Internal accumulation in firn and its significance for the mass balance of Storglaciären, Sweden

Abstract

The discussion on global change has led to increased interest in glacier mass balance since glaciers can be used as climatic indicators. To meet the need for high-quality mass-balance data requires critical examination of traditional mass-balance methods and their possible errors. One issue regarding mass-balance measurements that has received little attention is internal accumulation. Our study shows that internal accumulation in the firn layer of Storglaciären, Sweden, significantly affects the mass balance of the glacier. This occurs because the winter cold wave penetrates below the previous year’s summer surface and into underlying firn. We estimated internal accumulation from measurements of temperature and water content in firn. The depth of the 0°C isotherm correlated with snow depth and air temperature, so that low snow depth and low air temperature separately cause a deeper 0°C isotherm. We determined irreducible gravimetric water content in firn to 2–3%, which corresponds to an irreducible water saturation of 6–8%. Our value for firn is relatively high compared with that for snow, probably due to trapped water in isolated firn pores. Refreezing of percolating meltwater in spring accounted for ~30% of annual internal accumulation. The remaining 70% was due to re-freezing of retained capillary water in firn pores during winter. Disregarding internal accumulation would lead to underestimation of annual net mass balance by 0.04–0.06 m w.e., corresponding to 3–5% of annual accumulation of the entire glacier in an average year. Hence, internal accumulation potentially becomes a source for systematic error if not accounted in mass-balance measurements.

A plot-scale study of firn stratigraphy at Lomonosovfonna, Svalbard, using ice cores, borehole video and GPR surveys in 2012–14

“…The prominent density increase of ≈130 kg m (T) and density (ρ) of firn at the depth 1.5-5.3 m are −9°C and 440 kg m −3 , correspondingly (see Figs 3 and 4), relative heat capacity of ice (C) is 2050 J kg −1 K −1 , latent heat of water fusion (L) is 334 000 J kg −1 the calculation yields a value of 24 kg m , assuming the liquid water content of 5% (Schneider and Jansson, 2004). Apart from the uncertainty in the above estimates, the resulting inconsistency of 40 kg m −3 between the observed density increase and the sum of possible contributions from different densification processes can be explained as an effect of multiple melt-freeze cycles in spring and summer 2013 and/or lateral variability of firn density.…”

Density assumptions for converting geodetic glacier volume change to mass change

“…The processes governing firn compaction (pressure, aging) are active for both temperate and cold conditions thus allowing a similar description. For mountain glaciers, also refreezing of liquid water in the pore space can accelerate firn densification (Schneider and Jansson, 2004). Even in the case of temperate glaciers refreezing can be significant as winter temperatures tend to seasonally cool the uppermost firn layers below 0 • C (e.g.…”

Simulating melt, runoff and refreezing on Nordenskiöldbreen, Svalbard, using a coupled snow and energy balance model

“…In case not all the available percolating water refreezes in a layer, a small amount of water, called irreducible water, will be held by capillary forces while the remaining water percolates into the next layer. In accordance with Schneider and Jansson (2004), an empirical relation has been used to compute the maximum irreducible water content of a layer θ mi , i.e. the ratio of the mass of irreducible water to the total mass of the layer.…”

“…The latent heat release after refreezing raises subsurface temperatures and hence affects the heat flux from the surface into the ice. Refreezing below the previous year's summer surface in the accumulation zone, referred to as internal accumulation, has received considerable attention, since this term is disregarded by traditional mass balance observations (Trabant and Mayo, 1985;Schneider and Jansson, 2004;Reijmer and Hock, 2008). Other studies show the significance of refreezing in the timing and rate of englacial water transport (Pfeffer et al, 1991;Fountain, 1996;Jansson et al, 2003), which has substantial implications for basal dynamics (Zwally et al, 2002;Van de Wal et al, 2008;Schoof, 2010).…”