Thermal regime in steep permafrost rockwalls (Aiguille du Midi, 3842 m a.s.l., Mont Blanc massif) based on borehole data and 2D numerical modelling

• Main Authors: Magnin, Florence, Deline, Philip, Ravanel, Ludovic, Noetzli, Jeannette
• Other Authors: Environnements, Dynamiques et Territoires de Montagne (EDYTEM), Université Savoie Mont Blanc (USMB Université de Savoie Université de Chambéry )-Centre National de la Recherche Scientifique (CNRS), Glaciology, Geomorphodynamics and Geochronology, Department of Geography Zürich, Universität Zürich Zürich = University of Zurich (UZH)-Universität Zürich Zürich = University of Zurich (UZH)
• Format: Conference Object
• Language: English
• Published: HAL CCSD 2012
• Subjects: Active layer thickness; permafrost
• Online Access: https://hal.science/halsde-00878144

Since permafrost in steep rockwalls is mainly climatically controlled, the ongoing climate warming potentially strongly impacts frequency and magnitude of rockfalls in high-alpine rock faces in the near future. However, knowledge about the role of permafrost is still limited. This is especially because of measurement difficulties (invisibility of permafrost, remoteness and harshness in high mountain areas). In addition to local measurements, numerical models have thus been used in the past years to better understand the distribution and thermal conditions of permafrost and to investigate involved physical processes. To monitor permafrost in steep bedrock and to provide input and validation data for modeling, three 10-m- boreholes have been drilled in 2009 on the NW, NE and S faces of the Aiguille du Midi (AdM 3842 m a.s.l), a granitic peak in the Mont Blanc massif (France). Here, we study the thermal regimes at depth and heat diffusivity. Climatic parameters such as air temperature, wind speed and direction, incoming and outgoing solar radiations are also collected continuously by an automatic weather station. In this contribution, we present the first two years of data from the boreholes, in comparison with climatic parameters and outputs from a basic conductive 2D model. We gain a qualitative understanding of the processes governing the thermal regime of the central pillar of the Aiguille du Midi: (i) the importance of short-wave solar radiations on the most exposed rock faces, and air temperature control on other aspects, (ii) the impact of the snow cover, and (iii) the role of fractures on heat diffusivity and thermal regimes. 2010 and 2011 were respectively the coldest (mean annual air temperature: -9.1 C) and the warmest (-6.7 C) years since 2007 at the AdM. Solar radiations forcing is highlighted by fast thermal response of rockwalls experienced through the variation of the maximum Active Layer Thickness (ALT) from one year to another. In the SE face directly exposed to radiations, ALT was 2.7 m thicker ...