An analysis of balance velocities over the Greenland ice sheet and comparison with synthetic aperture radar interferometry

Balance velocities for the Greenland ice sheet have been calculated from a new digital elevation model (DEM), accumulation rates and an existing ice-t hickness grid, using a fully two-dimensional finite-difference scheme. The pattern of velocities is compared with velocities derived from synthetic-ap erture radar (SAR) interferometry for three different regions of the ice sh eet. Differences between the two estimates of velocity highlight the respec tive strengths and weaknesses of the datasets and techniques used. A compar ison with ten global positioning system-derived velocities indicates that t he balance-velocity scheme and input datasets used here provide a remarkabl y good representation of the velocity distribution inland from the margins. These balance-velocity data, therefore, could help constrain numerical ice -sheet models. The balance velocities were found to be unreliable dose to t he ice-sheet margins due to larger errors in ice thickness, surface slope a nd ablation rate in this region. Comparison of the balance velocities with SAR interferometry in the region of the "Northeast Greenland Ice Stream" in dicates the importance of the smoothing distance that must be applied to th e DEM before calculating balance velocities. A smoothing distance of 20 tim es the ice thickness gave good agreement between the two measures of veloci ty.