Processes in the percolation zone in southwest Greenland: challenges in modeling surface energy balance and melt, and the role of topography in the formation of ice slabs

Abstract

Increased surface melt in the percolation zone of Greenland causes significant changes in the firn structure, directly affecting the surface mass balance of the ice sheet and the amount and timing of meltwater runoff. Thick impermeable layers, referred to as ice slabs, are preventing melt water percolation and refreezing in the firn favoring lateral movement of water and direct runoff to the oceans. The objective of this dissertation is to enhance the understanding of these processes by modeling the surface energy balance and resulting melt, and investigating the spatial and temporal changes in firn surface properties and associated water movement in the percolation zone in southwest Greenland. Extensive fieldwork was carried out in this region between 2017 and 2019, including a collection of 19 shallow firn cores at several sites and the operation of two weather stations. A surface-energy balance model was forced with automatic weather station data from two sites (2040 and 2360 m a.s.l.). Extensive model validation and sensitivity analysis reveal that the skin layer formulation used to compute the surface temperature by closing the energy balance leads to a consistent overestimation of melt by more than a factor of two or three depending on the site. The results indicate that the energy available for melt is highly sensitive to small changes in surface temperature and suggests caution is needed in modeling Greenland melt from weather data. Furthermore, the spatial and temporal variability in air temperature bias of two regional climate models, MAR and RACMO, is assessed over the entire ice sheet. Model results are compared to 35 automatic weather stations over more than 25 years. Both models perform well in the ablation zone (< 1500 m a.s.l.) where most of the melt happens. However, a warm bias is found in both MAR and RACMO at the higher elevations percolation zone (> 1500 m a.s.l.). The seasonal evolution and interannual variability of near-surface firn characteristics in the percolation zone of southwest Greenland can be tracked with Sentinel-2 optical imagery. Fully saturated seasonal snow (blue slush) and lateral movement of water are strongly correlated with local topography. Furthermore there is evidence of water movement from higher to lower elevations, following surface slope, even after the halting of melt in the second half of August. This suggests that the formation of ice slabs is a self-sustained feedback process increasing the efficiency of the runoff networks in the percolation zone. Ice slabs form and become thicker in areas with smaller surface slope than the surroundings where melt water ponds on top of the impermeable layer, flows, and refreezes during fall, adding to the ice slab. This dissertation provides useful insights on the processes driving ongoing changes in the percolation zone of Greenland due to global warming. However, several questions remain still open. Melt is the main driver of changes. Accurately modeling it, solving the uncertainties in observed and modeled sensible and ground heat flux, is essential. Furthermore, more ground truth and field observations are necessary in the region where blue slush forms on top of ice slabs to quantitatively determine how much water leaves the ice sheet and how much instead refreezes thickening the ice slabs.