Weak Snow Research Articles

Numerical investigation of crack propagation regimes in snow fracture experiments

A snow slab avalanche releases after failure initiation and crack propagation in a highly porous weak snow layer buried below a cohesive slab. While our knowledge of crack propagation during avalanche formation has greatly improved over the last decades, it still remains unclear how snow mechanical properties affect the dynamics of crack propagation. This is partly due to a lack of non-invasive measurement methods to investigate the micro-mechanical aspects of the process. Using a DEM model, we therefore analyzed the influence of snow cover properties on the dynamics of crack propagation in weak snowpack layers. By focusing on the steady-state crack speed, our results showed two distinct fracture process regimes that depend on slope angle, leading to very different crack propagation speeds. For long experiments on level terrain, weak layer fracture is mainly driven by compressive stresses. Steady-state crack speed mainly depends on slab and weak layer elastic moduli as well as weak layer strength. We suggest a semi-empirical model to predict crack speed, which can be up to 0.6 times the slab shear wave speed. For long experiments on steep slopes, a supershear regime appeared, where the crack propagation speed reached approximately 1.6 times the slab shear wave speed. A detailed micro-mechanical analysis of stresses revealed a fracture principally driven by shear. Overall, our findings provide new insight into the micro-mechanics of dynamic crack propagation in snow, and how these are linked to snow cover properties.Graphical

Tragedies due to human folly: A learnt lesson from year 2023 Sikkim avalanche

In the Himalaya, natural hazards are common and their places, as well as timings, are difficult to predict. Rough terrain, unpredictable weather conditions and poor road networks of the Himalaya make the mitigation measures even more challenging as well as difficult. On April 4, 2023, 7 people have died and 13 were injured due to a small loose snow avalanche at 14th Mile, on the way to Nathula Pass, near Indo-China Border in Sikkim state (India). State government had advised the people, not to visit the area due to the poor weather conditions. However, people have ignored this precious advice and lost their lives. Along with Indo-China border, the region is famous for its scenic beauty. In addition, the region is one of the most difficult border areas with the limited infrastructural development for armed forces. The present study explores the causes and consequences of this killer avalanche from an environmental determinism. The results suggest that, due to the western disturbances, the region received heavy snowfall. Higher average temperature and flowing stream beneath snow, make the snow unstable. Weight of humans put pressure on the weak snow which led to formation of avalanche. Our finding suggests that peoples’ negligence about government advices have led to casualties. The purpose of this case study is to highlight the human folly which invites such unfortunate incidences across the globe. Further, this study emphasis the need of public-government partnership to minimize the impact of such upcoming hazards.

Stress measurements in the weak layer during snow stability tests

Abstract The snow compression test is a snow stability test where an isolated column of snow is progressively loaded by tapping on it to induce failure in a possible weak layer. The test result provides valuable information about the propensity of failure initiation within the snowpack. However, different persons might tap with different force and thus reduce the reproducibility of the test results. The aim of this work was to quantify the influence of different test persons and different snowpacks on snow compression test results. We therefore let 62 persons tap on a stress measurement plate during a workshop of the European Avalanche Warning Services. Moreover, in the field, we performed stress measurements during 116 snow compression tests with 13 persons at eight different locations in the Alps. Data on persons’ body features and snow properties were also collected. Our results show that the stresses that reach a weak snow layer due to tapping are influenced by both the snowpack as well as different persons. Still, the data's scattering is surprisingly small for lower loading steps and decreases with depth. Therefore, we can deduce that, especially when avalanche conditions are particularly dangerous, snow compression test results are quite reproducible.

