USCGC Healy Cruise

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The Sea-Ice Model
Xingren Wu EMC/NCEP/NOAA
Acknowledgements all EMC/NCEP members, in
particular, Dave Behringer, Robert Grumbine,
Yu-tai Hou, Mark Irredel, Steve Lord, Kenneth
Mitchell, Shrinivas Moorthi, Hua-Lu Pan, Diane
Stokes, Suranjana Saha, Jiande Wang, Jun Wang
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Outline

• Sea-ice
• Sea-ice in the climate system
• Sea-ice thermodynamics and dynamics
• Sea-ice in NCEP operational CFS
• Sea-ice in NCEP GFS
• Sea-ice models
• Sea-ice in NCEP new CFS

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Sea Ice
Sea ice is a thin skin of frozen water covering
the polar oceans. It is a highly variable feature
of the earths surface.
Nilas Leads First-Year Ice
Pancake Ice
Multi-Year Ice
Greece Ice
Melt Pond
Snow-Ice
Rafting
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NSIDC
Arctic sea-ice hits record low in 2007
9/16/2007
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Sea-Ice in the Climate System

• Sea-ice interacts strongly with both the
  atmosphere above and the ocean underneath in high
  latitudes.
• A sea ice cover significantly reduces the amount
  of solar radiation absorbed at the earths
  surface due to the marked changes in the surface
  albedo.
• The presence of extensive areas of sea ice
  suppresses heat loss by the ocean.
• The extent of sea ice is mainly influenced by,
  and has a significant effect on, the energy
  budget at the surface, and ocean-air energy
  exchange.

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Sea-Ice in the Climate System (cont.)

• The relationship between climate and sea ice is
  complex. Sea ice responds rapidly and sensitively
  to climatic change due to its positive
  feedback.
• Snow on sea ice is also a very important factor
  in shaping polar climate.
• In view of the varied impacts of the atmosphere,
  ocean and sea ice on each other, it is therefore
  important to include the sea-ice process in our
  weather and climate models.

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IPCC, 1995
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Issues related to sea-ice in GFS and CFS

• Data assimilation and Initial conditions
• Sea-ice models and coupling

• Initial condition issues
• Sea-ice concentration data are available
• Sea-ice thickness and velocity data are based on
  model spin-up values, so is snow thickness

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• Sea-ice model and coupling issues
• Ice thermodynamics
• Ice dynamics
• Ice model coupling to the atmosphere
• Ice model coupling to the ocean

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Ice Model Thermodynamics

• Based on the principle of the conservation of
  energy, determine
• Ice formation
• Ice growth
• Ice/snow melting
• Ice/snow temperature structure
• Leads (open water)

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Maykut and Untersteiner Model

• 1D vertical high-resolution sea-ice model
• Effects of snow cover, ice salinity, and internal
  heating due to penetration of solar radiation
• Thermodynamics of heat conduction through sea-ice
  between the atmosphere and the ocean
• Sea-ice formation and decay

Maykut, G.A. and N. Untersteiner. 1971. Some
results from a time-dependent thermodynamic model
of sea ice. J. Geophys. Res., 76, 1550-1575
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Ice Model Dynamics

• Based on the principle of the conservation of
  momentum, determine
• Ice motions
• Ice deformation
• Leads (open water)

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Hibler VP Model

• Five major dynamic forces in the momentum
  equation
• air stress at the top of sea-ice
• water stress below sea-ice
• gravitational stress from the tilt of sea surface
  (dynamic topography)
• coriolis force
• pressure stresses within ice
• Nonlinear viscous-plastic (VP) ice rheology

Hibler, W.D.III. 1979. A dynamic thermodynamic
sea ice model. J. Phys. Oceanogr., 9, 815-846
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Sea-Ice in current operational CFS

• Sea-ice is treated in a simple manner - 3 m depth
  with 100 concentration (i.e. no open water
  within the ice covered area). The surface
  temperature is predicted based on energy balance
  at the ice surface.
• Sea-ice climatology is used to update sea-ice
  change in CFS (with 50 cutoff for sea-ice cover).

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Sea-Ice in NCEP GFS (for NWP)

• A three-layer thermodynamic sea-ice model has
  been embedded into GFS since May 2005.
• Sea-ice concentration is prescribed.
• Sea-ice/snow thickness, the surface temperature
  and ice temperature structure are predicted.
• The heat and moisture fluxes and albedo are
  treated separately for ice and open water.

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Sea-Ice in new CFS thermodynamics

• Winton (2000) 3-layer thermodynamic model plus
  ice thickness distribution
• 2-layer of sea-ice and 1-layer of snow
• Fully implicit time-stepping scheme, allowing
  longer time steps
• 5 categories of sea-ice

Winton, M. 2000. A reformulated three-layer sea
ice model. J. Atmos. Ocean. Tech., 17(4), 525-531
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Sea-Ice in new CFS dynamics

• Hunke and Dukowicz (1997) elastic-viscous-plastic
  (EVP) ice dynamics model
• Improved numerical method for Hiblers
  viscous-plastic (VP) model
• Computionally more efficient than Hiblers VP
  model, suitable for fully coupled models

Hunke, E. C. and J. K. Dukowicz, 1997. An
elastic-viscous-plastic model for sea ice
dynamics. J. Phys. Oceanogr., 27, 1849-1867
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Sea-ice is one component of the new CFS
Fast loop ?a ?c ?i
Slow loop ?o
Fluxes
Tsfc Sea-Ice
X-grid
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Tripolar grid of Murray (1996) over the Arctic
for the sea-ice model
This avoids a singularity at the North Pole
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Preliminary Results

• Experiment GFS T126L64 coupled to 1x1 degree of
  sea-ice and ocean
• (1/3 in the tropics)
• (?a ?c ?i10 min, ?o1hr)
• Sea-Ice Results
• - Concentration
• - Thickness

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Arctic Sea-Ice Concentration March
CFS 20-year
95 90 80 60 40 20
OBS
95 90 80 60 40 20
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Antarctic Sea-Ice Concentration September
95 90 80 60 40 20
CFS 20-year
95 90 80 60 40 20
OBS
Ice less compact
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Annual Mean Arctic Sea-Ice Thickness
CFS 20-year
Sea-ice thickness climatology based on pre-1985
data. (Bourke and Garrett, 1987).
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Thank you!