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


Outline

• BCM components and configuration• Conservation• Physical parameterizations• Some results


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.


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.


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


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


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

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,

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


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


• 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

''


Mean salinity (upper figure) and temperature (lower figure) in asimulation with BCM


Turbulent fluxes

The ocean surface velocity is now included in the computation of turbulent fluxes.

sasaHpaaH TTCcQ uu

sasaEeaL qqCLQ uu

sasaDaC uuuu


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.


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.


Conceptual presentation of convective adjustment

Old scheme New scheme


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


Monthly mean SST and sea-ice cover from transient BCM simulation

March 1940 September 1940


Monthly mean SST and sea-ice cover from transient BCM simulation

September 1940March 1940


Monthly mean SSS from transient BCM simulation

September 1940March 1940


Monthly mean barotropic stream function from transient BCM simulation

March 1940 September 1940

nansen environmental and remote sensing center modifications of the micom version used in the bergen...

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