Balance Fluxes Computed Based on Different Altimeter-derived Topographies Relative to the Observed Ice Flow in the Greenland Ice Sheet

Balance fluxes, representing the ice flow that would correspond to a steady state ice sheet, are computed over the Greenland ice sheet. Input data are ice accumulation rate and altimeter-derived topography. Two topographic datasets derived from Ice, Cloud, and Land Elevation Satellite (ICESat) and European Remote-sensing Satellite (ERS-1&2) in 5km high-resolution grids are used, respectively, for comparison. To decide if the ice sheet is locally in a state of positive or negative mass budget, these balance fluxes must be compared with actual ice fluxes obtained from the observations of ice thickness and surface velocity. This comparison requires the ratio of depth-averaged velocity to surface velocity to convert the measured surface velocities to depth-averaged velocities. In this paper, the values of the ratio, depending on ice rheology, tempearture and other ice flow parameters, are determined from a 3D thermomechanical ice flow model with incorporation of anisotropic rheology and are used for comparing balance fluxes with available Global Positioning System (GPS) measurements (along 2000 metre contour) to evaluate the local ice-sheet state.