Using video detection of snow surface movements to estimate weak layer crack propagation speeds

Abstract Dry-snow slab avalanches release due to crack propagation in a weak snow layer under a cohesive snow slab. Crack propagation speeds can provide insights into the potential size of avalanches and inform fracture and avalanche release models. Despite their importance, slope-scale crack speed measurements from real avalanches are limited. Further, most existing slope-scale measurements utilize the appearance of slab fractures on the snow surface. However, we have no evidence that the appearance of surface cracking is a good indicator of the weak layer crack propagation tip. Here we present a novel method to estimate crack propagation speed from snow surface movements in avalanche videos. Our technique uses changes in frame pixel intensity, allowing us to detect the location of weak layer cracks well before slab fractures appear on the snow surface. We use field experiments and numerical simulations to validate our method before applying it to five avalanches. Our estimates show that cracks propagate faster up and down the slope than in the cross-slope direction; this suggests that different propagation regimes likely govern crack propagation up/down the slope, cross-slope and in flat terrain.

Modeling weak snow layer fracture in propagation saw test using an ice column model

ABSTRACT Fracture initiation and propagation in a weak snow layer are two primary processes of the slab avalanche formation process. This study proposes a model for the weak snow layer and investigates the fracture propagation process. The weak snow layer is conceptualized as columns of ice sandwiched between two strong layers of snow. The strong layers are modeled as linear elastic, whereas the ice is characterized as a damaging elastoplastic material. The effective mechanical properties of the model weak layer are examined using finite element analysis and are close to the snow properties reported in the literature. This model is used in numerical propagation saw tests (PSTs) to investigate the fracture propagation process in the weak snow layer. Critical crack length (CCL) and fracture propagation speed (FPS) in PST simulations are obtained by tracking the crack tip and are in good agreement with the previously reported results. An insight into the fracture propagation process in the weak snow layer is presented through energy variation analysis in PST simulations and shown that the FPS during dynamic fracture propagation varies with the top slab’s elastic modulus, the weak layer’s fracture energy, and inertia of the overlying slab.

Transition from sub-Rayleigh anticrack to supershear crack propagation in snow avalanches

Snow slab avalanches, characterized by a distinct, broad fracture line, are released following anticrack propagation in highly porous weak snow layers buried below cohesive slabs. The anticrack mechanism is driven by the volumetric collapse of the weak layer, which leads to the closure of crack faces and to the onset of frictional contact. Here, on the basis of snow fracture experiments, full-scale avalanche measurements and numerical simulations, we report the existence of a transition from sub-Rayleigh anticrack to supershear crack propagation. This transition follows the Burridge–Andrews mechanism, in which a supershear daughter crack nucleates ahead of the main fracture front and eventually propagates faster than the shear wave speed. Furthermore, we show that the supershear propagation regime can exist even if the shear-to-normal stress ratio is lower than the static friction coefficient as a result of the loss of frictional resistance during collapse. This finding shows that snow slab avalanches have fundamental similarities with strike-slip earthquakes.

An emerging impact of Eurasian spring snow cover on summer rainfall in Eastern China

Eurasian spring snow cover is widely considered as an important predictor of Asian summer monsoon rainfall, but its possible role in the formation of the north–south dipole structure of rainfall anomalies (NSDR)—a major mode of the eastern China summer rainfall variability—remains elusive. Here, we show that, there is a close connection between the western Eurasian spring snow cover (WESS) and NSDR during our research period 1967–2018, with less WESS tends to be accompanied by a wetter south-drier north pattern over eastern China, and vice versa. However, this relationship was not significant before the late 1990s, but has since become significant. Further analyses demonstrate that the shift in the WESS–NSDR relationship could be attributed to the modulation of summer North Atlantic Oscillation (SNAO). After the late 1990s, the WESS-related anomalous atmospheric circulations during summer are largely reinforced by the constructive superposition of those with same signs induced by SNAO, which in turn would intensify the impact of WESS and hence lead to a strong WESS–NSDR connection. In contrast, the influences of WESS are counteracted by those with opposite signs associated with SNAO before the late 1990s and thereby result in a weak snow–rainfall relationship. Our findings, along with the decline in Eurasian spring snow cover, provide a potential explanation for the recent ‘South Flood–North Drought’ pattern observed over eastern China.

