We have a new open-access study in the current volume of Annals of Glaciology that tracks the fate of meltwater in the relatively porous near-surface firn of the Greenland Ice Sheet using temperature sensors1 (available here). One of the main goals of this study was to understand what fraction of the meltwater produced at the ice sheet surface percolates vertically into the firn and locally refreezes, rather than leaving the ice sheet as runoff and contributing to sea level rise. The total retention capacity of all of Greenland’s firn could be a non-trivial buffer against sea level rise2.
For this particular study, we deployed firn temperature sensors at depths of up to 15 m at KAN_U. The sensors were automated to record data throughout the year, between our spring sites visits. KAN_U is located at 1840 m elevation in Southwest Greenland in the lower accumulation area. While KAN_U traditionally receives more mass from snowfall than it loses from melt, our study focused on the “extreme” 2012 melt season, which was the first year since records began that there was more meltwater runoff than snowfall at the site.
As refreezing meltwater releases a tremendous amount of latent energy, the location of refreezing meltwater within the firn can be inferred from temperature anomalies. We assessed temperature anomalies by comparing our observed firn temperatures against modeled firn temperatures, whereby the modeled temperatures only accounted for heat exchanged with the ice sheet surface via diffusion, not latent heat release. This allowed us to identify depths where firn temperatures were warmer than expected.
We found that despite 2012 being an extreme melt year, meltwater percolation and refreezing only occurred to 2.5 m depth during the melt season. It was only after the end of the melt season that some meltwater managed to percolate and refreeze in discrete bands at 5.5 and 8.5 m depth. This inference of relatively inefficient vertical meltwater percolation during the melt season appears to support the idea that thick and impermeable ice lenses that had previously formed within the firn during 2010 were inhibiting the percolation of 2012 meltwater3.
Maintaining the relatively sensitive automatic weather station needed to accurately measure firn temperatures and surface energy fluxes in the relatively harsh ice sheet environment was no easy task. It took a number of scientists and funding agencies, which are listed in the acknowledgement section of the paper, to make this study possible. The KAN_U weather station continues to report real-time climate data via the Programme for Monitoring of the Greenland Ice Sheet (PROMICE) data portal: www.promice.dk.
1Charalampidis, C., D. van As, W. Colgan, R. Fausto, M. MacFerrin and H. Machguth. 2016. Thermal tracing of retained meltwater in the lower accumulation area of the Southwestern Greenland ice sheet. Annals of Glaciology. doi:10.1017/aog.2016.2.
2Harper, J., N. Humphrey, W. Pfeffer, J. Brown and X. Fettweis. 2012. Greenland ice-sheet contribution to sea-level rise buffered by meltwater storage in firn. Nature. 491: 240-243.
3Machguth, H., M. MacFerrin, D. van As, J. Box, C. Charalampidis, W. Colgan, R. Fausto, H. Meijer, E. Mosley-Thompson and R. van de Wal. 2016. Greenland meltwater storage in firn limited by near-surface ice formation. Nature Climate Change. 6: 390–393.
— William Colgan (@GlacierBytes) May 19, 2016