Abstract
We present MPAS-Seaice, a sea-ice model which uses the Model for Prediction Across Scales (MPAS) framework and Spherical Centroidal Voronoi Tessellation (SCVT) unstructured meshes. As well as SCVT meshes, MPAS-Seaice can run on the traditional quadrilateral grids used by sea-ice models such as CICE. The MPAS-Seaice velocity solver uses the Elastic-Viscous-Plastic (EVP) rheology, and the variational discretization of the internal stress divergence operator used by CICE, but adapted for the polygonal cells of MPAS meshes, or alternatively an integral (“weak”) formulation of the stress divergence operator. An incremental remapping advection scheme is used for mass and tracer transport. We validate these formulations with idealized test cases, both planar and on the sphere. The variational scheme displays lower errors than the weak formulation for the strain rate operator but higher errors for the stress divergence operator. The variational stress divergence operator displays increased errors around the pentagonal cells of a quasi-uniform mesh, which is ameliorated with an alternate formulation for the operator. MPAS-Seaice shares the sophisticated column physics and biogeochemistry of CICE, and when used with quadrilateral meshes can reproduce the results of CICE. We have used global simulations with realistic forcing to validate MPAS-Seaice against similar simulations with CICE and against observations. We find very similar results compared to CICE with differences explained by minor differences in implementation such as with interpolation between the primary and dual meshes at coastlines. We have assessed the computational performance of the model, which, because it is unstructured, runs 70 % as fast as CICE for a comparison quadrilateral simulation. The SCVT meshes used by MPAS-Seaice allow culling of equatorial model cells and flexibility in domain decomposition, improving model performance. MPAS-Seaice is the current sea-ice component of the Energy Exascale Earth System Model (E3SM).
Highlights
Sea ice, the frozen surface of the sea at high latitudes, is an important component of the Earth climate system
These schemes are closely based on those implemented on the quadrilateral grid used in the CICE sea ice model, but adapted for the polygonal cells of Model for Prediction Across Scales (MPAS) meshes
While the variational scheme, with the alternate area denominator formulation, has excellent error characteristics for the stress divergence operator, the one-sided stencil of the variational strain rate operators results in 735 poor error characteristics
Summary
The frozen surface of the sea at high latitudes, is an important component of the Earth climate system. CICE approximates the sea-ice cover as a continuous fluid, and uses an Elastic-Viscous-Plastic (EVP) rheology to describe the relationship between stress and strain 25 in that fluid (Hunke and Dukowicz, 1997). This rheology adds a numerical elasticity to the Viscous-Plastic rheology of Hibler (1979) to allow explicit time-stepping and parallelization of the algorithm. Grid cells in MPAS meshes are polygons with four or more sides, rather than triangles This allows MPAS-Seaice to use the same mesh as either structured quadrilateral models such as CICE, or match the mesh used by unstructured ocean models such as MPAS-Ocean. We focus in this paper on simulations with quasi-uniform global meshes: variable resolution meshes will be considered in later publications
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