Dr. Benjamin Walter

We determined the compressive strength of weak layers of faceted crystals and depth hoar using artificially grown samples with a wide range of microstructural morphologies in a cold laboratory setup. Microcomputed tomography (μCT) imaging showed that the microstructures of the artificial samples were comparable to that of natural depth hoar. We performed compression experiments in a displacement controlled testing machine on 92 depth hoar samples with densities ranging from 150 kg m^-3 to 350 kg m^-3. The compressive strength spanned two orders of magnitude (1 kPa to 150 kPa) at strain rates of about 10^-3 s^-1 at -5 řC and followed a power law as a function of density. Several microstructural metrics such as the specific surface area (SSA), connectivity density, and correlation lengths obtained from μCT measurements exhibited a statistically significant relationship with compressive strength. Analysis of the residuals of the power law fit showed that in addition to density, horizontal correlation lengths also correlated with strength. However, in this study, density remained the dominant predictor of the compressive strength of depth hoar.

Natural weak layers exhibit a variety of different microstructures and densities, and thus show different mechanical behavior. Up to now, mechanical properties of snow have been mainly evaluated based on bulk proxies such as snow density, not accounting for relevant microstructural characteristics. To establish a link between the microstructure of weak layers and their mechanical properties, we tested weak layers of three different categories (faceted crystals and depth hoar, decomposed particles and rounded grains, and buried surface hoar) in displacement-controlled laboratory experiments using a uniaxial testing machine and recorded their microstructure using micro-computed tomography (µCT) imaging. The compressive strength of 295 tested samples covered three orders of magnitude (0.5 kPa to 1 MPa) for weak layer densities ranging from 65 kg/m^-3 to 470 kg/m^-3. As expected, our results show a strong correlation between weak layer density and strength. At the same time, we observe distinct scaling of different types of weak layers with density, highlighting the importance of snow microstructure for mechanical considerations. These results will help us to establish a link between density, microstructural properties and strength of weak snow layers, which is needed for the next generation of snow cover and avalanche release models.

Wind-blown snow is a frequent phenomenon in high-elevation and polar regions which impacts the surface energy and mass balance of these areas. Loose surface snow gets eroded and transported by wind, which influences the snow particles' physical properties (size, shape, optical properties) that determine the characteristics of the emerging wind-impacted snowpack layer. During airborne snow transport, the governing processes happen on the micro-scale while the particles are transported over long distances. The unfolding processes and the evolution of the particles' physical properties are thus difficult to observe in situ. Here, we used cold-laboratory ring wind tunnel experiments as an interim solution to study the governing processes during airborne snow transport with stable water isotopes as tracers for these micro-scale processes. Repeated analysis of airborne-sampled snow by micro-computed tomography (μCT) documented a growing and rounding of snow particles with transport time, with a concurrent decrease in specific surface area. Stable water isotope analysis of airborne snow and water vapour allowed us to attribute this evolution to the process of airborne snow metamorphism. The changes observed in the snow isotopic composition showed a clear isotopic signature of metamorphic deposition, which requires particle-air temperature gradients. These results question the validity of the thermal-equilibrium assumption between particles and air inside the saltation layer of wind-blown snow events, where the conditions are similar to the ones found in the wind tunnel. Our results thus refine the understanding of the governing processes in the saltation layer and suggest that the snow's isotopic composition can inform on local wind-blown snow events as the original snow isotope signal gets overprinted by airborne snow metamorphism. Within transport times of 3 h, we observed changes in the isotope signal of airborne snow of up to +1.47 ‰ in δ¹⁸O, ±5.7 ‰ in δD, and -6.1 ‰ in d-excess. Thus, airborne snow metamorphism has the potential to influence the climate signal stored in snow and ice core stable water isotope records.

The evolution of the surface snow microstructure under the influence of wind during precipitation events is hardly understood but crucial for polar and alpine snowpacks. Available statistical models are solely parameterized from field data where conditions are difficult to control. Controlled experiments which exemplify the physical processes underlying the evolution of density or specific surface area (SSA) of surface snow under windy conditions are virtually non-existent. As a remedy, we conducted experiments in a cold laboratory using a ring-shaped wind tunnel with an infinite fetch to systematically investigate wind-induced microstructure modifications under controlled atmospheric, flow and snow conditions and to identify the relevant processes. Airborne snow particles are characterized by high-speed imaging, while deposited snow is characterized by density and SSA measurements. We used a single snow type (dendritic fresh snow) for simulating different precipitation intensities, varied wind speeds at a height of 0.4 m from 3 to 7 m s^-1 (for fixed temperature) and varied temperatures from -24 to -2 °C (for fixed wind speed). The measured airborne impact trajectories confirm the consistency of our coefficient of restitution with large-scale saltation, rendering the setup suitable for realistically studying interactions between airborne and deposited snow. Increasing wind speeds resulted in intensified densification and stronger SSA decreases. The most drastic snow density and SSA changes in deposited snow are observed close to the melting point. Our measured densification rates as a function of wind speed show clear deviations from existing statistical models but can be re-parameterized through our data. This study, as a first of its kind, exemplifies a rich nonlinear interplay between airborne and deposited snow particles, which is discussed in view of a multitude of involved processes, i.e., airborne metamorphism, cohesion, particle separation and fragmentation.

