Abstract : K.3
Climatic trends for the last 45 winters in a French Alps region

Jean-Pierre Navarre, Gérald Giraud, Yves Durand, Daniel Goetz

Since the early 1990s Météo-France has used an automatic system of three numerical models that simulates meteorological parameters, snow cover stratigraphy and avalanche risks at various elevations, aspects and slopes for several mountainous massifs of the French Alps. Due to the lack of sufficient amount of directly observed long-term snow data, this SAFRAN-CROCUS-MEPRA model chain, usually applied for operational avalanche forecasting, is here used for retrospective snow and weather climate analyses during the 1958/2004 winter period for a region of the French Alps. The used method merges all the available observed information with guess-fields values issued from the newly reanalysed atmospheric data (ERA-40) of the European Centre for Medium-Range Weather Forecasts.

Since the middle of the 1980 years and below 2000 m, the winter air temperature has been rising near 2 °C. However, variations are large at varying altitudes with a positive trend near 1°C at 3000m. A main characteristic is also a large year-to-year variability with regard to the smoothed trend lines.

Precipitation trends are rather diverse making it hard to detect clear tendencies. As a general trend the winter precipitation is slightly rising (2mm per year). Year-to-year variability is big but trends remain uniform at varying elevations. Snow precipitations trends follow the temperatures changes with decreasing snowfalls below 2000 m.

Snow parameters in this region of the French Alps are characterised by a marked declining gradient from the north-western foothills to the south-eastern interior massifs. Enhanced at low elevations, trends in snow depth evolution are mainly negative in relation with temperature trends.

About avalanche activity, year-to-year variability is also big. No real trends are observed even if the cold snowfalls seem to be fewer these last years.

This climatology could then be used as reference for studies of snow depth evolution under the effect of a climatic change.