Performance and applicability of a 2.5D ice-flow model in the vicinity of a dome.

Three-dimensional ice flow modelling requires a lot of computing resources and observation data, such that 2D simulations are often preferable, at least when the stream lines are parallel; otherwise the lateral divergence of the flow should be accounted for (2.5D models). Assuming that the stream lines follow the steepest slope of the surface, the width variations of a flow tube are computed thanks to the surface curvature. The ability of the 2.5D models to account properly for a 3D state of strain and stress has not clearly been established, especially their sensitivity on how the ice surface curvature is determined (scanning window on a DEM), and on the geometry of the ice surface. In particular, these models might fail for divergent flows, and need to be more clearly defined. A twin experiment is here carried out, comparing 3D and 2.5D computed velocities, on three dome geometries, for several scanning windows and thermal conditions. The chosen scanning window used to evaluate the ice surface curvature should be comparable to the typical size of the measured ice relief. For isothermal ice, the error made by the 2.5D model is in the range 0-10% but for highly diverging flows the errors are 2 or 3 times higher and could lead to a non-physical reversed surface convexity at the dome. For non-isothermal ice, assuming a realistic temperature profile, the presence of a sharp ridge leads to a partly reversed velocity profile. The warmer bottom ice is more deformed than the upper ice, and this results in the non-verticality of the walls of the flow tube, violating the 2.5D assumptions. [ABSTRACT FROM AUTHOR]