A porous, up to 80 meters thick, snow layer called firn covers the Greenland ice sheet. When the ice sheet surface melts in the summer, the firn acts as a sponge, which retains gigatons of meltwater every year. The retained meltwater mostly freezes inside the firn, which prevents it from reaching the ocean and contributing to sea-level rise. But as the climate is becoming warmer, and the amount of meltwater is increasing, it is more relevant than ever to find out exactly how much meltwater the firn can actually retain.
“To understand how the firn’s retention process will function in a warming climate, we need computer models. But developing and testing these models is not an easy task. Various parameters control meltwater percolation and potential refreezing: the snow density and temperature, for example, or the amount of ice already contained in the snow. Various computer models are currently being used to simulate meltwater retention on the Greenland ice sheet. Yet, these models have never been evaluated and compared on the same weather data input,” says Baptiste Vandecrux, postdoc in the Department for Glaciology and Climate at The Geological Survey of Denmark and Greenland (GEUS) and lead author of the new study.
Computer models disagree
An international team of 23 researchers from 18 different research institutions have evaluated nine computer models at four sites on the Greenland ice sheet. The sites represent the various climatic regions of the ice sheet: from very cold and low-snowfall areas to relatively warm and high-snowfall areas.
“A striking and important finding of the study was that the snow models agree relatively well in the absence of melt. But the more melt was generated at the surface, the more models disagree on where the water infiltrates, whether it refreezes and is retained or whether it flows off towards the margin of the ice sheet and the ocean,” Baptiste Vandecrux says. He continues:
“This comparison exercise allows us to identify key issues that need to be resolved in future snow models. This will allow the improvement of these models and provide more accurate estimations of the current and future contribution of the Greenland ice sheet to sea-level rise.”
The study was published in the journal “The Cryosphere” and is available in open access at: https://doi.org/10.5194/tc-14-3785-2020