Réf. Martin & al. 2001 - A

Référence bibliographique complète
MARTIN, E. GIRAUD, G. LEJEUNE, Y. BOUDART, G. Impact of climate change on avalanche hazard. Annals of glaciology, 2001, n°32, p 163-167.

Abstract: The SCM software has been used to assess the climatology of the avalanche hazard and its sensitivity to climate change. A natural avalanche-hazard index based on MEPRA analysis was defined and validated against natural avalanche observations. A 15 year climatology then has allowed a comparison of avalanche hazard in the different French Alps massifs. The sensitivity to climate change can only be considered as a preliminary study, because of the very simple scenario. It appears that the avalanche hazard decreases slightly in winter, but the decrease is more pronounced in February and May-June. The relative importance of new-snow avalanches is expected to diminish while the relative proportion of wet-snow avalanches should increase. At this stage, one can hardly assess the impact of this scenario on the hazard of avalanches triggered by skiers or the frequency of major avalanche events, which are strongly related to extreme events.

Mots-clés

Avalanche hazard, models, simulations, climate change, French Alps


Organismes / Contact
Centre d'Etudes de la Neige / Centre National de Recherches Météorologiques / Météo France
eric.martin@meteo.fr

(1) - Paramètre(s) atmosphérique(s) modifié(s)
(2) - Elément(s) du milieu impacté(s)
(3) - Type(s) d'aléa impacté(s)
(3) - Sous-type(s) d'aléa
Temperatures and precipitations

Snowpack

Snow avalanche

 

Pays / Zone
Massif / Secteur
Site(s) d'étude
Exposition
Altitude
Période(s) d'observation

French Alps

All french Alps massifs    

1500-3000 m asl

1984-1998


(1) - Modifications des paramètres atmosphériques
Reconstitutions
 
Observations
For the 1984-1999 periode, precipitation is maximum in the northwest part (> 600 mm/ year at 1500 m asl) and decreases toward the southeast (about 200 mm/year). A secondary maximum is located in the extreme southeast, associated with the occurrence of Mediterranean lows.
Modélisations
 
Hypothèses
 

Informations complémentaires (données utilisées, méthode, scénarios, etc.)
Mean of snowfall analysed by SAFRAN at 1500 m asl

(2) - Effets du changement climatique sur le milieu naturel
Reconstitutions  
Observations
 
Modélisations

According to the climate change scenarios, the snow coverage is modified (especially at middle elevation). In the PT scenario (+ 1.8°C and + 10% precipitation), the snow cover duration is diminished by 30-40 days a-1 at 1500 m. At high elevation (3000 m), changes are small and snow coverage can be considered stable.

Hypothèses
 

Sensibilité du milieu à des paramètres climatiques
Informations complémentaires (données utilisées, méthode, scénarios, etc.)
Sensitivity of snow cover to precipitation, temperature, radiatons, etc.
SCM is used to simulate the snow coverage.

(3) - Effets du changement climatique sur l'aléa
Reconstitutions  
Observations
 
Modélisations

According to the simulated avalanche hazard for the 1984-1998 period, the number of days with a moderate or high avalanche hazard varies between 107 (Mt Blanc) and 28 (Ubaye) per year. The delineation follows the spatial distribution of precipitation. The lowest massifs of the Pre-Alps show fewer days with moderate or high avalanche index than their neighbours.

3 types of avalanche-hazard have been determined by MEPRA: new-snow (precipitation or fragmented particules), wet-snow (wet grain) and mixed (both types are present). In the north, new-snow avalanche hazard is predominant, while wet-snow avalanche hazard is predominant in the south, in conjunction with warmer and sunnier conditions. Days with mixed avalanches occur when the snow/rain limit is relatively high (1500 m or above). They are minimum in the southeast, where this type of situation is rarely encountered.

The interannual variability of the avalanche hazard is very high and the number of days with high or moderate avalanche hazard can double from a year to another. The lowest values are obtained during the driest winters and in this case, wet-snow avalanche hazard is at least equal to new-snow avalanche hazard. Precipitation is a key factor for natural avalanche hazard: the correlation between the number of days with moderate or high avalanche hazard and winter precipitation at 1500 m is significant (0.75), while correlation with winter temperature is low (0.45).

