Date of Award
Master of Science (MS)
There is widespread evidence for a rise in the Equilibrium Line Altitude (ELA) in many areas of the Greenland Ice Sheet. In a recent study (Ettema et al., 2009) West Greenland was found to have experienced a 3.9\% increase in area experiencing melt annually. In the wet snow and ablation zones, much of the melt water will enter the englacial hydrologic system via moulins, crevasses and surface fractures (Harper et al., 2012). Of this englacial water, a fraction may persist in the Cryo-Hydrologic System (CHS) (Rennermalm et al., 2012) long enough to refreeze, releasing latent heat and warming the background ice. Because the geometry of the CHS is difficult to constrain, we modeled the small-scale thermodynamics for several "end-member" scenarios that capture a range of plausible CHS geometries. In particular, we considered crevasses and deeper water bodies subject to one-time water filling, and crevasses that are filled annually and drain via a diffuse drainage system into a fracture network. We found that warming from shallow crevasses is largely driven by 1D horizontal conduction and that the warming is limited by the depth of crevasse penetration. Deep but not fully penetrating englacial water bodies can warm the lower layer of the ice sheet where the increase in the Flow Law Parameter has the greatest impact on ice velocity. The horizontal velocity gradient also caused stretching of deep englacial water bodies, which decreases refreezing time of liquid water at depth. Finally, drainage of crevasses through fracture networks was found to be an efficient mechanism to transport liquid water to depth. The small cross-sectional area, large surface area, and heterogeneity of the fractures provided the fastest and most efficient release of latent heat into the background ice. Based on the modeling results, we propose simple mathematical parameterizations that may be used to represent CHW in large-scale ice sheet models. We also share a new module written for the Community Ice Sheet Model (CISM) that solves the ice sheet thermodynamics using an enthalpy method and replaces a cold ice method module.
Macpherson, Brian Daniel, "Enthalpy-Based Models for Ice Sheets and Improving Understanding of Cryo-Hydrologic Warming" (2013). Civil Engineering Graduate Theses & Dissertations. 461.