Snow as a driving factor of rock surface temperatures in steep rough rock walls

Abstract

Observations show that considerable amounts of snow can accumulate in steep, rough rock walls. The heterogeneously distributed snow cover significantly affects the surface energy balance and hence the thermal regime of the rock walls. To assess the small-scale variability of snow depth and rock temperatures in steep north and south facing rock walls, a spatially distributed multi-method approach is applied at Gemsstock, Switzerland, combining 35 continuous near-surface rock temperature measurements, high resolution snow depth observations using terrestrial laser scanning, as well as in-situ snow pit investigations. The thermal regime of the rock surface is highly dependent on short-and longwave radiation, albedo, surface roughness, snow depth and on snow distribution in time and space. Around 2 m of snow can accumulate on slopes with angles up to 75°, due to micro-topographic structures like ledges. Hence, contrasts in mean annual rock surface temperature between the north and the south facing slopes are less than 4°C. However, significant small-scale variability of up to 10°C in mean daily rock surface temperature occurs within a few metres over the rock walls due to the variable snow distribution, revealing the heterogeneity and complexity of the thermal regime at a very local scale. In addition, multiple linear regression could explain up to 77% of near-surface rock temperature variability, which underlines the importance of radiation and snow depth and thus also of the topography. In the rock faces the thermal insulation of the ground starts with snow depths exceeding 0.2 m. This is due to the high thermal resistance of a less densely packed snow cover, especially in the north facing slope. Additionally, aspect induced differences of snow cover characteristics and consequently thermal conductivities are observed in the rock walls.

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

“…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). In contrast, Haberkorn et al (2015a) found that snow depths exceeding 0.2 m were enough to have an insulating effect on steep, bare bedrock. Such amounts are likely to accumulate in steep, high rock walls with a certain degree of surface roughness.…”

“…Such amounts are likely to accumulate in steep, high rock walls with a certain degree of surface roughness. Indeed, a warming effect of the snow cover on mean annual ground surface temperature (MAGST) was observed by Haberkorn et al (2015a) and Magnin et al (2015) in shaded rock walls, whilst in moderately inclined (45-70 • ) sun-exposed rock walls Hasler et al (2011) suggest a reduction of MAGST of up to 3 • C compared to estimates in near-vertical, compact rock due to snow persistence during the months with most intense radiation. Those observations emphasize the need to account for the strongly varying snow cover in thermal modelling of steep rock walls.…”

“…Recent studies based on terrestrial laser scanning (TLS) not only confirmed that snow accumulates in steep, rough rock walls with rock ledges (Haberkorn et al, 2015a;Sommer et al, 2015;Wirz et al, 2011) but also provided accurate snow depth distribution measurements for both rock temperature modelling and model verification. This is of great importance, since an accurately modelled snow cover evolution and its spatial patterns are crucial to correctly model the ground thermal regime (Fiddes et al, 2015;Hoelzle et al, 2001;Stocker-Mittaz et al, 2002) and assess contrasting influences of a heterogeneous snow cover on the ground thermal regime.…”

“…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.…”

“…They therefore suggested that air temperature and solar radiation are sufficient to model rock surface temperatures in near-vertical, compact, homogeneous rock walls. Rock walls are, however, often variably inclined, heterogeneous, fractured and thus partly snow-covered (Haberkorn et al, 2015a;Hasler et al, 2011;Sommer et al, 2015). Besides three-dimensional (3-D) subsurface heat flow and transient changes in steep bedrock thermal modelling (Noetzli and Gruber, 2009;Noetzli et al, 2007), the strongly variable spatial and temporal rock surface boundary conditions also need to be taken into account.…”