The Aiguille du Midi (Mont Blanc massif, European Alps): a unique high-Alpine site to study bedrock permafrost

Permafrost and its change in steep high-Alpine rock walls remain insufficiently understood because of the difficulties of in situ measurements. A large proportion of permafrost studies is mainly based on modelling, with a few existing instrumented sites and a resulting lack of process understanding. Yet, a number of rockfalls that occurred in the last decade in the Alps are likely related to climatically-driven permafrost degradation, as indicated by ice in starting zones, increased air temperature, and modelling studies. Starting off in the framework of the French-Italian PERMAdataROC project and presently under development within the EU co-funded project PermaNET (Permafrost long-term monitoring network: www.permanet-alpinespace.eu), our investigations at the Aiguille du Midi begin in 2005. The summit (3842 m a.s.l) is accessible from Chamonix by a cable car which was built at the end of the 1950s. Half a million tourists visit the site each year. Because of its elevation, geometry, and year-round accessibility to rock slopes of diverse aspects and to galleries, the site was chosen for: - Monitoring of the thermal regime in steep rock walls. Sensors with one or three thermistors were installed since 2005 at depths of 3, 10, 30 and 55 cm, and three 15-thermistor chains were set up in 10-m-deep boreholes this autumn, at all aspects and with slope angles in the range 60-90° (determining e.g. the presence and influence of snow). - Measurements of high altitude climatic data (air temperature and humidity, incoming and outgoing solar radiation, wind speed and direction) perpendicular to the rockwall surface, by movable automatic weather stations. Together with the rock temperature measurements, these data are used for physically-based model validation or statistical models construction of rock temperature distribution and variability in the rock walls. - Making an ‘in and out’ 3D-high-resolution DEM of the Aiguille by long-range (rock walls) and short-range (galleries) terrestrial laser scanning. - Surveying the distribution of permafrost in the rock mass and its seasonal evolution during the year using electrical resistivity tomography and laboratory testing of temperature-resistivity behaviour of the local granite. Complementary instrumentation and research (e.g. numerical modelling, laser scanning) are planned for the next months. The combination of process understanding, statistical analyses and/or modelling will help to improve our understanding of where, why and how permafrost degradation in mountains occurs. Secondly, we are interested in how a reduction in the uncertainty of data, process understanding and models may contribute to our predictive skill of corresponding effects.