The modelled liquid water balance of the Greenland Ice Sheet

Recent studies indicate that the surface mass balance will dominate the Greenland Ice Sheet's (GrIS) contribution to 21st century sea level rise. Consequently, it is crucial to understand the liquid water balance (LWB) of the ice sheet and its response to increasing surface melt. We therefore analyse a firn simulation conducted with the SNOWPACK model for the GrIS and over the period 1960-2014 with a special focus on the LWB and refreezing. Evaluations of the simulated refreezing climate with GRACE and firn temperature observations indicate a good model-observation agreement. Results of the LWB analysis reveal a spatially uniform increase in surface melt (0.16 m w.e.a(-1)) during 1990-2014. As a response, refreezing and run-off also indicate positive changes during this period (0.05 and 0.11 m w.e.a(-1), respectively), where refreezing increases at only half the rate of run-off, implying that the majority of the additional liquid input runs off the ice sheet. This pattern of refreeze and run-off is spatially variable. For instance, in the south-eastern part of the GrIS, most of the additional liquid input is buffered in the firn layer due to relatively high snowfall rates. Modelled increase in refreezing leads to a decrease in firn air content and to a substantial increase in near-surface firn temperature. On the western side of the ice sheet, modelled firn temperature increases are highest in the lower accumulation zone and are primarily caused by the exceptional melt season of 2012. On the eastern side, simulated firn temperature increases are more gradual and are associated with the migration of firn aquifers to higher elevations.