There are situations where Glen's Law is inadequate for modelling ice flow, especially as we ask our models to reflect more details of the natural system. In this case study I focus on the flow of ice in low-deviatoric-stress regimes, such as exist in the divide region of an ice sheet.
In ice, as with other polycrystalline materials, the dominant microscale flow mechanism changes with temperature, deviatoric stress, and grain size. A comprehensive flow law to describe ice deformation should include a term for each flow mechanism. From a microphysical perspective, this kind of flow law is essential for understanding and modelling ice behaviour. Translating this behaviour to the macroscale flow of ice sheets, on the other hand, requires a flow law that is simple to implement in an ice sheet model, yet still captures the effect of shifts in the flow mech anisms. I have modified Glen's Law to accommodate the shift in behaviour at low stress by adding a linear term:
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