²ÊÃñ±¦µä

Sommers, Aleah N ¹ ; Rajaram, Harihar ² ; Colgan, William T ³

¹ University of Colorado at Boulder, Department of Civil, Environmental, and Architectural Engineering
² University of Colorado at Boulder, Department of Civil, Environmental, and Architectural Engineering
³ Geological Survey of Denmark and Greenland

Rising sea level due to climate change is a significant global concern. Many coastal countries will incur dramatic costs as vital infrastructure is threatened, and small island nations are already being inundated. A large component of sea-level rise is due to the transfer of terrestrial ice from glaciers and ice sheets into the ocean. Since 1990, numerous outlet glaciers on the west coast of Greenland have displayed dramatic accelerations and frontal retreats, yielding substantial changes in ice geometry on the scale of decades or years, rather than centuries or millennia (Joughin et al. 2010). Yet other glaciers within the same geographic region have accelerated less rapidly or even decelerated over the same period (McFadden et al. 2011), and the mechanisms driving this heterogeneous response are poorly quantified.

Most recent changes in the surface mass balance and ice dynamics of the Greenland ice sheet have been restricted to elevations below 2,000m. Substantial computational efficiency can be gained by limiting numerical modeling efforts to this lower elevation periphery, where changes in ice sheet form and flow are most pronounced, rather than modeling the entire ice sheet from the main divide to the margin. Accurately modeling the lower elevations with this approach is dependent on prescribing accurate velocity and temperature profiles at the upstream boundary. The Program for Arctic Regional Climate Assessment (PARCA) provides reliable surface velocity data at 161 locations approximately circumscribing the 2,000m elevation contour of the Greenland ice sheet (Thomas et al. 2001), and the Ice2Sea project improved estimates (and constrained uncertainty) of ice thickness at the PARCA stake locations. Without corresponding velocity and temperature profiles, however, these data alone are insufficient to serve as upstream boundary conditions for lower elevation thermo-mechanical modeling.

Using a two-dimensional, enthalpy-based thermal flowline model, we generate velocity and temperature profiles across the ice sheet depth at the PARCA stake locations. While prescribing ice surface and bedrock elevation, observed surface velocities at the stake locations and the ice discharge calculated from surface mass balance serve as modeling targets. We employ an iterative procedure between mechanical and thermal calculations; ice velocities found by solving the momentum equation (via the Shallow Ice Approximation, which is valid for these high-elevation domains) inform the energy equation to solve for temperature and liquid water content, which then inform the velocity calculations, and so on until convergence.

Preliminary results suggest that observed surface velocities in some regions of Greenland can only be reproduced with a temperate bed at high elevations (in agreement with Aschwanden et al. 2012), and also indicate that model results are sensitive to other factors, such as interpolation and smoothing of ice surface elevation data.

Aschwanden, A., E. Bueler, C. Khroulev, and H. Blatter, 2012, An enthalpy formulation for glaciers and ice sheets, J. Glaciol.

Joughin, I., B. Smith, I. Howat, T. Moon, and T. Scambos, 2010, Greenland Flow Variability from Ice-Sheet-Wide Velocity Mapping. J. Glaciol.

McFadden, E.M., I.M. Howat, I. Joughin, B.E. Smith, and Y. Ahn, 2011, Changes in the dynamics of marine terminating outlet glaciers in west Greenland (2000-2009),J.Geophys. Res.

Thomas, R., B. Csatho, C. Davis, C. Kim, W. Krabill, S. Manizade, J. McConnell, and J. Sonntag, 2001, Mass balance of higher-elevation parts of the Greenland Ice Sheet, J. Geophys. Res. – Atmospheres