Abstract
Abstract. The microstructure of polycrystalline ice evolves under prolonged deformation, leading to anisotropic patterns of crystal orientations. The response of this material to applied stresses is not adequately described by the ice flow relation most commonly used in large-scale ice sheet models – the Glen flow relation. We present a preliminary assessment of the implementation in the Ice Sheet System Model (ISSM) of a computationally efficient, empirical, scalar, constitutive relation which addresses the influence of the dynamically steady-state flow-compatible induced anisotropic crystal orientation patterns that develop when ice is subjected to the same stress regime for a prolonged period – sometimes termed tertiary flow. We call this the ESTAR flow relation. The effect on ice flow dynamics is investigated by comparing idealised simulations using ESTAR and Glen flow relations, where we include in the latter an overall flow enhancement factor. For an idealised embayed ice shelf, the Glen flow relation overestimates velocities by up to 17 % when using an enhancement factor equivalent to the maximum value prescribed in the ESTAR relation. Importantly, no single Glen enhancement factor can accurately capture the spatial variations in flow across the ice shelf generated by the ESTAR flow relation. For flow line studies of idealised grounded flow over varying topography or variable basal friction – both scenarios dominated at depth by bed-parallel shear – the differences between simulated velocities using ESTAR and Glen flow relations depend on the value of the enhancement factor used to calibrate the Glen flow relation. These results demonstrate the importance of describing the deformation of anisotropic ice in a physically realistic manner, and have implications for simulations of ice sheet evolution used to reconstruct paleo-ice sheet extent and predict future ice sheet contributions to sea level.
Highlights
An essential component of any ice sheet model is the constitutive relation, which connects ice deformation rates and applied stresses
We have investigated some consequences of incorporating the flow properties of anisotropic ice into modelling flow in ice sheets and ice shelves
We have investigated the flow response to prolonged deformation under a constant or slowly changing stress regime and the associated development of an anisotropic crystal orientation fabric compatible with that deformation, as represented by the empirical, scalar, tertiary constitutive relation for ice with a compatible anisotropic crystal fabric of Budd et al (2013) – the Empirical Scalar Tertiary Anisotropy Regime (ESTAR) flow relation. Having implemented this flow relation in Ice Sheet System Model (ISSM), we made initial studies in the context of idealised experiments: for an embayed ice shelf, and in twodimensional models of grounded ice flow over varying topography and variable basal friction previously explored by ice flow modellers (Pattyn et al, 2008)
Summary
An essential component of any ice sheet model is the constitutive relation (or flow relation), which connects ice deformation rates and applied stresses. To account for the increased deformability associated with steady-state creep, a common adaptation of the Glen flow relation is the inclusion of a constant flow enhancement factor, EG, ε = EGA(T )τen−1σ Such a parameter is included in most large-scale ice sheet models Budd et al (2013) recently proposed a flow relation based on results from laboratory ice deformation experiments involving simple shear, compression, and combinations of these These experiments reached steady-state creep rates – referred to as tertiary flow. We refer to the generalised flow relation proposed by Budd et al (2013) as ESTAR (Empirical Scalar Tertiary Anisotropy Regime), since it is based on steady-state (tertiary) creep rates describing the deformation of ice with a flow-compatible induced anisotropy and features a scalar (collinear) relationship between the strain rate and deviatoric stress tensor components.
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