Subseasonal Forecast Skill of Snow Water Equivalent and Its Link with Temperature in Selected SubX Models

AbstractAccurate and skillful subseasonal forecasts have tremendous potential for sectors that are sensitive to hazardous weather and climate events. Analysis of prediction skill for snow water equivalent (SWE) and near-surface air temperature (T2m) is carried out for three (GEPS, GEFS, and FIM) global models from the subseasonal experiment (SubX) project for the 2000–14 period. The prediction skill of SWE is higher than the skill of T2m at week-3 and week-4 lead times in all models. The GEPS forecast tends to yield higher (lower) prediction skill of SWE (T2m) compared to the other two systems in terms of correlation skill score. The snow–temperature relationship in reanalysis is characterized by a strong negative correlation over most of the midlatitude regions and a weak positive correlation over high-latitude Arctic regions. All forecast systems reproduced well these observed features; however, the snow–temperature relationship is slightly weaker in the GEPS model. Despite the apparent lack of skill in temperature forecasts at week 4, all three models are able to predict the sign of temperature anomalies associated with extreme SWE conditions albeit with reduced intensity. The strength of the predicted temperature anomaly associated with extreme snow conditions is slightly weaker in the GEPS forecast compared to reanalysis and the other two models, despite having better skill in predicting SWE. These apparent disparities suggest that weak snow–temperature coupling strength in the model is one of the contributing factors for the lower temperature skill.

Micromechanical modeling of snow failure

Abstract. Dry-snow slab avalanches start with the formation of a local failure in a highly porous weak layer underlying a cohesive snow slab. If followed by rapid crack propagation within the weak layer and finally a tensile fracture through the slab, a slab avalanche releases. While the basic concepts of avalanche release are relatively well understood, performing fracture experiments in the laboratory or in the field can be difficult due to the fragile nature of weak snow layers. Numerical simulations are a valuable tool for the study of micromechanical processes that lead to failure in snow. We used a three-dimensional discrete element method (3-D DEM) to simulate and analyze failure processes in snow. Cohesive and cohesionless ballistic deposition allowed us to reproduce porous weak layers and dense cohesive snow slabs, respectively. To analyze the micromechanical behavior at the scale of the snowpack (∼1 m), the particle size was chosen as a compromise between low computational costs and detailed representation of important micromechanical processes. The 3-D-DEM snow model allowed reproduction of the macroscopic behavior observed during compression and mixed-mode loading of dry-snow slab and the weak snow layer. To be able to reproduce the range of snow behavior (elastic modulus, strength), relations between DEM particle and contact parameters and macroscopic behavior were established. Numerical load-controlled failure experiments were performed on small samples and compared to results from load-controlled laboratory tests. Overall, our results show that the discrete element method allows us to realistically simulate snow failure processes. Furthermore, the presented snow model seems appropriate for comprehensively studying how the mechanical properties of the slab and weak layer influence crack propagation preceding avalanche release.

Impact of Snow Initialization in Subseasonal‐to‐Seasonal Winter Forecasts With the Norwegian Climate Prediction Model

AbstractSnow initialization has been previously investigated as a potential source of predictability at the subseasonal‐to‐seasonal (S2S) timescale in winter and spring, through its local radiative, thermodynamical, and hydrological feedbacks. However, previous studies were conducted with low‐top models over short periods only. Furthermore, the potential role of the land surface‐stratosphere connection upon the S2S predictability had remained unclear. To this end, we have carried out twin 30‐member ensembles of 2‐month (November and December) retrospective forecasts over the period 1985–2016, with either realistic or degraded snow initialization. A high‐top version of the Norwegian Climate Prediction Model is used, based on the Whole Atmosphere Community Climate Model, to insure improved coupling with the stratosphere. In a composite difference of high versus low initial Eurasian snow, the surface temperature is strongly impacted by the presence of snow, and wave activity fluxes into the stratosphere are enhanced at a 1‐month lag, leading to a weakened polar vortex. Focusing further on 7 years characterized by a strongly negative phase of the Arctic Oscillation, we find a weak snow feedback contributing to the maintenance of the negative Arctic Oscillation. By comparing the twin forecasts, we extracted the predictive skill increment due to realistic snow initialization. The prediction of snow itself is greatly improved, and there is increased skill in surface temperature over snow‐covered land in the first 10 days, and localized skill increments in the mid‐latitude transition regions on the southern flanks of the snow‐covered land areas, at lead times longer than 30 days.

