Recent changes in avalanche activity in the French Alps and their links with climatic drivers: an overview

[Departement_IRSTEA]Eaux [TR1_IRSTEA]RIVAGEInternational audienceThis paper synthetizes our ongoing work on relations between natural avalanche activity and climate change in the French Alps and subregions. Firm results mainly concern occurrences, runout altitudes and high return period avalanches on long time scales (averages over “full” winters and winter-spring sub-seasons) since ~1950. Work in progress concerns extrapolation under future climate, shorter time scales (avalanche cycles), and more generally risk assessment under unstationarity. The strength and interest of the approach rely on the exceptional quality/quantity of avalanche records and snow and weather covariates available/used and on the development of specific statistical treatment methods

Time trends in avalanche activity and links with climatic drivers in the French Alps

International audiencePrevious studies described growing evidences or recent climate fluctuations in the alpine scale. A net warming is now well established since the end of the Little Ice Age, accelerated since 1985. One of the most direct impacts is a snow cover decrease at low and mid elevations, both in terms of local snow depth and snow cover duration, but also an increased variability, especially for winter temperatures. To study climatic variations in mountains area, snow avalanches are currently extremely rarely used for this purpose because their response to climatic fluctuations remains largely unknown. It is mainly due to the difficulty to assess its climatic control. Indeed, avalanche release has not a linear response to snowpack evolution ; moreover, controlling factor, such as temperature, can increase or decrease the snowpack stability. An other major limitation for the avalanche activity study is the difficulty of collecting data, especially during winter. Understanding the links between climate and avalanche activity is important: from an operational point of view (forecasting and risk management in mountains area) ; from a more fundamental interest: as natural avalanche activity is directly controlled by the quantity and quality of available snow cover, could it be therefore an attractive indicator of climate change

Sulfur cycle at Last Glacial Maximum: Model results versus Antarctic ice core data

International audienceFor the first time, an atmospheric general circulation and sulfur chemistry model is used to simulate sulfur deposition in Antarctica at the Last Glacial Maximum (LGM). Dimethylsulfide (DMS), emitted by phytoplankton, is the dominant source of atmospheric sulfur in Antarctica. Once in the atmosphere, it is oxidized into sulfur aerosols which are measured in ice cores. Such measurements allow for validating climate and chemistry models for glacial‐interglacial changes. Our glacial simulations test the effect of a recent re‐evaluation of glacial sea‐ice coverage on DMS sources and sulfur aerosol deposition. Using the present‐day oceanic concentrations of DMS, the model reproduces observed glacial and interglacial sulfur concentrations in the ice. This result suggests that climate change at the LGM did not greatly impact on DMS production in the oceanic sectors where sulfur aerosols deposited in central East Antarctica originate from

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

International audienceSnow avalanche activity is controlled to a large extent by snow and weather patterns. However, its response to climate fluctuations remains poorly documented. Previous studies have focused on direct extraction of trends in avalanche and winter climate data, and this study employs a time-implicit method to model annual avalanche activity in the French Alps during the 1958–2009 period from its most representative climatic drivers. Modelled snow and weather data for different elevations and aspects are considered as covariates that explain actual observed avalanche counts, modelled instability indexes, and a combination of both avalanche activity indicators. These three series present relatively similar fluctuations over the period and good consistency with historically harsh winters. A stepwise procedure is used to obtain regression models that accurately represent trends as well as high and low peaks with a small number of physically meaningful covariates, showing their climatic relevance. The activity indicators and their regression models seen as time series show, within a high interannual variability, a predominant bell-shaped pattern presumably related to a short period of colder and snowier winters around 1980, as well as a very slight but continuous increase between 1975 and 2000 concomitant with warming. Furthermore, the regression models quantify the respective weight of the different covariates, mostly temperature anomalies and south-facing snowpack characteristics to explain the trends and most of the exceptional winters. Regional differences are discussed as well as seasonal variations between winter and spring activity and confirm rather different snow and weather regimes influencing avalanche activity over the Northern and Southern Alps, depending on the season

fluctuations over 50 yr in the French Alps

Snow and weather climatic control on snow avalanche occurrenc

Micropollutants removal efficiency of stormwater control measures: comparison of centralized vs source control systems

