Kinetic Grain Growth in Firn Induced by Meltwater Infiltration on the Greenland Ice Sheet

The microstructure of polar firn governs its porosity, permeability, and compaction rate, and is thus critical to understanding surface elevation change, heat and gas exchange, and meltwater infiltration on ice sheets. Previous studies in high-elevation dry firn have identified two atmospheric drivers of kinetic grain growth, though both produce only millimetre-scale layers near the surface. Here, we demonstrate that meltwater infiltration in the percolation zone of the Greenland Ice Sheet (GrIS) produces centimetre- to decimetre-scale layers of kinetic grain forms, ranging from faceted crystals to depth hoar, persisting to depths of up to 16 m. We analysed subsurface temperature time series from a transect on the western GrIS to resolve thermal regimes associated with infiltration-driven kinetic grain growth. Two distinct mechanisms responsible for faceting were identified: one associated with the onset of the wet layer, the other with preferential meltwater flow events. For both mechanisms, elevated vapor fluxes were calculated and diminished grain sphericity was observed in SNOWPACK model simulations, implying each can facilitate kinetic grain growth. Wet layer onset was the dominant mechanism, producing pronounced reductions in sphericity and the most enduring faceted layers. Additionally, the rate of wetting front propagation influenced the longevity of faceted layers, with rapid infiltration preferentially producing lasting, lower-sphericity firn grains. As surface melt expands across the GrIS, constraining the influence of these faceted layers on meltwater storage, surface elevation change, and chemical transport will become increasingly important.