According to the climate change scenarios, the number of days with moderate or high avalanche hazard decreases in all massifs. Variations are higher in the north, where reference values are high. The highest relative variations are encountered in the Vercors, Chartreuse and Bauges massifs, where snow height and duration diminish drastically because of their lower elevation. In the other regions, the number of days with moderate or high avalanche hazard is diminished by 5-9 days/year. Partial scenarios indicate that this number increases with P and decreases with T. For example, in the Mt Blanc massif: presently 107 days a-1; PT 96 days a-1; P 120 days a-1; T 84 days a-1. The evolution of the mean avalanche-hazard index between November and June in the same massif is maximum in February and secondary in May. In the PT scenario, the partial index for wet snow and mixed avalanches increases systematically except in May and June (decrease in snow-cover duration). In contrast, the partial index for new-snow avalanches decreases. The full index decreases throughout the winter (except in March), but variations cannot be considered important when looking at the daily variability of this index. The maximum decrease can be seen in May and June because of the wet snow avalanches. The evolution of extreme events follows the evolution of mean parameters and the number of days with indexes higher than 7 decreases significantly in scenarios PT and T.

Hypothèses
 

Paramètre de l'aléa
Sensibilité du paramètres de l'aléa à des paramètres climatiques
Informations complémentaires (données utilisées, méthode, scénarios, etc.)
Avalanche occurrence Temperature and precipitation

The SAFRAN/Crocus/MEPRA (SCM) software has been used to assess the present avalanche activity and its modifications in a changed climate. An avalanche-hazard index has been constructed and validated against observations. Then the spatial and temporal variability of this index during the past 15 years has been discussed.

Avalanche activity is observed twice a day in the French snow and weather network by visual observation of past avalanches using a very simple code (no avalanche, small avalanches, 1, 2, 3-5, 6-10, more than 10 avalanches). At the massif scale (500-1000 km2) the avalanche activity has been summarized through an avalanche-hazard index, which allows a good estimation of natural avalanche release.

The MEPRA estimation of the avalanche hazard is available at a 3 hour time-step, with a vertical discretization of 300 m (1500-3000 m asl) and for six aspects (north, west, south, east, southeast and southwest). For each analysis, the MEPRA system deduces from the Crocus snowpack simulations additional characteristics (shear strength, ram resistance and grain types). After classifying the ram and stratigraphical profile, this model studies the natural mechanical stability of the snowpack.

In a first step, the usual stability index (shear strength/snow shear stress) is calculated for each layer of each simulated snowpack. Depending on the value and the temporal evolution of this index, a natural avalanche risk is deduced on a six-level scale (very weak, weak, moderate increasing, moderate decreasing, high and very high) completed with the avalanche types (fresh dry, fresh wet, fresh mixed, surface slab, surface wet, bottom wet). In case of wet snow, the calculated index is completed by a diagnostic based on the increase of wetted-layers depth. In a second step, the expert system interprets the snowpack structure to detect the possible release of a dry slab avalanche by a skier and then deduce a MEPRA accidental-avalanche risk in a four-level scale.

Several different indexes have been tested and correlations were calculated for winter 1986/87 in the Vanoise massif. The “daily maximum of the mean by aspect” has been chosen: the mean of the weighted indexes between 1500 and 3000 m is calculated for each aspect every 3h, and the final avalanche index is the maximum of all the calculated values. This index takes into account the exposure, as well as the fact that natural release of avalanches occurs most often when the daily instability is maximum. The correlation between the modelled and the observed index was 0.53 for 1986/87 and 0.63 for 1994/95 in the Vanoise massif. Further evaluations were made using contingency tables and 3 classes and their frequencies (weak: 75%, moderate: 15%, high: 10%) were defined. The corresponding thresholds were determined for the normalized avalanche-activity index and applied to the normalized modelled indexes.

SCM has been used to simulate the snow coverage and the avalanche hazard of the past 15 years (1984-1998) in the French Alps. The avalanche-hazard index was calculated all year round (except July and August) using the procedure previously defined. The thresholds for moderate and high avalanche-hazard index were calibrated using all massifs and 5 winter months (December-April). Thresholds found were 0.4 and 1.6. For the lowest massifs, the index was based on a limited number of elevations. Finally, the SCM software has been used to compare the January-February 1999 avalanche episodes to past extreme events. Episodes with an avalanche-hazard index higher than 7 (between high and very high) for at least 2 days and 2 massifs have been compiled.

The sensitivity of avalanche hazard to climate change was estimated with simple methods such as constant perturbations of the meteorological variables analysed by SAFRAN. Crocus and MEPRA were run perturbed data, and the results were compared to those of the reference run detailed previously. Because both a general warming and increased precipitation are expected for the next century, 3 runs (a full scenario and 2 partial ones) were made: a temperature rise of 1.8°C coupled with a precipitation increase of 10% (PT); a temperature rise of 1.8°C (T); and a precipitation rise of 10% (P). In experiments PT and T, the critical temperature (at which the precipitation turns from snow to rain) is fixed at 1.5°C.


(4) - Remarques générales

 


(5) - Syntèses et préconisations