The SeaIce Model Xingren Wu EMCNCEPNOAA Acknowledgements all

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 Xingren Wu: The Sea-Ice Model CFSRR: 11/7/2007

Outline v Sea-ice in the climate system v Sea-ice thermodynamics and dynamics v Sea-ice in NCEP operational CFS v Sea-ice in NCEP GFS v Sea-ice models v Sea-ice in NCEP new CFS Xingren Wu: The Sea-Ice Model CFSRR: 11/7/2007

Sea Ice Sea ice is a thin skin of frozen water covering the polar oceans. It is a highly variable feature of the earth’s surface. Greece Ice Melt Pond Xingren Wu: Pancake Ice Snow-Ice The Sea-Ice Model Nilas & Leads First-Year Ice Multi-Year Ice Rafting CFSRR: 11/7/2007

NSIDC Arctic sea-ice hits record low in 2007 9/16/2007 Xingren Wu: The Sea-Ice Model CFSRR: 11/7/2007

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 earth’s 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. Xingren Wu: The Sea-Ice Model CFSRR: 11/7/2007

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. Xingren Wu: The Sea-Ice Model CFSRR: 11/7/2007

IPCC, 1995 Xingren Wu: The Sea-Ice Model CFSRR: 11/7/2007

Issues related to sea-ice in GFS and CFS v Data assimilation and Initial conditions v Sea-ice models and coupling Initial condition issues ü Sea-ice concentration data are available § Sea-ice thickness and velocity data are based on model spin-up values, so is snow thickness Xingren Wu: The Sea-Ice Model CFSRR: 11/7/2007

Sea-ice model and coupling issues q q Xingren Wu: Ice thermodynamics Ice model coupling to the atmosphere Ice model coupling to the ocean The Sea-Ice Model CFSRR: 11/7/2007

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) Xingren Wu: The Sea-Ice Model CFSRR: 11/7/2007

Maykut and Untersteiner Model • 1 D 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 seaice 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 Xingren Wu: The Sea-Ice Model CFSRR: 11/7/2007

Ice Model: Dynamics Based on the principle of the conservation of momentum, determine: • Ice motions • Ice deformation • Leads (open water) Xingren Wu: The Sea-Ice Model CFSRR: 11/7/2007

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 Xingren Wu: The Sea-Ice Model CFSRR: 11/7/2007

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). Xingren Wu: The Sea-Ice Model CFSRR: 11/7/2007

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. Xingren Wu: The Sea-Ice Model CFSRR: 11/7/2007

Sea-Ice in new CFS: thermodynamics • Winton (2000) 3 -layer thermodynamic model plus ice thickness distribution ü ü ➢ 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 Xingren Wu: The Sea-Ice Model CFSRR: 11/7/2007

Sea-Ice in new CFS: dynamics • Hunke and Dukowicz (1997) elastic-viscousplastic (EVP) ice dynamics model ➢ ➢ Improved numerical method for Hibler’s viscous-plastic (VP) model Computionally more efficient than Hibler’s VP model, suitable for fully coupled models Hunke, E. C. and J. K. Dukowicz, 1997. An elastic-viscousplastic model for sea ice dynamics. J. Phys. Oceanogr. , 27, 1849 -1867 Xingren Wu: The Sea-Ice Model CFSRR: 11/7/2007

Sea-ice is one component of the new CFS GFS (LAND) Fast loop: Time Step Δa Δa= Δc= Δi Slow loop: Tsfc Sea-Ice Δo Fluxes Coupler Time Step Δc Atmosphere grid X-grid Sea-Ice Time Step Δi Xingren Wu: The Sea-Ice Model Ocean Time Step Δo CFSRR: 11/7/2007

Tripolar grid of Murray (1996) over the Arctic for the sea-ice model This avoids a singularity at the North Pole Xingren Wu: The Sea-Ice Model CFSRR: 11/7/2007

Preliminary Results üExperiment: GFS T 126 L 64 coupled to 1 x 1 degree of sea-ice and ocean (1/3 in the tropics) (Δa= Δc= Δi=10 min, Δo=1 hr) üSea-Ice Results - Concentration - Thickness Xingren Wu: The Sea-Ice Model CFSRR: 11/7/2007

Arctic Sea-Ice Concentration March CFS 20 -year 95 90 80 60 40 20 OBS 95 90 80 60 40 20 Xingren Wu: The Sea-Ice Model CFSRR: 11/7/2007

Antarctic Sea-Ice Concentration September 95 90 80 60 40 20 CFS 20 -year OBS Xingren Wu: 95 90 80 60 40 20 The Sea-Ice Model Ice less compact CFSRR: 11/7/2007

Annual Mean Arctic Sea-Ice Thickness Sea-ice thickness climatology CFS based on pre-1985 data. (Bourke and Garrett, 1987). Xingren Wu: The Sea-Ice Model 20 -year CFSRR: 11/7/2007

Thank you! Xingren Wu: The Sea-Ice Model CFSRR: 11/7/2007