Abstract
Abstract. Driven by temperature gradients, kinetic snow metamorphism plays an import role in avalanche formation. When gradients based on temperatures measured 10 cm apart appear to be insufficient for kinetic metamorphism, faceting close to a crust can be observed. Recent studies that visualised small-scale (< 10 cm) thermal structures in a profile of snow layers with an infrared (IR) camera produced interesting results. The studies found melt-freeze crusts to be warmer or cooler than the surrounding snow depending on the large-scale gradient direction. However, an important assumption within these studies was that a thermal photo of a freshly exposed snow pit was similar enough to the internal temperature of the snow. In this study, we tested this assumption by recording thermal videos during the exposure of the snow pit wall. In the first minute, the results showed increasing gradients with time, both at melt-freeze crusts and artificial surface structures such as shovel scours. Cutting through a crust with a cutting blade or shovel produced small concavities (holes) even when the objective was to cut a planar surface. Our findings suggest there is a surface structure dependency of the thermal image, which was only observed at times during a strong cooling/warming of the exposed pit wall. We were able to reproduce the hot-crust/cold-crust phenomenon and relate it entirely to surface structure in a temperature-controlled cold laboratory. Concave areas cooled or warmed more slowly compared with convex areas (bumps) when applying temperature differences between snow and air. This can be explained by increased radiative and/or turbulent energy transfer at convex areas. Thermal videos suggest that such processes influence the snow temperature within seconds. Our findings show the limitations of using a thermal camera for measuring pit-wall temperatures, particularly during windy conditions, clear skies and large temperature differences between air and snow. At crusts or other heterogeneities, we were unable to create a sufficiently planar snow pit surface and non-internal gradients appeared at the exposed surface. The immediate adjustment of snow pit temperature as it reacts with the atmosphere complicates the capture of the internal thermal structure of a snowpack with thermal videos. Instead, the shown structural dependency of the IR signal may be used to detect structural changes of snow caused by kinetic metamorphism. The IR signal can also be used to measure near surface temperatures in a homogenous new snow layer.
Highlights
Faceting as part of the kinetic snow metamorphism contributes to avalanche formation
Faceted crystals close to melt-freeze crusts were observed in the absence of gradients needed for kinetic metamorphism when measured with thermometers 10 cm apart (Jamieson, 2006; Smith and Jamieson, 2009)
For a temperature gradient of 11 K m−1 they calculated the latent heat flux to be less than 1 % of the total measured heat flux
Summary
Faceting as part of the kinetic snow metamorphism contributes to avalanche formation. Faceted crystals close to melt-freeze crusts were observed in the absence of gradients needed for kinetic metamorphism when measured with thermometers 10 cm apart (Jamieson, 2006; Smith and Jamieson, 2009). One explanation for the development of facets during the absence of gradients may be found in the coarse measuring resolution. The authors proposed that the warm crust resulted from increased snow internal temperature gradients and water vapour fluxes. They assumed a relatively small ice conduction at the crust which resulted in remnants of undissipated latent heat at the crust. In Shea et al (2012a) they found relatively cold crusts and related this observation to a reverse
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