Analysis and forecast of extreme new-snow avalanches: a numerical study of the avalanche cycles of February 1999 in France

Abstract

Snow and weather conditions typical of exceptional cycles of fresh-snow avalanches in the northern Alps are investigated using the numerical avalanche-hazard forecasting procedure of Météo-France. Sensitivity tests are performed on the events of February 1999 in the Chamonix France region, and resulting snowpack instability modeled at the massif scale is compared using adapted new indices and maps. Our results complete conclusions of earlier observation-based studies by providing new insights into the snow and weather conditions of February 1999. The large avalanches mainly resulted from large and very unstable fresh-snow accumulations. Moreover, the snowpack instability was increased locally by wind transport of light and fresh snow in February. The mechanical weaknesses resulting from the weather conditions prior to February were a key factor in explaining the unusual volumes of these avalanches. This study suggests that the operational numerical SAFRAN/Crocus/ MÉPRA (SCM) chain provides reliable forecasts of extreme new-snow avalanche situations at the massif scale, but that local-scale simulations are still needed to improve the efficiency of risk mitigation and civil protection policies.

Adding Expert Contributions to the Spatiotemporal Modelling of Avalanche Activity Under Different Climatic Influences

“…in this region. He also knows the location of the major avalanche paths and has specific terrain knowledge regarding the respective activity in different townships under different atmospheric flow conditions, for instance major winter storms (Rousselot et al, 2010;Eckert, Coleou, Castabrunet, Deschatres, Giraud and Galim, 2010). However, the map is a multi-dimensional object and the expert must make a single judgement about the entire map, by integrating all of his spatial knowledge.…”

Snow and weather climatic control on snow avalanche occurrence fluctuations over 50 yr in the French Alps

“…For instance, the famous avalanche cycle of February 1999 was due to heavy snowfalls and cold temperatures over the northern massifs. It included a major avalanche that killed 12 persons in Montroc, Chamonix valley Rapin and Ancey, 2000;Rousselot et al, 2010), and also caused widespread damage in Europe, including the Austrian and Swiss Alps (SLF Davos, 2000). Similarly, the December 2008 avalanche cycle in the eastern part of the Southern French Alps, generated by easterly fluxes early in the season, caused considerable traffic disturbances and damaged or destroyed equipment and buildings (Eckert et al, 2010c).…”

The detailed snowpack scheme Crocus and its implementation in SURFEX v7.2

“…Nevertheless, such models are best suited for reproducing the evolution of a snow season under the forcing of meteorological conditions, as demonstrated by the results of the Snow Model Intercomparison Project (Etchevers et al, 2004). Operationally, they are used in the field of avalanche risk forecasting, where the knowledge of detailed information on the vertical layering of the snowpack is critical (Durand et al, 1999;Rousselot et al, 2010). Regional or global simulations in coupled mode have been seldom carried out due to high computational costs .…”