Simulating avalanche problem types to assess avalanche climate zones in the French Alps

Snow avalanches result from a complex interaction of weather and terrain, where past weather as well as internal snow cover processes play an important role. Snow cover models account for these processes and simulate the snow cover at a level of detail that allows to describe snow instability. That information on snow instability is required to assess avalanche climates rather than snow climates – which represents a classification based on weather observations. Running the model SURFEX/Crocus with long-term meteorological data (S2M reanalysis) covering the winter seasons between 1958 and 2020, we eventually derived the avalanche problem types for the 23 massifs representing different regions of the French Alps at daily resolution. This allows us to create a climatology based on avalanche problem types and study differences between mountain regions in the French Alps.

In a first step, we applied a commonly used snow climate classification to understand how in the different Alpine regions seasonal characteristics fluctuate under continental or coastal influence. In some regions the majority of the winter seasons had coastal characteristics, whereas in other regions almost half of the seasons were classified continental. In a second step, we add snow instability information by simulating avalanche problem types. This allows us to explore snow instability patterns that result from the meteorological forcing that the snow climate classification summarizes but which the snow climate classification cannot fully capture. Across the regions persistent weak layers and wet snow were the most common avalanche problems types on days when the model expected natural release. Moreover, we applied a k-means clustering to the frequencies of new snow, persistent and wet snow avalanche problem types to identify similarities between in the Alpine regions. We assessed the clustering performance and found that 4 clusters separate well our data which includes information on duration and frequency of avalanche problem types. The clusters coincide with the geographic location of the regions, i.e. Northern, Southern, inner-alpine or front-range regions have specific characteristics that manifest in frequency of avalanche problem types.

We showed how avalanche problem types can be used as an additional descriptor to the snow climate classification to include snow instability specific information. As avalanche problem types identified spatial differences between different snow climate regions, we are now ready to analyze how those changes evolve with time in the different regions.