Nicole Clerx , Horst Machguth, Andrew Tedstone, Dirk van As

At high elevations on the Greenland ice sheet meltwater percolates and refreezes in place, and hence does not contribute to mass loss. However, meltwater generation and associated surface runoff is occurring from increasingly higher altitudes, causing changes in firn stratigraphy that have led to the presence of near-surface ice slabs. These ice slabs force meltwater to flow laterally instead of percolating downwards. Here we present a simple, physics-based quasi 2D-model to simulate lateral meltwater runoff and superimposed ice formation on top of ice slabs. Using an Eulerian Darcy flow scheme, the model calculates how far meltwater can travel within a melt season and when it appears at the snow surface. Results show that lateral flow is a highly efficient mechanism for runoff, as in any model grid cell lateral outflow is over 30 times larger than the amount of meltwater generated in situ. Superimposed ice formation can retain up to 40% of the available meltwater, and generally delays visible runoff. Validating the model against field or remote sensing data remains challenging, but the results presented here are a first step towards a more comprehensive understanding and description of the hydrological system in the accumulation zone of the southwestern Greenland ice sheet.