D4 Meteorological and Energy Instrumentation

Topic

Automatic weather stations installed directly on the surface of a glacier or ice sheet measure the atmospheric variables that control the surface's energy balance: air temperature and humidity, wind speed and direction, atmospheric pressure, precipitation, and the radiation incident upon the ice. Of all these atmospheric variables, the radiation balance at the ice surface is most directly linked to whether the ice melts or, conversely, accumulates more mass.

The eddy covariance method directly measures the turbulent fluxes of sensible and latent heat exchanged between the ice surface and the overlying atmosphere by analyzing the temporal correlation between rapid fluctuations in vertical wind speed and equally rapid fluctuations in air temperature or specific humidity. These turbulent sensible and latent heat fluxes are critical components of the ice surface energy balance; indeed, they determine the rates at which the ice directly sublimates into the air and melts.

A pyranometer measures the solar radiation reaching the ice surface, while a pyrgeometer measures the longwave infrared radiation received and emitted by that same surface; together, these instruments fully account for the radiation balance by quantifying each of its components separately. Once the radiation balance and the turbulent sensible and latent heat fluxes have been fully quantified, the ice surface energy balance determines the amount of surplus heat available to melt the ice. The expected behavior of this surface energy balance varies depending on the glacier zone: in the accumulation zone, the fraction of solar radiation reflected rather than absorbed by the ice surface governs the most significant component of the overall energy balance, whereas in the ablation zone, turbulent sensible and latent heat fluxes—measured using the previously described eddy covariance method—dominate the energy balance and determine the amount of ice melt.

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