TC The Cryosphere TC The Cryosphere 1994-0424 Copernicus Publications Göttingen, Germany 10.5194/tc-8-2335-2014 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) Larour E. 1 Utke J. 3 Csatho B. 4 Schenk A. 4 Seroussi H. https://orcid.org/0000-0001-9201-1644 1 Morlighem M. https://orcid.org/0000-0001-5219-1310 2 Rignot E. 1 2 Schlegel N. https://orcid.org/0000-0001-8035-448X 1 Khazendar A. 1 Jet Propulsion Laboratory – California Institute of Technology, 4800 Oak Grove Drive MS 300-323, Pasadena, CA 91109-8099, USA University of California Irvine, Department of Earth System Science, Croul Hall, Irvine, CA 92697-3100, USA Argonne National Lab, Argonne, IL 60439, USA Department of Geological Sciences, University at Buffalo, Buffalo, NY, USA 15 12 2014 8 6 2335 2351 Copyright: © 2014 E. Larour et al. 2014 This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/ This article is available from https://tc.copernicus.org/articles/8/2335/2014/tc-8-2335-2014.html The full text article is available as a PDF file from https://tc.copernicus.org/articles/8/2335/2014/tc-8-2335-2014.pdf

We present a new data assimilation method within the Ice Sheet System Model (ISSM) framework that is capable 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 on algorithmic differentiation to compute gradients of objective functions with respect to model forcings. It is applied to the Northeast Greenland Ice Stream, where surface mass balance and basal friction forcings are temporally inverted, resulting in adjusted modeled surface heights that best fit existing altimetry. This new approach allows for a better quantification of basal and surface processes and a better understanding of the physical processes currently missing in transient ice-flow models to better capture the important intra- and interannual variability in surface altimetry. It also demonstrates that large spatial and temporal variability is required in model forcings such as surface mass balance and basal friction, variability that can only be explained by including more complex processes such as snowpack compaction at the surface and basal hydrology at the bottom of the ice sheet. This approach is indeed a first step towards assimilating the wealth of high spatial resolution altimetry data available from EnviSat, ICESat, Operation IceBridge and CryoSat-2, and that which will be available in the near future with the launch of ICESat-2.

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