Investigating the release and flow of snow avalanches at the slope-scale using a unified model based on the material point method

Snow slab avalanches start with a failure in a weak snow layer buried below a cohesive snow slab. After failure, the very porous character of the weak layer leads to its volumetric collapse and thus closing of crack faces due to the weight of the overlaying slab. This complex process, generally referred to as anticrack, explains why avalanches that release on steep slopes can be triggered from flat terrain. On the basis of a new elastoplastic model for porous cohesive materials and the Material Point Method, we investigate the dynamics of mixed-mode anticracks, the subsequent detachment of the slab and the flow of the avalanche. In particular, we performed two and three dimensional slope scale simulations of both the release and flow of slab avalanches triggered either directly or remotely. We describe the fracture and flow dynamics on a realistic topography and focus on the volumetric plastic strain, stress invariants, propagation speed and flow velocity. Our simulations reproduce typical observations of “en-echelon” fractures and the propagation speed reached up to three times that measured in field experiments. In addition, slab fracture always started from the top in the Propagation Saw Test while it systematically initiated at the interface with the weak layer at the crown of slope-scale simulations in agreement with limited field observations. During the avalanche flow, snow granulation, erosion and deposition processes were naturally simulated and do not need additional implementations. Our unified model represents a significant step forward as it allows simulating the entire avalanche process, from failure initiation to crack propagation and to solid-fluid phase transitions, which is of paramount importance to forecast and mitigate snow avalanches.

Testing Unmanned Aircraft for Roadside Snow Avalanche Monitoring

An important part of the Norwegian Public Roads Administration’s (NPRA’s) mission is to monitor and react to snow avalanche hazards in steep areas above their roads. Small unmanned aircraft systems (UASs) are increasing capable and commercially available. The NPRA wanted to evaluate if this technology could support their avalanche program but had concerns about UASs’ ability to function and to provide usable information in typical field conditions. The NPRA hired vendors to fly nine different UAS aircraft on challenging avalanche surveillance missions in cold, windy weather in steep mountains. An evaluation team’s conclusions were that different aircraft could be effectively used for different aspects of avalanche monitoring and that cameras on UASs (both visible light and infrared) provided images useful for avalanche surveillance. A second test further evaluated if sensors and cameras carried on UASs could provide NPRA’s avalanche staff with usable snow depth, layering, and surface information. Data from UAS carrying visual cameras, used for both real-time viewing and photogrammetry (structure from motion), and ground penetrating radar (GPR) were compared with snow pack information collected using hand dug pits. The review team found the GPR output could identify the weak snow layers that cause avalanches but the raw data required time-consuming post-processing. Photogrammetry had considerable potential as it mapped snow surface conditions and, with a baseline survey, measured snow depth, both of which are valuable for avalanche assessment. Overall, the review team felt UAS technology held considerable promise to support transportation agencies’ roadside avalanche monitoring programs.

Numerical investigation of the mixed-mode failure of snow

The failure of a weak snow layer underlying a cohesive slab is the primary step in the release process of a dry snow slab avalanche. The complex and heterogeneous microstructure of snow limits our understanding of failure initiation inside the weak layer, especially under mixed-mode shear–compression loading. Further complication arises from the dependence of snow strength on the loading rate induced by the balance between bond breaking and bond formation (sintering) during the failure process. Here, we use the discrete element method to investigate the influence of mixed-mode loading and fast sintering on the failure of a weak layer generated using cohesive ballistic deposition. Both fast and slow loading simulations resulted in a mixed-mode failure envelope in good agreement with laboratory experiments. We show that the number of broken bonds at failure and the weak layer strength significantly decreases with increasing loading angle, regardless of the loading rate. While the influence of loading rate appears negligible in shear-dominant loading (for loading angles above 30^{circ }), simulations suggest a significant increase in the weak layer strength at low loading angles and low loading rates, characteristic of natural avalanches, due to the presence of an active sintering mechanism.

