Snow on steep permafrost rock walls
The timing, thickness and duration of the snow cover all influence rock temperature and therefore permafrost occurrence. Until present, it was often assumed that the snow cover is thin or non-existent in steep rock walls, due to gravity-induced effects. However, we observe that deep snow can indeed accumulate in very steep terrain. Measuring and modelling the snow cover in rock walls is therefore necessary to understand its effects on rock temperature.
In this project we investigate snow distribution and snow cover characteristics in various permafrost rock walls and their influences on the thermal regime of the rock. In parallel, the mechanical influences of snow and meltwater on rock wall stability are being studied by our project partners at the University of Bonn and the TU Munich. The overall aim of the project is to investigate the impact of the snow cover on changes in the rock thermal regime and thus on the conservation or degradation of permafrost in steep rock walls.
The study sites are steep North- and South-oriented rock walls in the Swiss Alps at Gemsstock (central Swiss Alps), Steintälli (Valais Alps) and Jungfraujoch (Bernese Alps). The latter is located in continuous permafrost terrain, whereas Gemsstock and Steintälli are at the lower fringe of permafrost, where thermal changes in the rock can rapidly affect permafrost distribution and rock wall stability.
2014 - 2017
Snow cover distribution
Snow cover distribution and thickness in rock walls is measured using a terrestrial laser scanner at different times in the course of winter. Snow depths are determined by comparing the winter laserscans with scans of the snow-free walls in summer, so-called high resolution digital elevation models. Automatic cameras register valuable information on weather conditions, snow distribution, snowmelt patterns, avalanches and ephemeral summer snow. Snow depths are also registered by automatic weather stations.
Our observations show that the local micro-topographic rock structure strongly influences snow cover distribution and that deep snow can accumulate on 70° to 80° steep slopes, particularly with step-like terrain. However, vertical and overhanging rock is generally snow-free but can be covered in snow and rime during storms, provoking short-term latent heat exchanges at the rock surface. Surprisingly, snow can disappear earlier in shady slopes if the rock surface in the sunny slopes is smoother with a more homogeneously distributed snow cover.
For the first time, snow pits have been dug in steep rock walls and these reveal strongly contrasting and aspect-dependent snow characteristics. Melt-forms and -crusts dominate on South-oriented slopes, whereas intense constructive snow metamorphism occurs on their North-facing counterparts. Wind redistribution and sloughing play an important role and the snow cover is often underlain by a layer of thick ice due to meltwater flowing down the bare rock surfaces and refreezing under the snow. Cavity hoar has been observed between strongly fractured rock and the snowpack, indicating that upward vapour transport occurs through fractures.
Near-surface rock temperatures are measured at 10 cm depth in vertical linear arrays over the rock walls with a distance of 3 m between each temperature logger. At the Gemsstock site, rock temperatures are also measured at depth, in a 40 m horizontal borehole which was drilled through the ridge from North to South in 2005.
The near-surface rock temperature measurements deliver a wealth of information on the timing of the snow cover, melt and freezing processes, the effects of snowmelt water and the influences of slope, aspect and surface roughness on rock temperature. The data obtained during snow-free periods displays the occurrence of huge diurnal temperature variations and multiple freeze-thaw cycles near the surface of the rock. In contrast, borehole temperature data indicate that the rock reacts with a 6 month delay at depth to temperature influences at the surface - but also that precipitation and snow melt water can infiltrate into fractures at the surface and influence temperatures at depth within hours.
Energy exchanges between the snow or atmosphere and the underlying rock - and consequently changes in rock temperature are simulated using the 1D model SNOWPACK. The aims are 1) to develop an efficient system to realistically simulate the snow cover and its impact on rock temperatures in terrain where in-situ measurements are unavailable and 2) to model future trends of changing temperature and snow cover scenarios.
The hydrothermal and hydromechanical influences of snow melt infiltration in fractured bedrock are modelled by our German project partners using the coupled model UDEC. Both models are driven and validated using various types of data obtained in the field.