Simulating Greenland Ice Slabs and Firn Aquifers with a 1D Firn Model

The Greenland Ice Sheet (GrIS) has been losing mass at an accelerating rate, primarily due to meltwater runoff to the ocean. Firn, a porous transition layer between snow and ice, has the potential to buffer the GrIS’s contribution to sea level rise by retaining this meltwater. In regions with low surface accumulation, such as the K-transect in Southwest Greenland, high surface melt leads to the formation of thick, near-impermeable ice slabs which decrease the capacity of firn to retain meltwater. In contrast, in regions with high surface accumulation, such as the Helheim glacier in Southeast Greenland, high surface melt causes the formation of firn aquifers.

In this research, we simulate ice slabs and firn aquifers with a one-dimensional firn model, called Timm’s Firn Model (TFM), along glacier flowlines in different climate forcing scenarios. Instead of the commonly-used, more computationally efficient bucket scheme, the TFM solves the Richards’ equation, which simulates the vertical water transport more physically. We investigate whether the TFM simulates an earlier onset, greater extent, and expansion of ice slabs and firn aquifers towards the interior of the GrIS. In addition, we offer a new detection method for ice slabs based on hydraulic conductivity and volumetric liquid water content, enabled by the modeling of liquid water movement with the Richards’ equation.

The results show that firn aquifers were already forming in the Helheim glacier region before the GrIS started rapidly losing mass. Furthermore, the TFM results indicate that, with warming, firn aquifers form earlier along the flowline, expanding towards the interior of the ice sheet. Firn aquifer formation is highly dependent on surface accumulation, with higher accumulation rates favouring formation.

We further find that ice slabs, though less extensive than firn aquifers, were present along the K-transect in Southwest Greenland before the GrIS’ rapid mass loss. With warming, ice slabs form earlier along the flowline and expand towards the interior, consistent with available observations. Three consecutive years of extensive melt lead to ice slab formation. However, decade-old ice in the subsurface firn leads to ice slab formation as well, by merging with newly refrozen layers.