Modelling evolution of Greenland Ice Sheet near-surface ice slab and its impact on runoff

In the accumulation regions of the Greenland Ice Sheet (GrIS), not all surface meltwater contributes to runoff. A significant portion is retained through refreezing within the underlying firn layer, a process that critically moderates the overall mass loss from the GrIS. The refreezing of percolating meltwater at shallow depths leads to densification of the near-surface and the formation of ice layers. The extent of meltwater refreezing is influenced by firn density and temperature, which together govern the permeability of the near-surface ice layers. The presence of shallow, thick ice layers (> 1m thick, also known as "ice slab") restricts the deeper percolation of meltwater, thereby promoting its conversion into runoff. For example, the formation of ice slab in GrIS has resulted in nearly a 30% increase in the area contributing to runoff generation since 2001. Therefore, modelling ice slab is essential for understanding the total mass loss of the GrIS, both in recent years and in future projections.

In this study, we present a high vertical resolution, physically distributed model that simulates surface mass balance, refreezing, ice layer formation, and runoff. A novel temperature-dependent criterion for ice layer permeability is incorporated that has been rigorously validated against field measurements from the Devon Ice Cap in the Canadian Arctic, where it demonstrates a strong agreement with point-scale observations of surface mass balance and vertical density profiles. We applied the model to the GrIS from 1999 to 2022, using a horizontal spatial resolution of 0.25° × 0.25°, a vertical resolution of 1 cm, and a temporal resolution of 15 minutes. The model simulations are calibrated using the SUMup archive of surface mass balance observations and validated against shallow core measurements of vertical density profiles. The high vertical resolution of the model provides insights into the process of ice slab evolution and impacts on runoff magnitudes and spatial distribution from the accumulation area of the GrIS. We analyze the model results to examine the relationship between the formation of ice slab and the runoff limit across the GrIS exploring sensitivities to changing climate.