Midterm Colloquium by Pia Nielsen-Englyst

Impact of albedo parameterizations on surface mass balance and runoff on the Greenland Ice Sheet

During the 21st century, the Greenland Ice Sheet has started to lose more mass than it gains due to warming temperatures. The ability of being able to quantify this mass loss of the Greenland ice sheet is essential in order to provide projections for the potential environmental, economic, and social repercussions and implications of the mass loss around the world.

The primary energy source for melting snow and ice is absorption of shortwave radiation. The amount of shortwave radiation, received at the surface, depends on the incident radiation and the albedo of the surface. The albedo, which is the ratio of reflected to incoming solar radiation at the surface, is one of the most crucial climate parameters. There are large differences between the albedo of fresh snow, old snow, melting snow and bare ice, and to get the timing, magnitude and spatial distribution of surface melting right in a climate model, it is vital to capture these differences. The main motivation behind this thesis is to capture these spatial and temporal differences in the surface albedo to create a better and more physically based albedo parameterization for the Greenland Ice Sheet.

Various choices of albedo formulations have been tested and fitted against in situ data provided by PROMICE weather stations. In the optimal albedo formulation, the albedo decays exponentially over time with a decay rate dependent on the temperature. A snowfall event will eventually refresh the albedo to a maximum value. The albedo scheme has been implemented and tested in a stand-alone version of the surface mass balance scheme implemented in the Danish regional climate model HIRHAM5 covering all of Greenland at 5 km resolution.

In order to validate the model, we need a comparison of variables at temporal and spatial scale against observations. Comparisons will be made with MODIS satellite derived albedo data and in-situ observations by PROMICE. Further, NSIDC (NASA National Snow and Ice Data Center) data on surface melt might be used.

Finally, the sensitivity to the albedo parameterization will be tested. This is done by running the same albedo scheme with higher and lower minimum albedo values and investigate the influence on e.g. surface mass balance, melt extent, and duration.

Supervisors: Christine Hvidberg, Peter Langen and Ruth Mottram