A Kinematic Model of the Surface Velocity of Helheim Glacier, Greenland

Velocity fields of Greenland's outlet glaciers are widely studied because outlet glaciers are large, fast moving, and involved in controlling the mass balance of the ice sheet. Helheim Glacier is among the most studied Greenland glaciers, yet relatively little is known about the parameters that describe its surface velocity field. We have developed a two dimensional kinematic model of the surface flow. The basic form of the model is established through physical considerations, and the least squares parameters are estimated by fitting to time series data from Global Positioning System (GPS) receivers on the surface of the glacier during the 2007 and 2008 summer seasons. We show that the velocity field is most strongly dependent on along-flow position, across-flow position, time elapsed from the beginning of the summer, and calving events. Using the model, we are able to separate the space-dependent velocity variations from the velocity variations of the entire network. We establish an empirical relationship between observed ice loss during calving and the corresponding step-wise increase of the velocity field in the glacier. We also find that the RMS residual decreases with time each summer once whole-network velocity variation has been accounted for, and we use this trend to estimate the accuracy of the GPS measurements.