Modelling lateral meltwater flow atop the GreenlandIce Sheet’s near-surface ice slabs

The Greenland ice sheet is losing mass. Thereby, the location of the runoff limit, the highest elevation from which meltwater finds its way off the ice sheet, plays an important role. Above the runoff limit all meltwater refreezes and does not contribute to mass loss. In recent years surface runoff has increasingly occurred from higher elevations, thereby expanding the area of mass loss: between 1985 and 2020, the maximum runoff limit rose by on average 194 metres, expanding the visible runoff area by around 29%.

The observed rise in the runoff limit might be related to intensive meltwater refreezing within the firn which leads to the formation of thick ice layers, also called ice slabs. Our field experiments, carried out at around 1750 m a.s.l. on the K-Transect, have shown that meltwater generated over ice slabs is generally forced to flow laterally: initially through a near-surface slush matrix and then forming streams and rivers. It remains unclear, however, how much of the meltwater contributes to runoff, and which percentage refreezes and contributes to ice slab formation or expansion.

Here we present a conceptual quasi 2D-model of runoff, that simulates lateral meltwater flow on top of an ice slab using firn hydrological properties measured on the southwest Greenland ice sheet. We adapted a gridded linear-reservoir runoff routing model to calculate (i) the distance meltwater can travel within one melt season, and (ii) when meltwater breakthrough at the snow surface (i.e. slush formation) occurs. First results provide insight into the evolution of the water table height over time that matches observations made during our summer field campaign. We are exploring ways to incorporate meltwater refreezing, to better understand ice slab evolution and their impact on the fate of meltwater between vertical percolation, refreezing and lateral runoff.