Higher-order fabric structure in a coupled, anisotropic ice-flow model
Ice-crystal fabric can induce mechanical anisotropy that significantly affects flow, but ice-flow models generally do not include fabric development or its effect upon flow. Here, we incorporate a new spectral expansion of fabric, and more complete description of its evolution, into the ice-flow model Elmer/Ice. This approach allows us to model the effect of both lattice rotation and migration recrystallization on large-scale ice flow. The fabric evolution is coupled to flow using an unapproximated non-linear orthotropic rheology that better describes deformation when the stress and fabric states are misaligned. These improvements are most relevant for simulating dynamically interesting areas, where recrystallization can be important, tuning data are scarce and rapid flow can lead to misalignment between stress and fabric. We validate the model by comparing simulated fabric to ice-core and phase-sensitive radar measurements on a transect across Dome C, East Antarctica. With appropriately tuned rates for recrystallization, the model is able to reproduce observations of fabric. However, these tuned rates differ from those previously derived from laboratory experiments, suggesting a need to better understand how recrystallization acts differently in the laboratory compared to natural settings.
Read more: Lilien, D., N. Rathmann, C. Hvidberg, A. Grinsted, M. Ershadi, R. Drews and D. Dahl-Jensen, 2023. Simulating higher-order fabric structure in a coupled, anisotropic ice-flow model: Application to Dome C. Journal of Glaciology, 1-20. doi:10.1017/jog.2023.78
- Created on .
- Last updated on .
- Hits: 844
Theoretical and numerical work has shown that under certain circumstances grounding lines of marine-type ice sheets can enter phases of irreversible advance and retreat driven by the marine ice sheet instability (MISI). Instances of such irreversible retreat have been found in several simulations of the Antarctic Ice Sheet. However, it has not been assessed whether the Antarctic grounding lines are already undergoing MISI in their current position. Here, we conduct a systematic numerical stability analysis using three state-of-the-art ice sheet models: Úa, Elmer/Ice, and the Parallel Ice Sheet Model (PISM). For the first two models, we construct steady-state initial configurations whereby the simulated grounding lines remain at the observed present-day positions through time. The third model, PISM, uses a spin-up procedure and historical forcing such that its transient state is close to the observed one. To assess the stability of these simulated states, we apply short-term perturbations to submarine melting. Our results show that the grounding lines around Antarctica migrate slightly away from their initial position while the perturbation is applied, and they revert once the perturbation is removed. This indicates that present-day retreat of Antarctic grounding lines is not yet irreversible or self-sustained. However, our accompanying paper (Part 2, 
Modeling the evolution of glaciers and ice sheets under climate variability requires accurate estimation of ice flow velocities by numerical models. One of the most challenging components of these models is the representation of basal sliding at the rock-ice interface, generally described by relationships between stress and sliding speed. These relationships are mainly derived from laboratory experiments and theoretical studies, and their ability to represent nature still needs to be evaluated. Because direct observation beneath glaciers and ice sheets is difficult, few studies have attempted to validate sliding models from natural scale observations. In this context, our study provides a rare constraint, based on observations on an alpine glacier, on the law that should be used to model glacier sliding on clean bedrock (so-called hard-bed glaciers). We show that a simple power law performs well in explaining long-term glacier behavior for a power exponent of ∼3.1. We suggest that this exponent should be adopted in non-linear power laws incorporated into ice flow models that perform future projections of hard-bedded glaciers and ice sheet evolution.
