Floating sea-ice regulates the transfer of heat and momentum between the atmosphere and the ocean. It also provides a barrier to the exchange of freshwater, gases and aerosols. Sea-ice is acutely sensitive to changes in climate, and as the planet warms retreating sea-ice will reflect less solar radiation back to space, representing one of the strongest positive climate feedbacks in the Earth system.
The sea-ice model in UKESM is the CICE model (Community Ice Code, Hunke and Lipscombe 2010), developed at the US Los Alamos National Laboratory with collaborators around the world. The model shares the tripolar grid used by the ocean model, thereby avoiding a singularity at the North Pole. The scientific configuration of CICE used in UKESM1 was developed by Met Office, CPOM, BAS and NOC as a component of the physical climate model HadGEM3-GC3.1. In this configuration the sea-ice dynamics are modelled using the elastic-viscous-plastic rheology scheme of Hunke and Dukowicz (1997).
The model’s ice thermodynamics, controlling the flux and storage for heat through the ice, uses 5 ice thickness categories and one open water category and also multiple layers in the vertical, with 4 ice layers and one snow layer (Bitz & Lipscomb, 1999). Unusually for a climate model, the exchange of heat and water between the ice and atmosphere is computed by the land surface model JULES rather than in the sea-ice component itself. This allows the exchange to be calculated with updated atmospheric conditions every atmosphere timestep (~20 minutes) rather than every coupling timestep (3 hours).
This sea-ice model also includes a new prognostic scheme to simulate the impact of melt ponds on reflected sunlight (Flocco et al, 2010). During the melt season, these ponds accumulate on the surface of the ice and can cover up to 50% of the sea-ice area. This significantly reduces the surface albedo and enhances the rate of melting. The albedo calculation also now uses separate albedos for visible and near-infrared radiation.
M. Bitz and W. H. Lipscomb. An Energy-Conserving Thermodynamic Model of Sea Ice. Journal of Geophysical Research 104, 15,669-16,677, 1999.
Hunke and J. K. Dukowicz. An elastic-viscous-plastic model for sea ice dynamics. J. Phys. Oceanogr., 27:1849–1867, 1997.
Hunke, E. C. and Lipscomb, W. H.: CICE: the Los Alamos sea ice model documentation and software users’ manual, Version 4.1, LA-CC-06-012, Los Alamos National Laboratory, N.M., 2010.
Flocco, D., D. L. Feltham, and A. K. Turner (2010), Incorporation of a physically based melt pond scheme into the sea ice component of a climate model, J. Geophys. Res., 115, C08012, doi:10.1029/2009JC005568.