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)

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.

Projections of Future Sea Level Contributions from the Greenland and Antarctic Ice Sheets: Challenges Beyond Dynamical Ice Sheet Modeling

“…A path forward for obtaining a time-evolving state would be to combine existing observational time series into a reanalysis, similar to what is being done in physical oceanography to provide the best possible ocean state estimates (Menemenlis et al, 2008). A few studies have been performed by assimilating time series of observations (Goldberg and Heimbach, 2013;Larour et al, 2014). They show that forecasting is much improved when models are initialized not with a snapshot of observations but with longer time series.…”

Committed retreat of Smith, Pope, and Kohler Glaciers over the next 30 years inferred by transient model calibration

“…Transient calibration of a model of an Antarctic ice stream with temporally resolved plan-view data has not previously been carried out, though we point out that Larour et al (2014) used methods similar to those used in this study to infer surface mass balance over the Northeast Greenland Ice Stream over a 6-year period from laser altimetry. No future projections were made in their study.…”

“…However, gradient calculation is more complicated, now requiring a time-dependent adjoint model, which can be derived via the continuous-form adjoint of the model equations, as has been done for simplified ocean models (Tziperman and Thacker, 1989), or by means of algorithmic differentiation (AD; Griewank and Walther, 2008). Used extensively in ocean modeling (e.g., Heimbach et al, 2005;Wunsch and Heimbach, 2013), the use of AD tools in land ice modeling is becoming increasingly common (Heimbach and Bugnion, 2009;Goldberg and Heimbach, 2013;Larour et al, 2014).…”

Response of the large-scale subglacial drainage system of Northeast Greenland to surface elevation changes

“…We use a simple model approach and are therefore not aiming to, or indeed capable of, capturing the present-day variations and dynamics of the region, since this variability is coupled to several complex processes not included in our model (cf. Larour et al, 2014). Rather, we focus on the sensitivity of the drainage basin to realistic fluctuations in surface elevation over timescales of decades to millennia and the corresponding change in hydropotential.…”