Ice, Snow, or Very Cold Water? Making sense of the Greenland Ice Sheet Surface layers

Polar Portal scientists make an important step to more accurately determine how much snow melts on Greenland – and what happens to the water after ice and snow melts.

Polar Portal scientists are busy people. They work on multiple projects and one of these has just produced some valuable results, published recently in the journal Frontiers of Earth Science.

“We now really feel we have a handle on what is going on in the snow pack in Greenland” said Dr. Ruth Mottram, a climate scientist who runs the HIRHAM5 climate model at DMI and is a co-author on the new study.

The paper is an outcome of the Retain project, funded by the Danish Council for Independent Research, and is focused on the surface snow layers of the Greenland ice sheet. “With the assistance of a lot of measurements made literally over decades, and the incomparable PROMICE automatic weather station data, we have been able to both develop and then check our model to unprecedented detail. Our results now clearly show how well the model is able to not only work out how much snow is falling and how much snow and ice is melting each year,  but also what happens to the water”, she continued.

Lead author and polar portal contributor, Dr. Peter Langen from DMI who did much of the model development work, explains. “When snow melts at the surface, the water doesn’t just run-off straight away. It usually trickles into the snow pack where it can refreeze into thin ice lenses. In some places the snow acts like a giant sponge, soaking up water but not really letting it drain away until a critical amount is in the snow layers. This makes it tricky to assess just how much water is actually going into the ocean. In our new paper we have tested lots of different ways to understand this mathematically”.

Snow pits like this provide a crucial source of information on how much snow has fallen and where meltwater has percolated down and refrozen (image credit: Ruth Mottram)
Snow pits like this provide a crucial source of information on how much snow has fallen and where meltwater has percolated down and refrozen (image credit: Ruth Mottram)
Frozen ice lenses in the snowpack (image credit: Jason Box)
Frozen ice lenses in the snowpack (image credit: Jason Box)

 

The real breakthrough has come with the increasing number of observations that have allowed the scientists to independently check how well the computer models are able to simulate the Greenland ice sheet. Project co-Principal Investigator, Professor Jason Box at GEUS is clear that not only the weather stations that GEUS-PROMICE have been installing and maintaining, but also the years of fieldwork, using a range of techniques from radar to shallow cores and snow pits, have been vital for being able to develop and assess the new model. “The Greenland ice sheet is really going through a period of change. We saw that last year with the extremely early start to melting. This can really make it challenging to do fieldwork just when we need long-term records more than ever. With these new model parameters we can now see how important these early melt events can be”.

DTU PhD student Baptiste Vandecrux concurs “On the ACT-16 Arctic circle traverse across the Greenland ice sheet last year, we experienced some pretty challenging conditions due to the early melt event, but this makes our work on the model and the observations even more interesting and important”.

The early melt in 2016 was reported by polar portal scientists on the 12th April 2016. Based on this work, it was argued already then that it would not have much overall impact on how much ice Greenland would lose. As co-author Dr. Robert Fausto from GEUS puts it, “When we compare the model results with sensors in the snow or with observations from snow pit we can see that much of this meltwater in the early part of the season stays within the snowpack either as liquid water or as ice lenses, which means we can really estimate quite well how much water is retained in the ice sheet”.

It’s not all good news however, “the bad news is that observations and models also show that widespread melting and the formation of thick ice layers in the snow, effectively creating an ice lid, preventing further drainage down deep into the snowpack, so later in the year we get much more runoff. It’s important to understand these processes as we need to know when the melt happens and where the water goes. This also helps us to quantify how much the surface melt of Greenland is contributing to sea level rise at the present day, and how quickly this will likely increase in the future”.

The Retain project also has a benefit for the Polar Portal as we can use the upgraded model to extend and improve the daily estimated surface mass balance.  We expect to introduce this new data over the next few weeks.

Full reference :

Langen, P.L., Fausto, R.S., Vandecrux, B., Mottram, R.H., Box, J.E., 2017. Liquid Water Flow and Retention on the Greenland Ice Sheet in the Regional Climate Model HIRHAM5: Local and Large-Scale Impacts. Front. Earth Sci. 4. doi:10.3389/feart.2016.00110 http://journal.frontiersin.org/article/10.3389/feart.2016.00110/full

This is a contribution to the special issue in Frontiers in Science on Melt Water Retention Processes in Snow and Firn on Ice Sheets and Glaciers: Observations and Modeling, curated by Retain project members.

Learn more about the Danish Council for Independent Research funded Retain project here: http://retain.geus.dk/ (Grant no. 4002-00234).

Polar Portal is collaboration between DMI, GEUS, DTU-Space and DTU-Byg with funding from the Danish Energy Agency (Energistyrelsen)