Drifting snow studies in the SLF boundary-layer wind-tunnel

Wind is able to relocate large amount of snow within short time. Deposited wind transported snow increases avalanche hazard as larger amount of snow can be released leading to larger and sometimes catastrophic avalanches. Experts say: Wind is the master-builder of avalanches.

Fig. 1: High-speed imaging of saltating snow in the SLF boundary-layer wind-tunnel.

Wind transports snow in three modes: creep, saltation and turbulent suspension. During creep, also called rolling or reptation, the moving snow particles remain very close to the surface. In general, they stay in direct contact with the ground but occasionally undertake short hops of a few (tenths) millimetres. During saltation, drifting snow particles follow ballistic trajectories with maximum heights of ~ 10 - 20 cm and lengths of up to a few metres. Turbulent suspension refers to the entrainment of snow into the atmospheric boundary-layer by turbulent eddies. In this mode, snow is transported over long distances in the atmosphere. Sometimes a forth mode is distinguished as modified saltation which encompasses the transition between saltation and turbulent suspension. In general, saltation is considered to be the most important transport mode in terms of snow mass flux.

In the SLF boundary-layer wind-tunnel the characteristics of saltation are investigated. In particular, the snow particle size distribution, the snow mass concentration and the intermittency of snow transport are addressed. Additionally, particle trajectories of drifting snow, i.e. their ejection/impact velocities and angles, heights, distances, and curvatures are studied. To this end, experiments are performed in the wind-tunnel for various wind velocities and snowpacks of different age and grain characteristics. The experiments are performed either with a shadowgraphic imaging technique (Shadowgraphy) or high-speed particle tracking velocimetry technique (PTV).

Fig. 2: Photograph of snowpack and measurement system setup in the wind-tunnel (left), shadowgraphy image with saltating snow particles (middle), and particle size recognition (right).