Abstract
Abstract. 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.
Highlights
Global mean sea level (GMSL) rise observations show an overall budget in which freshwater contribution from the polar ice sheets represents a significant portion (Church and White, 2006, 2011; Stocker et al, 2013), which is increasing (Velicogna, 2009; Rignot, 2008) relative to thermosteric expansion and contribution from terrestrial glaciers (Gardner et al, 2013)
We presented a new data assimilation system within the Ice Sheet System Model (ISSM) framework that is capable of assimilating surface altimetry data from missions such as ICESat into reconstructions of transient ice flow
An application to the Northeast Greenland Ice Stream was provided, where surface mass balance and basal friction forcings were temporally inverted, resulting in significant improvements in the best fit to observations. This new approach allows for a better understanding of which processes can be characterized by altimetry, and it illustrates the need for combining different data sets such as altimetry and satellitederived surface velocities into inversions of basal friction and surface mass balance
Summary
Global mean sea level (GMSL) rise observations show an overall budget in which freshwater contribution from the polar ice sheets represents a significant portion (Church and White, 2006, 2011; Stocker et al, 2013), which is increasing (Velicogna, 2009; Rignot, 2008) relative to thermosteric expansion and contribution from terrestrial glaciers (Gardner et al, 2013). In order to quantify the contribution of polar ice sheets to GMSL in the near future, accurate mass balance projections must be carried out, which can either be based on extrapolation of current trends (Velicogna and Wahr, 2006; Velicogna, 2009; Shepherd and Wingham, 2007; Rignot et al, 2011) or supported by transient ice-flow models that are physically validated against data Such models, as demonstrated in the SeaRISE (Sea-level Response to Ice Sheet Evolution) and ice2sea intercomparison projects, are not fully capable of capturing the present-day trends (Bindschadler et al, 2013; Nowicki et al, 2013a, b), which hinders our ability to project them into the future with a high degree of confidence.
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