Failure mechanics of snow layers through image analysis

This paper is devoted to analyze the micro-mechanical behavior of snow layers subjected to shear force. A number of cold laboratory tests were performed on reconstructed snow samples on a specially designed direct shear apparatus and the loading process leading to the failure was recorded by a high-speed camera. The specimens were prepared under isothermal conditions and immediately tested in order to avoid sintering between the snow grains. Image analysis techniques (digital image correlation and particle image velocimetry) were applied on the camera records in order to obtain the snow strain history (during the loading phase) and the velocity field after the failure. Sequences of jamming and unjamming phases with the formation of vortices were observed. These observations confirm that snow layer failure process is similar to the one observed in other natural materials (e.g., sands). These results give an insight into triggering and failure propagation in a weak snow layer.

Stress Concentrations in Weak Snowpack Layers and Conditions for Slab Avalanche Release

AbstractDry‐snow slab avalanches release due to the formation of a crack in a weak layer buried below cohesive snow slabs, followed by rapid crack propagation. The onset of rapid crack propagation occurs if stresses at the crack tip in the weak layer overcome its strength. In this study, we use the finite element method to evaluate the maximum shear stress τmax induced by a preexisting crack in a weak snow layer allowing for the bending of the overlaying slab. It is shown that τmax increases with increasing crack length, slab thickness, slab density, weak layer elastic modulus, and slope angle. In contrast, τmax decreases with increasing elastic modulus of the slab. Assuming a realistic failure envelope, we computed the critical crack length ac for the onset of crack propagation. The model allows for remote triggering from flat (or low angle) terrain. Yet it shows that the critical crack length decreases with increasing slope angle.

Acoustic emission signatures prior to snow failure

ABSTRACTSnow slab avalanches are caused by cracks forming and propagating in a weak snow layer below a cohesive slab. The gradual damage process leading to the formation of the initial failure within the weak layer (WL) is still not entirely understood. To this end, we designed a novel test apparatus that allows performing loading experiments with large snow samples (0.25 m2) including a WL at different loading rates and simultaneously monitoring the acoustic emissions (AE) response. By analyzing the AE generated by micro-cracking, we studied the evolution of the damage process preceding snow failure. At fast loading rates, the exponent of the AE energy distribution (b-value) gradually changed, and both the energy rate and the inverse waiting time increased exponentially with increasing load. These changes in AE signature indicate a transition from small to large events and an acceleration of the damage processes leading to brittle failure. For the experiments at slow loading rate, these changes in the AE signature were not or only partially present, even if the sample failed, indicating a different evolution of the damage process. The observed characteristics in AE response provide new insights on how to model snow failure as a critical phenomenon.

Global Distribution of Snow Precipitation Features and Their Properties from 3 Years of GPM Observations

Abstract Using a 3-yr Global Precipitation Mission (GPM) Ku-band Precipitation Radar (KuPR) dataset, snow features (SFs) are defined by grouping the contiguous area of nonzero solid precipitation. The near-surface wet bulb temperatures calculated from ERA-Interim reanalysis data are used to verify that SFs are colder than 1°C to omit snowfall that melts before reaching the surface. The properties of SFs are summarized to understand the global distribution and characteristics of snow systems. The seasonal and diurnal variations of SFs and their properties are analyzed over Northern and Southern Hemispheric land and ocean separately. To quantify the amount of snow missed by the GPM KuPR and the amount of snow underestimated by the CloudSat Cloud Profiling (CPR), 3-yr KuPR pixel-level data are compared with 4-yr CloudSat CPR observations. The overall underestimation of snowfall during heavy snow events by CPR is less than 3% compared to the combined CPR and KuPR estimates. KuPR underestimates about 52% of weak snow. Only a small percentage of SFs have sizes greater than 10 000 km2 (0.35%), maximum near-surface reflectivity above 30 dBZ (5.1%), or echo top above 5 km (1.6%); however, they contribute 40%, 49.5%, or 30.4% of the global volumetric snow detected by KuPR. Snow in the Northern Hemisphere has stronger diurnal and seasonal variation compared to the Southern Hemisphere. Most of the SFs over the ocean are found with relatively smaller, less intense, and shallower echo tops than over land.

