European Ice Sheet Modelling Initiative (EISMINT) model intercomparison experiments with first-order mechanics

The European Ice Sheet Modelling Initiative (EISMINT) intercomparison experiments with thermomechanical coupling are repeated with an ice sheet model that applies the first-order approximation for computing the flow field. The experiments impose radially symmetric boundary conditions. Most of the previous results have shown the loss of implied radial symmetry, i.e., the formation of distinct, regularly spaced spokes of cold ice in the simulated basal temperatures. A similar result is also observed in the presented first-order model results. The computed velocity components scatter widely along the marginal grid points. This indicates that the spokes may be triggered by the poor representation of the margin with a regular grid, where the steep gradients in the surface enhance the numerical errors. Additionally, the generally applied second-order discretization scheme tends to decouple even and odd numbered grid points, thus leading to wavy solutions with a wavelength of two grid cells. These patterns strongly suggest that the loss in radial symmetry is a numerical artifact.