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Schweizer J.,Institute for Snow and Avalanche Research | Bruce Jamieson J.,University of Calgary
Annals of Glaciology | Year: 2010

Information on snowpack instability is crucial for assessing avalanche risk in backcountry operations as well as for operational forecasting of the regional avalanche danger. Since slab avalanche release requires both fracture initiation and fracture propagation in a weak snowpack layer, field observations should ideally provide reliable information on the probability or propensity of both fracture processes. Even simple field observations that do not require digging a snow pit can provide useful information. Traditional snowpack tests include the shovel shear test, the shear frame test, the compression test (CT) and the rutschblock test (RB). Interpretation of the test results for the CT and RB has been improved by considering the appearance or type of the fracture in addition to the score. More recently, two tests have been developed that focus on fracture propagation rather than initiation: the extended column test (ECT) and the propagation saw test (PST). We compare the sensitivity, specificity and unweighted average accuracy of various stability tests. Comparative studies indicate that the RB, ECT and PST have comparable accuracy. For most test methods the unweighted average accuracy of a single test was 70-90% depending on the dataset. Test methods such as the RB, ECT and PST, which fracture an area large enough to include fracture propagation, are generally more accurate than test methods that fracture smaller areas (e.g. the CT). The threshold-sum method was also less accurate. Even with very experienced observers for the RB, ECT and PST an error rate of at least about 5-10% has to be expected. Performing a second, adjacent test on the same slope improves test reliability. Source

Hirashima H.,Snow and Ice Research Center | Yamaguchi S.,Snow and Ice Research Center | Sato A.,Snow and Ice Research Center | Lehning M.,Institute for Snow and Avalanche Research
Cold Regions Science and Technology | Year: 2010

Liquid water movement in snow is an important aspect of wet snow metamorphism and is vital for forecasting wet snow avalanches. However, despite its importance, liquid water movement is over-simplified in the current version of the numerical model SNOWPACK, yielding an inadequate simulation of the water content profile. In general, estimations of liquid water flux in porous materials are based on hydraulic conductivity and suction. This paper presents a water transport model based on the van Genuchten formulation, with parameters obtained from gravity drainage column experiments. Simulations that try to describe the capillary barrier between layers of different grain sizes lead to long computation times because of the small time increments required. In this study, a new algorithm was developed and incorporated in the water transport model in order to simulate the capillary barrier using relatively long time increments (60. s). The model was then used to simulate water movement between snow layers of different grain size. The results confirm that a water-saturated layer forms at the boundary between fine and underlying very coarse snow, which is consistent with observations.The new water transport model was incorporated in SNOWPACK, which then produced an unstable water-saturated layer at the boundary between different grain sizes. Simulation results from the SNOWPACK model were compared with measurements. The main achievement in this study is that the natural capillary barrier was documented and then reproduced using the modified SNOWPACK model. Nevertheless, much work has still to be done to get fully satisfactory results for the reproducibility of grain size and liquid water content. © 2010 Elsevier B.V. Source

Satyawali P.K.,Snow and Avalanche Study Establishment | Schneebeli M.,Institute for Snow and Avalanche Research
Annals of Glaciology | Year: 2010

A method for automated and fast classification of snow texture would be useful for applications where snow structure must be quantified. Large numbers of field measurements were carried out on natural snow in order to investigate small-scale variations of the micro-penetration force. Snow characterization was done for snow from the Himalaya and the Alps, using a high-resolution snow penetrometer (SnowMicroPen). Measurements of snow resistance at equal intervals of 4 mm were geostatistically evaluated. The range parameter (correlation length, or CL) of penetration force was estimated for all major snow classes from the sample semivariogram. Average CL was lowest for new snow and highest for melt-freeze snow. For major snow classes, CL was found to increase with snow density. Ground-perpendicular and ground-parallel snow profiles were also obtained for homogeneous snow, and CL was estimated along these directions. New snow showed larger CL in the ground-parallel direction, and depth-hoar snow showed larger CL in the ground-perpendicular direction. Based on CL, the directional anisotropy was calculated. An attempt was also made to show the relationship between CL and texture index. The semivariogram was used to estimate the fractal dimension. Both CL and fractal dimension were found to be potential parameters to describe snow. Source

Mitterer C.,Institute for Snow and Avalanche Research | Hirashima H.,Japan National Research Institute for Earth Science and Disaster Prevention | Schweizer J.,Institute for Snow and Avalanche Research
Annals of Glaciology | Year: 2011

Wet-snow avalanches are difficult to forecast, as the change from stable to unstable snow conditions occurs rapidly in a wet snowpack, often in response to water production and movement. Snow stratigraphy plays a vital role in determining flux behaviour. Capillary barriers or melt-freeze crusts can impede and divert water horizontally over large areas and thus may act as a failure layer for wet-snow avalanches. We present a comparison of measured and modelled liquid water content, θ w, and snow stratigraphy during periods of wet-snow instabilities. Special attention is given to the reproducibility of capillary barriers, ponding of water on melt-freeze crusts and the timing of first wetting and of water arrival at the bottom of the snowpack, because these factors are believed to play a major role in the formation of wet-snow avalanches. In situ measurements were performed in the vicinity of automatic weather stations or close to recent wet-snow avalanches in order to compare them with model results. The simulations are based on two different water flux models incorporated within the 1-D snow-cover model SNOWPACK. The comparison of the two model runs with observed θ w and stratigraphy revealed that both water-transport models reproduced the ponding of water on melt-freeze crusts. However, in both models melt-freeze crusts were transformed to normal melt forms earlier than observed in nature, so still existing ponding was not captured by the models. Only one of the models was able to reproduce capillary barriers in agreement with observations. The time of the first wetting at the surface was well predicted, but the simulated arrival time of the wetting front at the bottom of the snowpack differed between the simulations; it was either too early or too late compared with the observation. Source

Grunewald T.,Institute for Snow and Avalanche Research | Lehning M.,Institute for Snow and Avalanche Research
Annals of Glaciology | Year: 2011

The spatial distribution and the local amount of snow in mountainous regions strongly depend on the spatial characteristics of snowfall, snow deposition and snow redistribution. Uniform altitudinal gradients can only represent a part of these influences but are without alternative for use in larger-scale models. How well altitudinal gradients represent the true snow distribution has not been assessed. We analyse altitudinal characteristics of snow stored in two high-alpine catchments in Switzerland. Peak winter snow depths were monitored using high-resolution airborne laser scanning technology. These snow depths were transferred to snow water equivalent by applying simple density estimations. From these data, altitudinal gradients were calculated for the total catchment areas and for selected subareas characterized by different accumulation patterns. These gradients were then compared with gradients resulting from automated snow depth measurements obtained from several snow stations on different height levels located in the catchments, and with estimations from climatological precipitation gradients. The analysis showed that neither precipitation gradients nor flat-field stations estimate catchment-wide snow amounts accurately. While the climatological gradient showed different trends for different areas and years, the snow stations tended to overestimate mean snow amounts. Source

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