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Sunday, July 14, 2013

DMI's Surface Mass Budget of the Greenland Ice Sheet

The link to this site will go up in the left-hand column of this blog, for our convenience.

Current Surface Mass Budget of the Greenland Ice Sheet

Here you can follow the daily surface mass balance on the Greenland Ice Sheet. The snow and ice model from one of DMI’s climate models is driven every six hours with snowfall, sunlight and other parameters from a research weather model for Greenland, Hirlam-Newsnow. We can thereby calculate the melting energy, refreezing of melt water and sublimation (snow that evaporates without melting first). The result of this is a change in the snow and ice from one day to the next and this change is shown below. All numbers are in water equivalent, that is, the amount of water the snow and ice would correspond to if it was melted.

Left: Map of the surface mass balance today (in mm water equivalent per day). Right: The average surface mass balance for today’s calendar date over the period 1990-2011.

In the above figure, we show the daily surface mass balance (on the left) and you can see where it has snowed and melted (incl. sublimate) on the ice sheet over the last 24 hours. For comparison, the map to the right shows the average value for the same calendar date over the period 1990-2011. This historical average is based on weather from a somewhat different model and the two are therefore not always strictly comparable.
The surface mass balance is calculated over a year from September 1st to August 31st (the end of the melt season). The figure to the right shows the sum of all the daily changes from September 1st up to today. Next year on September 1st the map will be reset and we start over. This accumulated map illustrates how much the surface mass balance has contributed in each point across the ice sheet. 

The figure below shows the total daily contribution from all points on the ice sheet (top) and the same accumulated from September 1st to now (bottom). The blue curves show this season’s surface mass balance in gigatons (Gt: 1 Gt is one billion tons and corresponds to 1 cubic kilometer of water), and for comparison the mean curves from the historical model run are shown with two standard deviations on either side. Note that the accumulated curve does not end at 0 at the end of the year. Over the year, it snows more than it melts, but calving of icebergs also adds to the total mass budget of the ice sheet. Satellite observations over the last decade show that the ice sheet is not in balance. The calving loss is greater than the gain from surface mass balance, and Greenland is losing mass at the rate of about 200 Gt/yr.

Map of the accumulated surface mass balance (in mm water equivalent) from September 1st to now.

Top: The total daily contribution to the surface mass balance from the entire ice sheet (blue line, Gt/day). Bottom: The accumulated surface mass balance from September 1st to now (blue line, Gt). The dark grey line shows the corresponding average number from the climatology along with two standard deviations on each side (light grey). In the bottom panel, we also show the season 2011-12 (red) which had very high summer melt in Greenland.
Ice flow
Due to gravity, ice flows slowly outwards like dough on a kitchen counter. When snow falls on top of the ice sheet year after year, the layers below are slowly compressed into ice. In the central part of the ice sheet, where little if any melt occurs, new layers will therefore continually be added. The ice does not grow in height, however, since the extra ice is balanced by the flow away from the center. Further out towards the coast we find the equilibrium line, where snowfall and melt are exactly balanced. Below the equilibrium line, there is more melt than snowfall and here the net mass loss is countered by the flow coming out from the center of the ice sheet. Here it is the ice sheet itself which melts.

For an ice sheet that neither grows or shrinks, there is at all points averaged over the year a balance between
  • the amount of snow that falls and is compressed to ice
  • the amount of snow and ice that melts or evaporates (sublimates) and
  • the amount of ice that flows away due to the ice motion
The two first contributions make up the surface mass balance. For the ice sheet as a whole, there is a balance between the surface mass balance and the amount of ice that calves into the ocean as icebergs. 

If climate changes, the surface mass balance may change such that it no longer matches the calving and the ice sheet can start to gain or lose mass. This is important to keep track of, since such a mass loss will lead to global sea level rise. As mentioned, satellites measuring the ice sheet mass have observed a loss of around 200 Gt/year over the last decade.

Greenland Climate Research Centre collaborates with Danish Climate Centre at DMI on research in both atmospheric impact on the Greenland Ice Sheet and the ice flow itself and its interaction with the rest of the climate system.

Three processes determine whether the ice sheet grows or diminishes. Accumulation of snow on top increases mass. In time, the snow is transformed to ice that flows down through the ice sheet and out towards the margins. Melt in the lower regions of the ice sheet and iceberg calving from glaciers reduces the mass. If mass loss exceeds mass gain the ice sheet will shrink. Graphics Diego Winterborg.

For further information please contact climate scientist Peter L. Langen, Danish Climate Centre,

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