Abstract. As in any environmental system, modeling instabilities within the glacial system is a numerical challenge of potentially high real-world relevance. Differentiating between the impacts of physical system processes and numerical noise is not straightforward. Here we use an idealized North American geometry and climate representation (similar to the HEINO experiments, Calov et al., 2010) to examine the numerical sensitivity of ice stream surge cycling in glaciological models. Through sensitivity tests, we identify some numerical requirements for a robust model configuration for such contexts. To partly address model-specific dependencies, we use both the Glacial Systems Model (GSM) and Parallel Ice Sheet Model (PISM). We show that modeled surge characteristics are resolution-dependent though converging (decreasing differences between resolutions) at higher horizontal grid resolutions. Discrepancies between high and coarse horizontal grid resolutions can be reduced by incorporating a resolution-dependent basal temperature ramp for basal sliding thermal activation. Inclusion of a diffusive bed thermal model reduces the surge cycling ice volume change by ∼33 % as the additional heat storage dampens the change in basal temperature during surge events. The inclusion of basal hydrology, as well as a non-flat topography, leads to increased ice volume change during surge events (∼20 and 17 %, respectively). Therefore, these latter three components are essential if one is endeavoring to maximize physical fidelity in ice stream surge cycle modeling. An abrupt transition between hard bedrock and soft sediment, as in the HEINO experiments, leads to ice stream propagation along this boundary but is not the cause of the main surge events.
<p>Some ice sheets and glaciers experience long quiescent periods interspersed with short periods of rapid ice advance, such as the binge-purge-type cycling hypothesized to be associated with Heinrich Events. Modeling ice stream activation/de-activation, however, is numerically challenging given the relatively abrupt changes at surge onset and the high ice velocities. In spite of this, a number of high-profile modeling papers have explored Heinrich events and ice surges, but generally with very limited consideration of numerical aspects. Here we test the ability of the 3D Glacial Systems Model (GSM) and Parallel Ice Sheet Model (PISM) to simulate binge-purge-type surges and explore the stability of the simulations with respect to relevant numerical and discretization uncertainties.&#160;</p><p>We find surge characteristics exhibit a resolution dependency but converge at higher horizontal grid resolutions (order 5 km). In accordance with theoretical and experimental work, our model results suggest that the thermal activation of basal sliding should start below the pressure melting point. A resolution-dependent basal temperature ramp for the thermal activation of basal sliding as well as a subglacial hydrology model can reduce the discrepancies between high and coarse horizontal grid resolutions. Furthermore, incorporating a bed thermal and at least a minimal complexity subglacial hydrology model significantly affects surge characteristics and is, therefore, essential for modeling large-scale ice stream cycling.</p>
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