UCLA

The Greenland Ice Sheet is a major contributor to global sea level rise, with recent mass loss dominated by meltwater runoff from the ablation zone, i.e. areas of the ice sheet where annual mass losses exceed gains. In this zone, the winter snowpack melts entirely each summer exposing bare glacier ice. Observations of Greenland’s ablation zone suggest the exposed bare ice surface is comprised of low-density ice termed “weathering crust” that may store meltwater, potentially reducing meltwater runoff export to surrounding oceans. Climate models are the primary tools used to forecast future Greenland mass loss, but these models treat the ablation zone as impermeable high-density ice with no meltwater retention capacity. Recent evidence suggests that climate models overpredict meltwater runoff from the ablation zone, which may be linked to weathering crust presence, but diagnosing climate model predictions is difficult because observations of meltwater runoff on the ice sheet surface are extremely rare and weathering crust presence is undocumented. This dissertation presents the results of four investigations that address this problem by pairing field observations of hydrologic and radiative properties of bare ice collected in Greenland’s ablation zone with numerical modeling and analysis of climate model output. The results of these investigations reveal the presence of low-density weathering crust on Greenland’s bare ice ablation zone surface and the potential for non-trivial meltwater runoff retention within weathering crust on Greenland’s bare ice ablation zone surface. New estimates of spectral radiation attenuation coefficients are quantified and directly applied to a numerical model of spectral and thermodynamic heat transfer in bare ice. This model successfully simulates meltwater runoff from a supraglacial catchment on Greenland’s southwest ablation zone surface. Model results suggest that nocturnal refreezing of meltwater stored within weathering crust occurs in Greenland’s ablation zone, potentially reducing runoff up to 32% on annual timescales. These findings imply a reinterpretation of refreezing on bare ice as an important control on Greenland’s ablation zone surface mass balance and the need to represent this process in climate model predictions of future Greenland mass loss.