Abstract
Abstract. Variations in intrinsic bed conditions that affect basal sliding, such as the distribution of deformable sediment versus hard bedrock, are important boundary conditions for large-scale ice-sheet models, but are hard to observe and remain largely uncertain below the modern Greenland and Antarctic ice sheets. Here a very simple model-based method is described for deducing the modern spatial distribution of basal sliding coefficients. The model is run forward in time, and the basal sliding coefficient at each grid point is periodically increased or decreased depending on whether the local ice surface elevation is too high or too low compared to observed in areas of unfrozen bed. The method considerably reduces large-scale errors in Antarctic ice elevation, from several 100s to several 10s of meters in most regions. Remaining ice elevation errors over mountain ranges such as the Transantarctics are further improved by parameterizing the possible effect of sub-grid topography in the basal sliding law, representing sliding in deep valleys. Results are compared with modern velocity data, and various sensitivity tests are described in Appendices.
Highlights
One major uncertainty in modeling continental ice sheets is the distribution of bed properties that determine the rate of sliding at the ice–bed interface
A number of previous studies have attempted to deduce basal stresses or sliding coefficients under modern ice sheets and glaciers, pioneered by MacAyeal (1992, 1993) and adapted for instance by Vieli and Payne (2003) and Joughin et al (2004). Those studies were applied to limited regions using the Shelfy Stream Approximation (SSA) equations appropriate for stretching flow, and relatively sophisticated control methods to rigorously account for the nonlocal nature of the dynamics
Unlike the more sophisticated control or adjoint methods used in previous inverse studies, the method is local and ignores the spatial connectivity of ice dynamics
Summary
One major uncertainty in modeling continental ice sheets is the distribution of bed properties that determine the rate of sliding at the ice–bed interface. A number of previous studies have attempted to deduce basal stresses or sliding coefficients under modern ice sheets and glaciers, pioneered by MacAyeal (1992, 1993) and adapted for instance by Vieli and Payne (2003) and Joughin et al (2004) Those studies were applied to limited regions using the Shelfy Stream Approximation (SSA) equations appropriate for stretching flow, and relatively sophisticated control methods to rigorously account for the nonlocal nature of the dynamics. Price et al (2011) applied a simpler method to Greenland, and Le Brocq et al (2009) linked a similar method with a basal hydrology model for West Antarctica All these studies are based on fitting modeled ice velocities to observed surface or balance velocities, with ice thicknesses and elevations prescribed to modern observed. Appendices A to G present various topics including sensitivities to modern bedrock topography and other uncertain model inputs
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