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Wind can transport large quantities of snow and thus greatly influence the distribution of snow in alpine and polar regions. In areas where drifting snow accumulates, avalanches can form. SLF researchers in Davos are therefore using a 14-meter-long wind tunnel to investigate how wind moves snow. They want, for example, to understand better the way the transport of snow varies according to the strength of the wind and the characteristics of the snow cover. This is why they create wind under controlled conditions with speeds of up to 70 kilometers per hour and realistic turbulence immediately above a natural snow cover.
The doctoral student, Philip Crivelli, is studying the shapes erosion takes when it is formed by wind on a snow surface. For this he uses an instrument that was originally developed as a movement sensor for a computer-game console. With the infrared sensor, the exact positions of objects in space can be determined. Philip uses this capability to measure the snow surface in the wind tunnel three-dimensionally at a high resolution, and then follows how it changes over time. The researchers also use other measurement methods, such as a snow-particle counter and ‘shadowgraphy’. The latter involves illuminating snow particles from one side of the wind tunnel and filming them from the other side with a high-speed camera. From the resulting sequence of shadow images, the researchers can then determine the speed, size and spatial distribution of the transported snow particles.
Snow transport is a complex phenomenon
With this experimental procedure, Philip and his colleagues have been able, for the first time, to follow in detail how wind erodes and deposits snow particles. Turbulence close to the ground is one important factor. Another factor is the state of the snow cover because impacting particles can knock other particles loose from the snow cover. Since the individual snow particles are bonded together with fine bridges of ice, a certain force is required to break these bonds before the wind can transport the individual particles away. Finally, the changing snow surface also influences the transport of snow. The measurements in the wind tunnel will enable researchers to better describe the complex transport of snow in weather and climate models in future. (Martin Heggli, Diagonal 1/17)
www.slf.ch/more/wind_tunnel