Abstract
Abstract. Observed snow stratigraphy and snow stability are of key importance for avalanche forecasting. Such observations are rare and snow cover models can improve the spatial and temporal resolution. To evaluate snow stability, failure initiation and crack propagation have to be considered. Recently, a new stability criterion relating to crack propagation, namely the critical crack length, was implemented into the snow cover model SNOWPACK. The critical crack length can also be measured in the field with a propagation saw test, which allows for an unambiguous comparison. To validate and improve the parameterization for the critical crack length, we used data from 3 years of field experiments performed close to two automatic weather stations above Davos, Switzerland. We monitored seven distinct weak layers and performed in total 157 propagation saw tests on a weekly basis. Comparing modeled to measured critical crack length showed some discrepancies stemming from model assumption. Hence, we replaced two variables of the original parameterization, namely the weak layer shear modulus and thickness, with a fit factor depending on weak layer density and grain size. With these adjustments, the normalized root-mean-square error between modeled and observed critical crack lengths decreased from 1.80 to 0.28. As the improved parameterization accounts for grain size, values of critical crack lengths for snow layers consisting of small grains, which in general are not weak layers, become larger. In turn, critical weak layers appear more prominently in the vertical profile of critical crack length simulated with SNOWPACK. Hence, minimal values in modeled critical crack length better match observed weak layers. The improved parameterization of critical crack length may be useful for both weak layer detection in simulated snow stratigraphy and also providing more realistic snow stability information – and hence may improve avalanche forecasting.
Highlights
Snow slab avalanches are hazardous and can threaten people and infrastructure
To assess whether the parameterization for the critical crack length implemented in SNOWPACK can be used to automatically identify critical weak layers, we investigated whether the five lowest values in the vertical profile of the critical crack length in SNOWPACK corresponded to the critical weak layers tested in the field
During the calm weather period starting in mid-November, the near-surface snow transformed into a layer of faceted crystals, forming a persistent weak layer that was buried by snow in mid-December (FC141216)
Summary
Snow slab avalanches are hazardous and can threaten people and infrastructure. Each year, around 100 avalanche fatalities occur in the European Alps (Techel et al, 2016). Whether avalanche release is likely largely depends on snow layering, in particular the complex interaction between slab layers and a so-called weak layer (Schweizer et al, 2008). Such weak layers often form near or at the snow surface and, if subsequently covered by a snowfall, can sometimes persist throughout the season. Dry-snow slab avalanches start with a failure in the weak layer resulting in a macroscopic crack. If this crack reaches a critical size, the crack will rapidly propagate outward To assess snow instability requires information on the spatial distribution of slab and weak layer properties and how cracks form and propagate
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