Cold-laboratory experiments to observe meltwater and ice layer interactions in snowpacks

Abstract

The fate of percolating surface meltwater encountering ‘impermeable’ ice layers is uncertain in the accumulation zone of the Greenland Ice Sheet (GrIS). Often, ice layers are considered to retard meltwater and cause lateral runoff. However, modelled and field-based observations in the percolation zone of the GrIS have suggested ice layers are not necessarily impermeable and meltwater can breakthrough, percolating to deeper depths of snow/firn and consequently inferring a greater refreezing capacity within the accumulation zone. The physical and thermal conditions which control the permeability of ice layers remain unclear and effective parameterisation of these processes is lacking for snow/firn modelling of melt, refreezing and runoff. Here we present repeat cold-laboratory experiments which seek to understand how meltwater interacts with thin ice layers (5 to 20 mm) for two differing thermal regime contexts whereby the surrounding snow/firn thermal regime is either (i) below or (ii) at the melting point. We find that under extreme melt regimes, ice layers continually retard wetted fronts of percolating meltwater when the thermal regime of the snowpack is below the melting point. This barrier results in the snowpack at depth remaining at least ~1oC cooler than snow above the ice layer which is saturated with meltwater. We also find that the ice layer forces ~35% of the percolating meltwater to runoff, cooling the overlying snow and increasing the refreezing capacity of the snow closer to the snowpack surface. The remaining ~65% of meltwater ponds and later refreezes on the ice layer, thickening the impenetrable surface. When the thermal regime of the surrounding snow/firn is at the melting point, we find that meltwater is able to pond without refreezing, resulting in the ice layer failing and allowing deeper percolation into the snowpack. These findings suggest that the thermal regime of a snowpack is crucial for the structural integrity of an ice layer and thus the permeability of a snowpack. Consequently, these findings have implications for parameterising meltwater runoff and ice layer integrity in snow and firn models which incorporate ‘impermeable’ barriers in their domains. Future work will continue to explore similar experiments with thicker ice layers (~60 mm) to determine whether ice layer breakthrough is primarily a function of snow/firn thermal regime and/or ice layer thickness.