Thermal characteristics of permafrost in the steep alpine rock walls of the Aiguille du Midi (Mont Blanc Massif, 3842 m a.s.l)

Abstract

Abstract. Permafrost and related thermo-hydro-mechanical processes are thought to influence high alpine rock wall stability, but a lack of field measurements means that the characteristics and processes of rock wall permafrost are poorly understood. To help remedy this situation, in 2005 work began to install a monitoring system at the Aiguille du Midi (3842 m a.s.l). This paper presents temperature records from nine surface sensors (eight years of records) and three 10 m deep boreholes (4 years of records), installed at locations with different surface and bedrock characteristics. In line with previous studies, our temperature data analyses showed that: micro-meteorology controls the surface temperature, active layer thicknesses are directly related to aspect and ranged from < 2 m to nearly 6 m, and that thin accumulations of snow and open fractures are cooling factors. Thermal profiles empirically demonstrated the coexistence within a single rock peak of warm and cold permafrost (about −1.5 to −4.5 • C at 10 m depth) and the resulting lateral heat fluxes. Our results also extended current knowledge of the effect of snow, in that we found similar thermo-insulation effects as reported for gentle mountain areas. Thick snow warms shaded areas, and may reduce active layer refreezing in winter and delay its thawing in summer. However, thick snow thermo-insulation has little effect compared to the high albedo of snow which leads to cooler conditions at the rock surface in areas exposed to the sun. A consistent inflection in the thermal profiles reflected the cooling effect of an open fracture in the bedrock, which appeared to act as a thermal cutoff in the sub-surface thermal regime. Our field data are the first to be obtained from an Alpine permafrost site where borehole temperatures are below −4 • C, and represent a first step towards the development of strategies to investigate poorly known aspects in steep bedrock permafrost such as the effects of snow cover and fractures.

Challenges and solutions for long-term permafrost borehole temperature monitoring and data interpretation

“…The data obtained are also essential for calibrating and validating numerical simulations (e.g. Haberkorn et al, 2015a, b;Magnin et al, 2015;Noetzli et al, 2010).…”

Mountain permafrost and associated geomorphological processes: recent changes in the French Alps

“…), NW (3738 m) and NE (3745 m) faces of the Central Piton. Each of them was equipped with a chain of 15 thermistors (accuracy ± 0.1°C) measuring temperature between 30 cm and 10 m in depth every 3 h (Magnin et al, 2015b). The mean annual temperatures at 10 m indicated a warm permafrost in the S face (-1.5°C) and a cold one in the NW (-4.5°C, Fig.…”

“…Since 2005, significant efforts have been made to study both the thermal state of permafrost, which reflects past and current climates in high mountain areas, and the associated geomorphological dynamics. Instrumented boreholes, subsurface thermal measurements, and monitoring of morphodynamics are used to characterise the status of the permafrost in rock walls (Magnin et al, 2015b) or in surficial deposits (Bodin et al, 2009;Schoeneich et al, 2014) in the French Alps and its response to ongoing climate change. Most of this research is part of the French observation and permafrost monitoring network PermaFRANCE (Schoeneich et al, 2010).…”

Distributed snow and rock temperature modelling in steep rock walls using Alpine3D

“…Measuring rock wall temperatures (e.g. Gruber et al, 2004a;Haberkorn et al, 2015a;Hasler et al, 2011;Magnin et al, 2015;PERMOS, 2013) and in a further step modelling the spatial permafrost distribution in steep rock walls is therefore of great importance.…”

“…The highly variable spatial and temporal distribution of the snow cover strongly influences the ground thermal regime of steep rock faces (Haberkorn et al, 2015a, b;Magnin et al, 2015) due to the high surface albedo and low thermal conductivity of the snow cover, as well as energy consumption during snow melt (Bernhard et al, 1998;Keller and Gubler, 1993;Zhang, 2005). In gently inclined, blocky terrain, effective ground surface insulation from cold atmospheric conditions was observed and modelled for snow depths exceeding 0.6 to 0.8 m (Hanson and Hoelzle, 2004;Keller and Gubler, 1993;Luetschg et al, 2008).…”