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Scientific Background

Micrometeorology on the Greenland Ice Sheet

Science Background

There is much interest in the response of the Greenland ice sheet to climate change. For the investigation of this response, reliable parameterizations of the surface exchange of momentum, heat and moisture are required. Of special importance is the ablation zone of the ice sheet, where the yearly meltwater production exceeds the yearly snow- and rainfall. The determination of the melt rate in the ablation zone is important for the prediction of sea-level rise. Apart from this, the melting conditions in summer are attractive for boundary-layer research since they offer the opportunity to perform investigations over a vast homogeneous area where the surface temperature is accurately known. On the other hand, the melting conditions cause a number of notorious problems for carrying out measurements, and observations in this area have long remained scarce for this reason.

What Were We Measuring and Why?
Typically, due to aerodynamic drag, there is a wind gradient in the wind flow just a few hundred meters

above the the earth's surface - the surface layer of the planetary boundary layer. Wind speed increases with increasing height above the ground, starting from zero due to the no-slip condition. Flow near the surface encounters obstacles that reduce the wind speed, and introduce random vertical and horizontal velocity components at right angles to the main direction of flow. This turbulence causes vertical mixing between the air moving horizontally at one level and the air at those levels immediately above and below it, which is important for the transport of heat and moisture.

Note: turbulence is a very effective mechanism for quantities that can be transported through molecular mechanisms (heat, moisture, pollutants). If there was no turbulence, molucular diffusion would be the only available mechanism. This is a rather ineffective mechanism. An intuitive example: putting cream in coffee. If you are very careful and pour the cream in slowly, it will float on the coffee and remain separate. Eventually, of course, the cream will diffuse into the coffee and make a uniform light brown mixture, but you may have to wait a month or two. Or, you can pick up your spoon and give the contents of the cup a stir, when the mixing will take place in seconds. The reduction in velocity near the surface is a function of surface roughness, so wind velocity profiles are quite different for different terrain types.

Note: an ice sheet is special in this respect because of it very low surface roughness (we'll come back to that later)

Elements of Energy Balance


A Unique Moment