-Zhu International Research Centre, Beijng
Nansen Environmental andRemote Sensing Center
Outline
• BCM components and configuration• Conservation• Physical parameterizations• Some results
Nansen Environmental andRemote Sensing Center
BCM components and configuration
MICOM• Based on version 2.8• Horizontal resolution of ~1.5° with enhanced meridional resolution near the equator and the northern hemisphere grid singularity located over Siberia.• 35 model layers• Reference pressure at the surface• Dynamic-thermodynamic sea ice modules included
ARPEGE• Spectral model.• Hybrid vertical coordinate (terrain following/pressure).• Can use semi-Lagrangian time integration.• Horizontal resolution: T63 (~2.8°)• 31 model layers.
Nansen Environmental andRemote Sensing Center
Main motivations for MICOM improvements
• Get rid of flux adjustments or at least reduce the magnitude of the adjustments.• Improve the Southern Ocean sea-ice extent and volume.• Improve Arctic sea-ice volume and summer extent.• Reduce model drift.• Add features required by IPPC.
Nansen Environmental andRemote Sensing Center
MICOM conserves total mass, but not mass within each layer!
Red dotted lines:After layer advection
Blue lines:After bottom pressure restoration
Conservation of layer mass
Nansen Environmental andRemote Sensing Center
00ˆ bbe ppp
ne
nnD
ne
ne pctpp FF ,ˆˆ 1
nn
upwind
b
kkkDkk p
ptpp FFF ~,~ˆ
Method 1 to reduce the bottom pressure error
Nansen Environmental andRemote Sensing Center
Method 2 to reduce the bottom pressure error
iinki
nk ApM ,
knki
niBii
nkin
iB
iBk ppAp
pp
M 1,
1,
1,1
,
0, ,ˆ
1,1
,
0,1
, ˆˆ
n
kiniB
iB
k
nkn
ki ppp
MMp
Layer mass before advection:
Layer mass after advection and with of correction to restore thebottom pressure:
Apply correction to layer thickness so layer mass is conserved:
k
nki
niB pp 1
,1
, ˆˆ . Then typically 0,
1,
0,
1,ˆ iB
niBiB
niB pppp Let
Nansen Environmental andRemote Sensing Center
Advection/diffusion of temperature and salinityin an isopycnic layer
)( SKSutS
h
),( ST
In the continuous case:• Isopycnic advection does not lead to density change• Isopycnic diffusion increases density (cabbeling)
)( TKTutT
h
Nansen Environmental andRemote Sensing Center
• Density changes due to lateral diffusion is compensated by diapycnal fluxes.
• Layer density can deviate from the target density, but the latter is used in computing the pressure gradient.
• The remaining density deviation from target density is kept small by relaxing the water mass towards a water mass with correct density.
ST
,
)S,T)S,T RRRRRR ,
SSTT '''
R
R
R
R SSTT
''
Nansen Environmental andRemote Sensing Center
Mean salinity (upper figure) and temperature (lower figure) in asimulation with BCM
Nansen Environmental andRemote Sensing Center
Turbulent fluxes
The ocean surface velocity is now included in the computation of turbulent fluxes.
sasaHpaaH TTCcQ uu
sasaEeaL qqCLQ uu
sasaDaC uuuu
Nansen Environmental andRemote Sensing Center
Left/right figure show snapshots of SST (C°) without/with ocean surfacevelocities included in the computation of turbulent fluxes.
Time series of SST in the region [5S,5N] and[120W,90W]. Green curve is climatology whileblue/red curve is without/with ocean surfacevelocitiy dependent turbulent fluxes.
Nansen Environmental andRemote Sensing Center
Left figure is monthly mean SST with region and coloring as previous figure.
Right figure show eastward surface velocity in the same region. Coloring as before.
Nansen Environmental andRemote Sensing Center
Conceptual presentation of convective adjustment
Old scheme New scheme
Nansen Environmental andRemote Sensing Center
Nansen Environmental andRemote Sensing Center
Diapycnal mixing
Old code:• Background mixing (diffusivity ~1/N)
New code:• Background mixing (diffusivity ~1/N)• Shear instability mixing (Ri dependent, Large et. al 1994)• Gravity current entrainment (Ri dependent, Turner 1986)• Cabbeling (due to lateral mixing)• Momentum mixing (Pr=10)
Absorption of short-wave radiation below mixed layer
Nansen Environmental andRemote Sensing Center
Monthly mean SST and sea-ice cover from transient BCM simulation
March 1940 September 1940
Nansen Environmental andRemote Sensing Center
Monthly mean SST and sea-ice cover from transient BCM simulation
September 1940March 1940
Nansen Environmental andRemote Sensing Center
Monthly mean SSS from transient BCM simulation
September 1940March 1940
Nansen Environmental andRemote Sensing Center
Monthly mean barotropic stream function from transient BCM simulation
March 1940 September 1940