Abstract
Abstract. Retention and refreezing of meltwater are acknowledged to be important processes for the mass budget of polar glaciers and ice sheets. Several parameterizations of these processes exist for use in energy and mass balance models. Due to a lack of direct observations, validation of these parameterizations is difficult. In this study we compare a set of 6 refreezing parameterizations against output of two Regional Climate Models (RCMs) coupled to an energy balance snow model, the Regional Atmospheric Climate Model (RACMO2) and the Modèle Atmosphérique Régional (MAR), applied to the Greenland ice sheet. In both RCMs, refreezing is explicitly calculated in a snow model that calculates vertical profiles of temperature, density and liquid water content. Between RACMO2 and MAR, the ice sheet-integrated amount of refreezing differs by only 4.9 mm w.e yr−1 (4.5 %), and the temporal and spatial variability are very similar. For consistency, the parameterizations are forced with output (surface temperature, precipitation and melt) of the RCMs. For the ice sheet-integrated amount of refreezing and its inter-annual variations, all parameterizations give similar results, especially after some tuning. However, the spatial distributions differ significantly and the spatial correspondence between the RCMs is better than with any of the parameterizations. Results are especially sensitive to the choice of the depth of the thermally active layer, which determines the cold content of the snow in most parameterizations. These results are independent of which RCM is used to force the parameterizations.
Highlights
The surface mass balance (SMB) of a glacier is defined as the sum of all processes adding mass to the surface minus all processes removing mass: SMB = dt (C + RF − surface sublimation (SUs) − sublimation of drifting snow (SUds) − erosion by drifting snow (ERds) − RU) . (1) 1 yrThe most important contribution to accumulation is snowfall (C), with additional contributions of condensation and freezing of rainfall (RF)
First a comparison will be made of the parameterizations as formulated in their original papers forced by and compared to both models, RACMO2 and Modele Atmospherique Regional (MAR), in the absence of observations
In this study we applied several parameterizations that calculate the annual amount of refreezing to the Greenland ice sheet
Summary
Refreezing is an important process: it increases the temperature and density of the snow/firn and delays and reduces runoff, it reduces melt in the ablation zone since it delays bare ice exposure, and impacts mass balance profiles since it enhances mass accumulation around the equilibrium line and in the percolation zone above. Bøggild (2007) and Wright et al (2007) focussed on estimating superimposed ice formation, while Schneider and Jansson (2004) and Reijmer and Hock (2008) discussed the impact of refreezing on the glacier mass balance. A few observational studies on water infiltration are available as well (Marsh and Woo, 1984; Pfeffer and Humphrey, 1996; Humphrey et al, 2012) These observational studies show the importance of heterogeneous infiltration (piping) in the process of water infiltration and heating of cold firn. We discuss the impact and sensitivity of the values of the different input parameters to the parameterizations
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