Thermal diffusivity of permafrost in the Swiss Alps determined from borehole temperature data

Abstract. Mountain permafrost is warming and thawing globally due to climate change. Its mechanical properties largely depend on ground temperature, whereby the primary process of heat transfer in frozen ground is heat conduction. Thermal diffusivity quantifies the rate of heat propagation in a material and is thereby a key thermal property, but no empirical values for mountain permafrost substrates are currently available. In this study, we derive the thermal diffusivity of different mountain permafrost landforms and substrates in the Swiss Alps empirically for the first time. To do so, we perform a linear regression analysis of the heat diffusion equation and validate the derived thermal diffusivity with inversions of numerical and analytical solutions. As a data basis, we systematically analyze data from the 29 temperature boreholes of the Swiss Permafrost Monitoring Network PERMOS, which allows us to investigate the natural variability of thermal diffusivity in space and time and derive a well-constrained range of thermal diffusivity in mountain permafrost (25- to 75-percentile range: 1.1–3.3 mm² s^-1) and the overlying active layer (25- to 75-percentile range: 0.8–2.4 mm² s^-1). While we find only small but significant (p<0.01) differences in diffusivity between the landforms for all three approaches, strong spatio-temporal variations are identified. Our results complement our understanding of the thermal properties of permafrost and thus directly offer potential implications for the development and application of new ground temperature and energy-balance models. Furthermore, we discuss the potential to indirectly identify short-term non-conductive heat fluxes by isolating discrepancies between observations and model predictions of temperature rate variations with time. The quantification of non-conductive heat fluxes is still poorly constrained due to their strongly non-linear nature and the inherent challenges in their measurement. Non-conductive heat fluxes point to the presence of water and/or air circulation in the permafrost. Water can significantly influence the mechanical properties of permafrost substrates. The dynamics of unstable slopes are increasingly being driven by water infiltration related to ice loss within the permafrost. Therefore, our method and results open new possibilities in permafrost science, hydrogeology, natural hazard studies, and practical applications such as high-mountain construction technology.