Peering beneath the powder: Using radar to understand avalanches

High-resolution radar images from Switzerland’s experimental test site show that snow temperature is a key factor in classifying avalanche behavior.

By Terri Cook

Avalanches are major natural hazards for people and infrastructure in snowy, mountainous regions. During the last 40 years, they have caused an average of about 100 fatalities per year in the European Alps and many more worldwide. But although the dangers that avalanches pose have long been apparent, our incomplete understanding of their complex behavior has limited the development of physical models necessary to design effective mitigation measures.

To help advance our understanding of these hazards, Köhler et al. report the results of six seasons of avalanche observations at the WSL Institute for Snow and Avalanche Research’s Vallée de la Sionne test site in western Switzerland. Using a custom-designed, high-resolution radar system, the team was able to monitor flow processes in the underlying dense region of avalanches that is usually obscured by a cloud of powdery snow.

The results show that the 77 imaged events displayed a wide variety of flow behaviors, including several stopping mechanisms mainly determined by the snow’s temperature and liquid water content, which help control the snow’s cohesiveness. Warm snow, for example, tends to stop near the front of an avalanche and cause the following snow to pile up, whereas cold snow typically stops toward the avalanche’s tail, allowing only a small portion of the material to reach the final runout.

By combining the radar data with information on snow cover conditions, the team identified seven distinct avalanche flow regimes. Whereas small- to medium-sized avalanches typically exhibit one regime, the researchers found that larger avalanches—which experience a large range of snow conditions along their track—tend to be more complex, transitioning between flow regimes and/or exhibiting multiple flow regimes at the same time.

By demonstrating the extensive variety of behaviors that avalanches can exhibit, the Vallée de la Sionne data demonstrate the limits of current avalanche propagation models, which are typically based upon the basic distinction between dense and powder avalanches. In addition to serving as a valuable source of data for advancing physical avalanche models, this study will be of interest to researchers studying similar types of flows, such as turbidity currents. This study clearly points out the need for additional research to effectively model the different flow regimes and the transitions between them.

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