Inferred basal friction and surface mass balance of the Northeast Greenland Ice Stream using data assimilation of ICESat (Ice Cloud and land Elevation Satellite) surface altimetry and ISSM (Ice Sheet System Model)

摘要

We present a new data assimilation method within the Ice Sheet System Model (ISSM) framework that iscapable of assimilating surface altimetry data from missions such as ICESat(Ice Cloud and land Elevation Satellite)into reconstructions of transient ice flow. The new method relies onalgorithmic differentiation to compute gradients of objective functions withrespect to model forcings. It is applied to the Northeast Greenland IceStream, where surface mass balance and basal friction forcings are temporallyinverted, resulting in adjusted modeled surface heights that best fitexisting altimetry. This new approach allows for a better quantification ofbasal and surface processes and a better understanding of the physicalprocesses currently missing in transient ice-flow models to better capturethe important intra- and interannual variability in surface altimetry. Italso demonstrates that large spatial and temporal variability is required inmodel forcings such as surface mass balance and basal friction, variabilitythat can only be explained by including more complex processes such assnowpack compaction at the surface and basal hydrology at the bottom of theice sheet. This approach is indeed a first step towards assimilating thewealth of high spatial resolution altimetry data available from EnviSat,ICESat, Operation IceBridge and CryoSat-2, and that which will be available in thenear future with the launch of ICESat-2.