International audienceMany studies have analysed and quantified stormwater runoff polluting impact in water bodies. In the meantime municipalities encourage the use of stormwater control measures (SCM) to reduce waterflows and pollutant contamination. In this context, the Micromegas project aims at i) determining how to evaluate the efficiency of SCMs regarding micropollutant (MP) removal and ii) comparing the efficiency of centralized "end of pipes" SCMs versus source control systems. Their efficiency is determined through in-situ sampling, the quantification of 54 MP and the comparison of concentration loads at the outlet of a retention basin (centralized system) and three source control systems (a swale, a trench and a porous pavement) with respectively water from the supplying pipe network and the outlet of a similar impervious asphalted site. This article presents our first results regarding these four sites efficiency as well as the monitoring and sampling procedures and the comparison methodology

Projected changes of snow conditions and avalanche activity in a warming climate: the French Alps over the 2020-2050 and 2070-2100 periods

International audienceProjecting changes in snow cover due to climate warming is important for many societal issues, including the adaptation of avalanche risk mitigation strategies. Efficient modelling of future snow cover requires high resolution to properly resolve the topography. Here, we introduce results obtained through statistical downscaling techniques allowing simulations of future snowpack conditions including mechanical stability estimates for the mid and late 21st century in the French Alps under three climate change scenarios. Refined statistical descriptions of snowpack characteristics are provided in comparison to a 1960-1990 reference period, including latitudinal, altitudinal and seasonal gradients. These results are then used to feed a statistical model relating avalanche activity to snow and meteorological conditions, so as to produce the first projection on annual/seasonal timescales of future natural avalanche activity based on past observations. The resulting statistical indicators are fundamental for the mountain economy in terms of anticipation of changes. Whereas precipitation is expected to remain quite stationary, temperature increase interacting with topography will constrain the evolution of snow-related variables on all considered spatio-temporal scales and will, in particular, lead to a reduction of the dry snowpack and an increase of the wet snowpack. Overall, compared to the reference period, changes are strong for the end of the 21st century, but already significant for the mid century. Changes in winter are less important than in spring, but wet-snow conditions are projected to appear at high elevations earlier in the season. At the same altitude, the southern French Alps will not be significantly more affected than the northern French Alps, which means that the snowpack will be preserved for longer in the southern massifs which are higher on average. Regarding avalanche activity, a general decrease in mean (20-30 %) and interannual variability is projected. These changes are relatively strong compared to changes in snow and meteorological variables. The decrease is amplified in spring and at low altitude. In contrast, an increase in avalanche activity is expected in winter at high altitude because of conditions favourable to wet-snow avalanches earlier in the season. Comparison with the outputs of the deterministic avalanche hazard model MEPRA (Modele Expert d'aide a la Prevision du Risque d'Avalanche) shows generally consistent results but suggests that, even if the frequency of winters with high avalanche activity is clearly projected to decrease, the decreasing trend may be less strong and smooth than suggested by the statistical analysis based on changes in snowpack characteristics and their links to avalanches observations in the past. This important point for risk assessment pleads for further work focusing on shorter timescales. Finally, the small differences between different climate change scenarios show the robustness of the predicted avalanche activity changes

Comparaison croisée de données météorologiques et avalancheuses pour la caractérisation de cycles avalancheux: l'exemple de Décembre 2008 dans le sud est des Alpes Françaises.

International audienceIn December 2008, an intense avalanche cycle occurred in the eastern part of the southern French Alps. Southerly atmospheric fluxes that progressively evolved into an easterly return caused important snowfalls with return periods up to 10 years. Cold temperatures and drifting snow had important aggravating effects. The return period for the number of avalanches was above 50 years in two massifs and some of the avalanche had very long runouts that exceeded historical limits recorded in the French avalanche atlas. Using this case study, this paper illustrates and discusses how avalanche reports, snow and weather data and results from numerical modelling of the snow cover call be combined to analyse abnormal temporal clusters of snow avalanches. For instance, it is shown how statistical techniques developed in other fields can be used to test the significance of different explanatory factors, extract spatio-temporal patterns, compare them with previous cycles and quantify the magnitude/frequency relationship at different scales