Viscosity of melt-affected snow/firn derived from laboratory-based uniaxial compression experiments.

Enhanced climate warming across Arctic glaciers, ice caps and the Greenland Ice Sheet has intensified melt and refreezing processes, producing more widespread regions of melt-affected, near-surface (< 12 m depth) firn within accumulation areas. Accurately simulating this near-surface region is important, as it governs the evolving capacity of ice sheets to retain meltwater through refreezing and thus strongly influences surface mass balance. Although over short timescales (days to months) refreezing of meltwater dominates changes in the density of melt-affected firn, over longer timescales (decades to centuries), densification through vertical compaction of refrozen firn remains significant. Whilst there is a substantial body of both field and theoretical work addressing the issue of dry firn densification by compaction of snow into firn and ice, i.e. in the absence of melting and refreezing, there is a need for better understanding of densification by compaction in melt-affected snow / firn increasingly found across accumulation areas of ice sheets and glaciers.

 

Firn densification can be defined via a constitutive equation, calculating the vertical compression rate due to stress from the overburden pressure dependent on firn viscosity. Laboratory studies have demonstrated the potential of controlled experiments to constrain the viscosity of dry snow of varying densities.  However, similar experimental constraints do not yet exist for refrozen, melt-affected firn. In this study we extend such laboratory approaches to refrozen snow/firn to quantify their effective viscosities. We undertook confined uniaxial compression tests on snow/firn samples with known grain sizes and densities (c. 400 – 600 kg m^-3), simulating expected ranges of overburden pressures to c. 12 m depth to establish the stress-strain rate relationships that define near-surface firn viscosities. Our experiments indicate an exponential increase in viscosity with density. The viscosity values calculated here for melt-affected snow/firn are broadly consistent with the lower envelope of viscosities reported in previous studies on cold dry firn, though they exceed those made in the field for temperate firn. The relationship of viscosity to snow/firn density allows our findings to be incorporated into large-scale firn densification and surface mass balance models, where resolving detailed microstructural processes and form is often impractical. By providing empirically constrained viscosity estimates tied directly to density, this work bridges a gap between field observations of melt-affected firn and the parameterizations required by regional and continental-scale models for more robust future projections of near-surface refreezing capacity of ice sheets and glaciers.