Snow fracture in relation to slab avalanche release: critical state for the onset of crack propagation

Abstract. The failure of a weak snow layer buried below cohesive slab layers is a necessary, but insufficient, condition for the release of a dry-snow slab avalanche. The size of the crack in the weak layer must also exceed a critical length to propagate across a slope. In contrast to pioneering shear-based approaches, recent developments account for weak layer collapse and allow for better explaining typical observations of remote triggering from low-angle terrain. However, these new models predict a critical length for crack propagation that is almost independent of slope angle, a rather surprising and counterintuitive result. Based on discrete element simulations we propose a new analytical expression for the critical crack length. This new model reconciles past approaches by considering for the first time the complex interplay between slab elasticity and the mechanical behavior of the weak layer including its structural collapse. The crack begins to propagate when the stress induced by slab loading and deformation at the crack tip exceeds the limit given by the failure envelope of the weak layer. The model can reproduce crack propagation on low-angle terrain and the decrease in critical length with increasing slope angle as modeled in numerical experiments. The good agreement of our new model with extensive field data and the ease of implementation in the snow cover model SNOWPACK opens a promising prospect for improving avalanche forecasting.

Mechanical behaviour of weak snow layers: modelling a porous structure of sintered grains

Weak layers of snow are thin layers of low density and cohesion that consist of a complex network of sintered ice grains. These layers are very fragile and their collapse is considered to be the primary cause of dry slab snow avalanche release. This study reports on an attempt at modelling the mechanical response of these weak snow layers with discrete element (DEM) simulations, using x-ray tomographical images of real snow samples as input data on the microstructure of the material. An original method for representing irregular grain shapes in the DEM is first introduced. The method is based on utilizing the medial axis concept to represent an arbitrary grain shape with an optimal set of overlapping spheres. A thorough study of the effect of the grain approximating technique parameters on the key geometrical features of the grains is then carried out. Finally, the functionality of the model is demonstrated by performing an oedometric test on an image of a real snow sample.

Temporal evolution of crack propagation propensity in snow in relation to slab and weak layer properties

Abstract. If a weak snow layer below a cohesive slab is present in the snow cover, unstable snow conditions can prevail for days or even weeks. We monitored the temporal evolution of a weak layer of faceted crystals as well as the overlaying slab layers at the location of an automatic weather station in the Steintälli field site above Davos (Eastern Swiss Alps). We focussed on the crack propagation propensity and performed propagation saw tests (PSTs) on 7 sampling days during a 2-month period from early January to early March 2015. Based on video images taken during the tests we determined the mechanical properties of the slab and the weak layer and compared them to the results derived from concurrently performed measurements of penetration resistance using the snow micro-penetrometer (SMP). The critical cut length, observed in PSTs, increased overall during the measurement period. The increase was not steady and the lowest values of critical cut length were observed around the middle of the measurement period. The relevant mechanical properties, the slab effective elastic modulus and the weak layer specific fracture, overall increased as well. However, the changes with time differed, suggesting that the critical cut length cannot be assessed by simply monitoring a single mechanical property such as slab load, slab modulus or weak layer specific fracture energy. Instead, crack propagation propensity is the result of a complex interplay between the mechanical properties of the slab and the weak layer. We then compared our field observations to newly developed metrics of snow instability related to either failure initiation or crack propagation propensity. The metrics were either derived from the SMP signal or calculated from simulated snow stratigraphy (SNOWPACK). They partially reproduced the observed temporal evolution of critical cut length and instability test scores. Whereas our unique dataset of quantitative measures of snow instability provides new insights into the complex slab-weak layer interaction, it also showed some deficiencies of the modelled metrics of instability – calling for an improved representation of the mechanical properties.