12th Congress INTERPRAEVENT 2012 Grenoble / France – Extended Abstracts www.interpraevent.at
TIME TRENDS IN AVALANCHE ACTIVITY AND LINKS WITH CLIMATIC DRIVERS IN THE FRENCH ALPS
Hélène Castebrunet 1and Nicolas Eckert2
INTRODUCTION
To study climate fluctuations in mountainous areas, long weather data series are seldom, especially at high elevations (Beniston et al., 1997), and using proxy indicators such as glacier mass balances is now quite well accepted. Snow avalanches are strongly controlled by climatic parameters, but currently extremely rarely used for this purpose. However, the question of the response of snow avalanche activity to climate fluctuations is crucial for hazard assessment, at least on the long range.
Hence, the aim of this work is to quantify the evolution of different variables characterising avalanche activity in the French Alps over the last decades, and its links with climate.
DATA AND METHODS
The work is based on a chronicle, the « Enquête Permanente sur les Avalanches » (EPA) describing the avalanche events on approximately 3900 determined paths in the French Alps and Pyrenees, with quantitative (runout altitudes, deposit volumes, etc.) and qualitative (flow regime, release cause, etc.) data. Despite numerous sources of uncertainty to be kept in mind, this database offers a unique opportunity to quantify the recent evolution of snow avalanches.
Snow and weather covariates are from the Safran-Crocus-Mepra model chain (Météo France) used since the early 1990s to simulate meteorological parameters, snow cover stratigraphy, and avalanche danger at various elevations, aspects, and slopes for 23 massifs in the French Alps. Output of such simulations are precious to characterise the winter climate of the French Alps over the period of interest (Durand et al., 2009a; 2009b).
Fig. 1 Annual fluctuations of avalanche counts per winter for a mean path of the French Alps. A) Annual trend and exceptional winters. B) Low frequency trend. C) Mean frequency trends.
1 Hélène Castebrunet. Météo France Centre d’Etude de la Neige – Cemagref UR UTGR.
2 Nicolas Eckert. Cemagref, UR ETGR, 38 402 Saint Martin d’Hères, France (email: nicolas.eckert@©cemagref.fr)
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Adapted statistical methodologies, mainly variance decompositions and regressions, have been developed to extract the predominant temporal patterns from the avalanche data (Eckert et al., 2010a;
2010b) and relate them to similar evolution in the snow and weather covariates.
RESULTS
Regarding avalanche occurrences, no significant and continuous trend exists over the last 50 years. On the other hand, a nice low frequency bell shaped trend is detected, with a maximum between 1976/77 and 1985/86, highlighting a short period of relatively harsher winters. At higher frequencies, the harsh winters of the beginning of the 1950’s are also highlighted. However, the interannual variability around these trends is strong, linked with the interannual variability of snow and weather conditions.
Thus, with a "classical" detection threshold, the reference winters 1950/51, 1977/78, 1985/86, 1994/95 and 1998/99 are detected as abnormally strong in terms of avalanche activity (Figure 1). These annual fluctuations can be very well explained by a relatively simple regression model, both in terms of trends and peaks (Figure 2). Predominant control variables are temperature and snowpack characteristics for south facing slopes. Similar patterns can be detected, even enhanced in terms of amplitude of the low frequency trend, in avalanche runout elevations, which offers an interesting complementary view of the evolution of snow avalanche magnitude.
Fig. 2 Annual fluctuations of standardized avalanche counts per winter for a mean path of the French Alps versus regression model, and regression model smoothed over 5 and 20 year time windows.
REFERENCES
Beniston, M., Diaz, H. F., Bradley, R. S. (1997).Climatic change at high elevation sites: an overview.
Clim. Change 36 : 233-251.
Durand, Y., Laternser, M., Giraud, G., Etchevers, P., Lesaffre, L., Mérindol, L. (2009a). Reanalysis of 44 year of climate in the French Alps (1958–2002): methodology, model validation, climatology, and trends for air temperature and precipitation. JAMC 48 (3) : 429-449.
Durand, Y., Laternser, M., Giraud, G., Etchevers, P., Mérindol, L., Lesaffre, B. (2009b). Reanalysis of 47 Years of Climate in the French Alps (1958–2005): Climatology and Trends for Snow Cover.
JAMC 48 (12): 2487–2512.
Eckert, N., Parent, E., Kies, R., Baya, H. (2010a). A spatio-temporal modelling framework for assessing the fluctuations of avalanche occurrence resulting from climate change: application to 60 years of data in the northern French Alps. Clim. Change 101: 515-553.
Eckert, N., Baya, H., Deschâtres, M. (2010b). Assessing the response of snow avalanche runout altitudes to climate fluctuations using hierarchical modeling: application to 61 winters of data in France. J. Climate 23: 3157-3180.
Keywords: snow avalanches, winter climate change, time trends, statistical analyses
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