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. The microstructure of polycrystalline iceevolves under prolonged deformation, leading to anisotropic patterns ofcrystal orientations. The response of this material to applied stresses isnot adequately described by the ice flow relation most commonly used inlarge-scale ice sheet models – the Glen flow relation. We present apreliminary assessment of the implementation in the Ice Sheet System Model(ISSM) of a computationally efficient, empirical, scalar, constitutiverelation which addresses the influence of the dynamically steady-stateflow-compatible induced anisotropic crystal orientation patterns that developwhen ice is subjected to the same stress regime for a prolonged period –sometimes termed tertiary flow. We call this the ESTAR flow relation. Theeffect on ice flow dynamics is investigated by comparing idealisedsimulations using ESTAR and Glen flow relations, where we include in thelatter an overall flow enhancement factor. For an idealised embayed iceshelf, the Glen flow relation overestimates velocities by up to 17 % whenusing an enhancement factor equivalent to the maximum value prescribed in theESTAR relation. Importantly, no single Glen enhancement factor can accuratelycapture the spatial variations in flow across the ice shelf generated by theESTAR flow relation. For flow line studies of idealised grounded flow overvarying topography or variable basal friction – both scenarios dominated atdepth by bed-parallel shear – the differences between simulated velocitiesusing ESTAR and Glen flow relations depend on the value of the enhancementfactor used to calibrate the Glen flow relation. These results demonstratethe importance of describing the deformation of anisotropic ice in aphysically realistic manner, and have implications for simulations of icesheet evolution used to reconstruct paleo-ice sheet extent and predict futureice sheet contributions to sea level.

Implementing an empirical scalar constitutive relation for ice with flow-induced polycrystalline anisotropy in large-scale ice sheet models