Co-chairs: Stephen Cornford and Hilmar Gudmundsson
The following description has been adapted from Asay-Davis et al. (2015, submitted).
The previous Marine Ice Sheet Model Intercomparison Projects, MISMIP and MISMIP3d, tested the capabilities of ice sheet models to simulate advance and retreat cycles under changes in ice softness and basal sliding, respectively. MISMIP tested flowline models in one horizontal dimension (1HD), while MISMIP3d required modeling in two horizontal dimensions (2HD). Each experiment taught the community a great deal about the numerical behavior of ice-sheet models of various types as well as the similarities and differences in the results they produced. In particular, the MISMIP results showed that steady-state grounding-line positions could differ markedly depending on the resolution, type of stress approximation, and discretization methods employed. Follow-up studies found that models with fixed grids (as opposed to those that track the grounding line in time) and without sub-grid-scale parameterizations of the grounding line require grounding-line resolution on the order of hundreds of meters to accurately reproduce grounding-line dynamics.
It was clear in discussions of a follow-up intercomparison exercise that the MISMIP3d experimental design had three shortcomings as a test of 2HD marine ice sheet models. First, it started from a steady state that was invariant in the cross-flow direction—that is, 1HD—and did not involve significant lateral stresses. Second, the initial grounding lines of the shallow-shelf approximation (SSA) models were around 80 km downstream from the Stokes models, but the grounding line only moved about 20 km in the perturbation experiment. That left an obvious question entirely unanswered: in a realistic simulation with the model parameters chosen to match geometry and velocity derived from observations, and thus with prescribed initial conditions, does the SSA provide a good approximation to the Stokes model? Third, grounding line migration was driven by changes to the basal traction field, rather than the ice shelf melting that is thought to be the dominant driver of present-day grounding-line retreat in West Antarctica.
MISMIP+ has been designed to address each of the shortcomings above. Regarding the first, the chosen geometry, based on Gudmundsson et al. (2012), results in strong lateral stresses that buttress the ice stream, and, given particular parameter choices, results in a stable grounding line crossing a retrograde slope, a configuration not possible in 1HD. Regarding the second, modelers are free to choose certain model parameters so that their initial grounding line is close to that of a reference model, and in preliminary tests two models that bracketed the high resolution MISMIP3d results have been found to have grounding lines within a few kilometers of one another in steady state. Finally, extensive grounding line retreat is driven by sub-shelf melt rates.
The MISMIP+ experiments are initialized by running the model with no melting to steady state. The first MISMIP+ experiment (Ice0) continues the run with no melting for one hundred years to demonstrate that the initial state remains steady over this time period. The second experiment (Ice1) explores the response of the ice sheet to a strong melt perturbation by prescribes a depth-dependent parameterization of basal melting with a peak melt rate of ~80 m/yr, driving 100 (optionally 1000) years of grounding-line retreat. After 100 years, the melt rate is set back to zero, allowing the grounding line to re-advance over 100 (optionally 900) years. The third experiment (Ice2) emulates a large calving event by prescribing a strong melt rate (100 m/yr) beyond a prescribed calving front for 100 (optionally 1000) years, followed by a removal of the perturbation for an additional 100 (optionally 900) years.