Surface Meltwater Runoff and Retention in the Accumulation Zone of the Greenland Ice Sheet

Surface meltwater transport on the Greenland Ice Sheet has important implications for mass balance. Melt runoff is the principal loss term in the surface mass balance, and any meltwater that reaches the englacial and subglacial hydraulic systems can contribute to sliding and dynamic losses from the ice sheet. In the accumulation zone of a glacier or ice sheet, some fraction of melt is retained each year by percolation into the underlying cold snow and subsequent refreezing of that melt. The runoff limit is defined as the elevation below which all surface melt is able to leave the ice sheet and count as mass loss. Most ice sheet mass balance models make use of meltwater retention parameterizations to determine the runoff limit, but the runoff limit has never been directly measured. Lateral routing of meltwater is important for runoff calculations because any melt produced at the surface must be able to leave the ice sheet to count as mass loss. A physically based snowpack model combining day-time surface melt percolation with nighttime refreezing was used to simulate the effects of multi-year cycles of melt, infiltration and refreezing on snowpack physical properties and downslope flow of meltwater. Increasing accumulation during the melt season leads to a higher frequency of melt layers in the near-surface snow and a greater fraction of surface melt traveling downslope. The temperature conditions in the snowpack control the lateral distance this melt can travel. Previous meltwater retention models have taken into account only seasonal melt and refreeze cycles, but the daily cycles in this model indicate that nighttime refreezing has a large effect on lateral melt transport by inhibiting the distance melt can travel down the ice sheet. Recent field data and snow temperature measurements from the Greenland Climate Network indicate the snowpack refreezes most nights during the melt season in the accumulation zone. Our results suggest the maximum lateral distance melt can travel is only a few kilometers per melt season. These findings indicate much less runoff from the accumulation zone of Greenland than previously estimated.