The intermittency regions of powder snow avalanches.

Powder snow avalanches are typically composed of several regions characterized by different flow regimes. These include a turbulent suspension cloud of fine particles, a dense basal flow and an intermittency frontal region, which is characterized by large fluctuations in impact pressure, air pressure, velocity and density, but whose origin remains unknown. In order to describe the physical processes governing the intermittency region, we present data from four large powder snow avalanches measured at the Vallée de la Sionne test site in Switzerland, which show that the intermittency is caused by mesoscale coherent structures. These structures have a length of 3–14 meters and a height of 10 m or more. The structures can have velocities as much as 60% larger than the avalanche front speed and are characterized by an air/particle mixture whose average density can be as high as 20 kg m−3. This average density increases the drag on large granules by a factor of up to 20 compared to pure air, so that each structure can maintain denser snow clusters and single snow granules in suspension for several seconds. The intermittency region has importance for the dynamics of an avalanche, as it provides an efficient mechanism for moving snow from the dense layer to the powder cloud, but also for risk assessment, as it can cause large forces at large heights above the basal dense layer.