|Complex Greenland outlet glacier flow captured|
|investigators:||A. Aschwanden, M. Fahnestock, and M. Truffer|
The paper is based on PISM simulations of 600 m grid resolution over the entire Greenland ice sheet. All parts of the ice sheet, and each outlet glacier in particular, see the same physics. The quality of this flow model for 29 major outlet glaciers is assessed by comparison with present-day-observed surface velocities at cross-flow near-ocean profiles, often called “flux gates”. The main result is that the majority of the outlet glaciers show strong correlation between modeled and observed velocity. The paper demonstrates that outlet glacier flow can be captured with high fidelity if ice thickness is well-constrained and if vertical shearing as well as membrane stresses are included in the model. While it is not clear that solving the full-stress configuration would improve the fit, it is clear that the shallow hybrid model can be applied at higher resolution and for longer-duration runs. Inversion of surface properties for individual glaciers is not essential to reproduce the overall flow pattern. Spatial variability in flow can be explained in large part by the spatial variability in ice thickness.
The paper is based on PISM simulations with grid resolution down to 600 m over the entire Greenland ice sheet. To start, each of an initial ensemble of 14 lower-resolution (1500 m) experiments has a single ice-sheet-wide value for all parameters. The best of these, in an ice-sheet-wide measure, is re-run at the 600 m resolution and various coarser resolutions. The quality of this flow model for 29 outlet glaciers is assessed; each outlet glacier sees the same physics. The main result is that the majority of the outlet glaciers show strong correlation between modeled and present-day-observed velocity, when it is compared along cross-flow and near-ocean profiles.
Before this paper one might suppose, based on the most prominent literature on the subject, that a detailed, measurably-accurate, outlet-glacier-resolving model of the present-day velocity of an entire ice sheet was dependent both on removing shallow assumptions from the stress balance and on tuning a very large number of basal parameters. Both of these “required” properties would be very bad news for the prospect of using ice sheet simulations to do science! On the one hand, Stokes models are computationally-expensive, while on the other hand only present-day, and not past or future, data are available to set all these basal parameters through inversion.
Such a pessimistic view turns out to be substantially false. Aschwanden et al. (2016) show that four things do matter: (i) an accurate map of bedrock topography, (ii) a stress regime in which viscous membrane stresses are part of the balance with basal sliding resistance, (iii) an energy-conservation-driven basal stress model derived (conceptually) from a model of a wet, pressurized, deformable basal layer, and (iv) high model resolution over all areas of the ice sheet where sliding is possible and/or steep/rough basal topography exists.
NASA IceBridge missions, and the mass-conserving-bed technology of Morlighem et al (2014), are shown by this paper to represent major progress on item (i). Items (ii) and (iii) are properties of the PISM continuum model, and item (iv) of its implementation as parallel-scalable software. Certainly all of these “things that matter” are improvable. More-complete stress balances and the use of inversion of present-day velocities will both be essential to improvements. The main idea remains, however: if the modeled flowing ice has the right bottom geometry, and if the dynamical model has certain key features, then the resulting dynamics are already inside the ballpark!
See the official announcement for complete details.
The Antarctic Research Centre, Victoria University of Wellington, New Zealand, is offering a FULLY FUNDED scholarship for an enthusiastic and talented Ph.D student to undertake numerical ice-sheet modelling research. Experiments will focus on better understanding and simulating the processes involved in ice-sheet – ocean interactions. Such processes determine the basal mass balance of marine-based ice-sheets such as the West Antarctic Ice Sheet, and as such, control the pattern and timing of grounding-line migrations.
Collaborating with scientists at a variety of New Zealand (VUW, GNS, NIWA) and Australian (UNSW, UTAS, AAD) institutions, the researcher will use present-day glaciological and oceanographic observations as primary constraints to a suite of model simulations that will explore the sensitivity of Antarctic ice-sheets to changes in ocean circulation. A key aspect of the project lies in trying to identify and quantify thresholds and feedback mechanisms that may either accelerate or inhibit ice-shelf melt. The ultimate aim of the project is to build on recent work to provide more robust simulations of ice-shelf and ice-sheet changes under future scenarios of perturbed atmospheric and oceanographic conditions.
The research project will span a range of temporal and spatial scales, but will primarily use the Parallel Ice Sheet Model and will focus initially on the Ross Ice Shelf. The successful applicant may also have the opportunity to spend time in Antarctica acquiring new data.
Skills: Applicants must have a strong background in geophysics, maths or other numerical Earth Sciences. Experience working in a UNIX / Linux environment, including shell scripting, is essential. Programming abilities in any of the usual languages and experience with high-performance computing facilities would also be extremely useful.
Applications: We wish to have the successful applicant starting no later than July 2016, and therefore request completed applications by 18th December 2015.
For details of the application process or to lodge an expression of interest, contact Dr. Nick Golledge (email@example.com) as soon as possible.
PISM is jointly developed at the University of Alaska, Fairbanks (UAF) and the Potsdam Institute for Climate Impact Research (PIK). For more about the team see the UAF Developers and PIK Developers pages.
UAF developers, who are in the Glaciers Group at the GI, are supported by NASA's Modeling, Analysis, and Prediction and Cryospheric Sciences Programs (grants NAG5-11371, NNX09AJ38C, NNX13AM16G, NNX13AK27G) and by the Arctic Region Supercomputing Center.