Modeling transient three-dimensional temperature fields in mountain permafrost

Abstract

Permafrost is a common and important thermal phenomenon in the subsurface of high mountains. Its degradation due to climate change may lead to adverse effects, such as decreasing stability of infrastructure or an increase in rock falls from permafrost slopes. Research intensively deals with assessing and modeling mountain permafrost conditions, since they are not directly visible and extrapolation of measurements is difficult in steep and inhomogeneous terrain. So far, focus of mountain permafrost modeling studies was typically on the thermal conditions of the near-surface. The objective of this thesis is to describe and analyze mountain permafrost in greater depths (i.e., below the active layer), with a main focus on its three-dimensional distribution and transient effects from past and possible future climate variations. We design a modeling procedure that includes the processes in the atmosphere, at the surface, and in the subsurface, and which is based on the combination of existing approaches. To calculate subsurface temperature fields we determine ground surface temperatures with a distributed surface energy balance model based on climate time series. The result is used as upper boundary condition in a heat conduction scheme. For the simulation of scenarios of future permafrost conditions, we use climate time series constructed from Regional Climate Model results. The modeling procedure is tested in a number of validation steps, namely sensitivity studies and comparisons with field data. Due to the complex situation in nature, we first apply the modeling procedure to a number of idealized test cases with simplified topography, typical surface and subsurface characteristics, and different climate scenarios. The subsequent application to real topography includes the characterization of permafrost conditions at borehole sites and rock fall starting zones. The results indicate a three-dimensional distribution pattern of mountain permafrost, which is mainly influenced by topography and spatially and temporally variable surface temperatures. Isotherms incline steeply and strong lateral heat fluxes exist, which minimizes the influence of the geothermal heat flux. Transient signals from past climate variations in current permafrost temperatures derive mainly from the last glacial period and the major fluctuations during the past millennium. Temperature depressions in high mountains are smaller than in flat terrain, because multi-lateral warming in steep topography accelerates the pace of a surface temperature signal intruding into the subsurface. Simulations of future conditions point to subsurface temperature fields that substantially deviate from stationary conditions and are characterized by deep-reaching and long-term perturbations, generally rising temperatures, and an increase in both volume and vertical extent of warm permafrost zones. In the European Alps, near-surface permafrost on steep south-exposed slopes will have disappeared even in the highest peaks within the next two centuries, but substantial permafrost volumes will remain at depth for centuries to millennia. Three-dimensional and transient modeling tools are important for the comprehensive analysis of mountain permafrost conditions, since in complex mountain topography nearsurface conditions do not provide sufficient information. For future research the influence of variable surface and subsurface characteristics on the subsurface thermal field, the interpretation of temperatures recorded in boreholes in mountain topography, and the re-analysis of past rock fall events from permafrost slopes are important topics.