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Computer models are necessary for understanding and predicting marine ice sheet behaviour. However, there is uncertainty over implementation of physical processes at the ice base, both for grounded and floating glacial ice. Here we implement several sliding relations in a marine ice sheet flow-line model accounting for all stress components and demonstrate that model resolution requirements are strongly dependent on both the choice of basal sliding relation and the spatial distribution of ice shelf basal melting.Sliding relations that reduce the magnitude of the step change in basal drag from grounded ice to floating ice (where basal drag is set to zero) show reduced dependence on resolution compared to a commonly used relation, in which basal drag is purely a power law function of basal ice velocity. Sliding relations in which basal drag goes smoothly to zero as the grounding line is approached from inland (due to a physically motivated incorporation of effective pressure at the bed) provide further reduction in resolution dependence.A similar issue is found with the imposition of basal melt under the floating part of the ice shelf: melt parameterisations that reduce the abruptness of change in basal melting from grounded ice (where basal melt is set to zero) to floating ice provide improved convergence with resolution compared to parameterisations in which high melt occurs adjacent to the grounding line.Thus physical processes, such as sub-glacial outflow (which could cause high melt near the grounding line), impact on capability to simulate marine ice sheets. If there exists an abrupt change across the grounding line in either basal drag or basal melting, then high resolution will be required to solve the problem. However, the plausible combination of a physical dependency of basal drag on effective pressure, and the possibility of low ice shelf basal melt rates next to the grounding line, may mean that some marine ice sheet systems can be reliably simulated at a coarser resolution than currently thought necessary.

Methods: The topographic data was produced using subversion revision number r2825 of the BISICLES ice-sheet model (https://commons.lbl.gov/display/bisicles/BISICLES) configured to use the shallow shelf approximation (SSA, a commonly used approximation of the Stokes equation appropriate for ice shelves and fast flowing ice streams). The simulation used to produce the topography was the Ice1r followed by the Ice1ra experiment, as described in Sec. 2.2 of Asay-Davis et al. (2015). The bedrock topography for the experiment is described by an analytic function in Sec. 2.1 of Asay-Davis et al. (2015). In these experiments, BISICLES was first run to steady state without basal melting, then was allowed to retreat for 100 years forced by a basal melt parameterization (Ice1r) and finally was allowed to re-advance for 100 years with the melt parameterization turned off (Ice1ra). BISICLES makes use of adaptive mesh refinement (AMR) to focus resolution in fast flowing regions and near grounding lines. In the simulations, resolution was allowed to vary from 4 km to 1 km, and the topographic snapshots have been interpolated to simple rectangular mesh with a constant resolution of 1 km. List of datasets included in this data publication: Ocean1_input_geom_v1.01.nc: Topography data from the beginning of the Ice1r experiment, the topographic boundary conditions for the Ocean0 and Ocean1 experiments. Ocean2_input_geom_v1.01.nc: Topography data from the end of the Ice1r experiment (year 100), the topographic boundary conditions for the Ocean2 experiment. Ocean3_input_geom_v1.01.nc: 101 yearly snapshots of the topography data from the Ice1r experiment (years 0 to 100), the topographic boundary conditions for the Ocean3 experiment. Ocean4_input_geom_v1.01.nc: 101 yearly snapshots of the topography data from the Ice1ra experiment (years 100 to 200), the topographic boundary conditions for the Ocean4 experiment. inputs.spin: Configuration file for the initial BISICLES spinup to steady state without melting inputs.melt: Configuration files for the Ice1r and Ice1ra evolution shortshallow.py: Python script for computing the melt parameterization in the Ice1r and Ice1ra experiments. addMasksToBisicles.py: Python script use to compute the floating, grounded and open ocean fractions based on the topography data and the assumption of exact flotation of the ice shelf. This data set includes four sets of ice-shelf topography data for use in the ISOMIP+ experiments described in Sec. 3 of Asay-Davis et al. (2015), to which this data set is supplementary material. The data set contains four NetCDF files, each of which includes six spatially two-dimensional fields: the ice upper surface, ice draft (ice sheet lower surface), bed topography, the fraction of open ocean, the fraction of floating ice and the fraction of grounded ice.