|Organization of ice flow by localized regions of elevated geothermal heat flux|
|investigators:||M. L. Pittard, B. K. Galton-Fenzi, J. L. Roberts, C. S. Watson|
|journal:||Geophysical Research Letters|
Geothermal flux is one input to a thermo-mechanically coupled ice flow model such as PISM, with significant impact on both ice softness and basal lubrication. Maps of geothermal flux under present-day ice sheets come from nontrivial geophysical inversions, based on seismic and/or magnetic observations, which generate non-unique and (inevitably) smoothed maps. For example, solutions by Shapiro & Fitzwoller (2004) and Fox Maule et al (2005) are familiar to Antarctic ice sheet modelers. However, measurements on ice-free continents show geothermal flux has strong spatial variations including concentrated highs (hot spots).
A model like PISM can, at least, demonstrate the effects on ice flow of small-spatial-scale variations in geothermal flux. This paper studies the Lambert-Amery glacial system in East Antarctica, where a variety of evidence indicates high heat flux regions of at least 120 mW per square meter. Localized regions of elevated geothermal flux are tested in PISM simulations. The results show significant effects on slow-moving ice, with influence extending both upstream and downstream of the geothermal anomaly. Fast-moving ice is relatively unaffected. This contrast suggests that the effect of geothermal flux on ice softness may dominate the lubrication effect.
PETSc 3.7 was released on April 25, 2016. We are currently working on making PISM compatible with PETSc 3.7 and will announce it here as soon as possible.
In the meantime, please install petsc 3.6.4 from here. PISM version 0.7 (
stable0.7 branch) works with any PETSc 3.5.X and higher.
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!
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.