Snow cornices growing on the leeward side of mountain ridges are common in alpine and polar regions during snow seasons. These structures may crack and fall, leading to an increase in avalanche danger. Although cornice formation has been observed in wind tunnel tests and the field, knowledge gaps still exist regarding the formation mechanism. This is particularly true with respect to wind conditions which favor cornice formation. To characterize the wind effects as the main factor for cornice growth, we carried out ring wind tunnel (RWT) experiments in a cold laboratory under various wind conditions. We quantitatively investigated the growth rate of the cornice in the horizontal and vertical direction as well as the airborne particle concentration. The results show that cornices only appear under a moderate wind speed range (1-2 times the threshold wind speed). The cornice growth rates in length and thickness are mainly determined by the combined effects of mass accumulation and erosion. The lower-limit wind speed for cornice growth is approximately equal to the threshold wind speed for snow transport. The upper limit of wind speed is when the erosion rate is higher than the deposition rate. The length growth rate of the cornices reaches a maximum for wind speeds approximately 40% higher than the threshold wind speed. Moreover, a conceptual model for interpreting the cornice accretion mechanism is proposed based on the mass conservation and the results of the RWT experiments. The estimated suitable wind condition for cornice growth and formation are in good agreement with field observations in Gruvefjellet, Svalbard. Based on the physics of drifting snow, our results provide new insights into snow cornice formation and improve understanding of cornice processes that can influence avalanche activity. The experimental results and the conceptual model can be used in future snow cornice simulation and prediction work for cornice-induced avalanches.

Modelling and forecasting wind-driven redistribution of snow in mountainous regions with its implications on avalanche danger, mountain hydrology or flood hazard is still a challenging task often lacking in essential details. Measurements of drifting and blowing snow for improving process understanding and model validation are typically limited to point measurements at meteorological stations, providing no information on the spatial variability of horizontal mass fluxes or even the vertically integrated mass flux. We present a promising application of a compact and low-cost radar system for measuring and characterizing larger-scale (hundreds of metres) snow redistribution processes, specifically blowing snow off a mountain ridge. These measurements provide valuable information of blowing snow velocities, frequency of occurrence, travel distances and turbulence characteristics. Three blowing snow events are investigated, two in the absence of precipitation and one with concurrent precipitation. Blowing snow velocities measured with the radar are validated by comparison against wind velocities measured with a 3D ultra-sonic anemometer. A minimal blowing snow travel distance of 60-120 m is reached 10-20 % of the time during a snow storm, depending on the strength of the storm event. The relative frequency of transport distances decreases exponentially above the minimal travel distance, with a maximum measured distance of 280 m. In a first-order approximation, the travel distance increases linearly with the wind velocity, allowing for an estimate of a threshold wind velocity for snow particle entrainment and transport of 7.5-8.8 m s^-1, most likely depending on the prevailing snow cover properties. Turbulence statistics did not allow a conclusion to be drawn on whether low-level, low-turbulence jets or highly turbulent gusts are more effective in transporting blowing snow over longer distances, but highly turbulent flows are more likely to bring particles to greater heights and thus influence cloud processes. Drone-based photogrammetry measurements of the spatial snow height distribution revealed that increased snow accumulation in the lee of the ridge is the result of the measured local blowing snow conditions.

We present a simple model based on already existing and widely used equations for estimating particle mass fluxes on surfaces sheltered by live vegetation. Wind-tunnel measurements of vertical profiles of mass flux in three different dense live plant canopies, and as a function of the spatially averaged skin friction velocity u_τ', provide the baseline set of data. For the bare-sand surface, the total mass flux Q shows the typical b(u_τ'−u_τt')³ increase with increasing skin friction velocity u_τ', where b is a constant and u_τt' is the threshold at the onset of particle erosion. Similar relations, however, with different values for b and u_τt' compared to the bare-sand surface were found for experiments with 5.25 and 24.5 plants m^–2 and can be explained by the spatial variations of u_τ' for the canopy cases. Based on the resulting parameters b and u_τt', which are found to be functions of the roughness density λ, we present a final simple relation Q(λ, u_τ') used for estimating the total mass flux for surfaces sheltered by live vegetation.

Shadowgraphy was employed to study snow saltation in boundary-layer wind tunnel experiments with fresh, naturally deposited snow. The shadowgraphy method allowed for a temporally and spatially high-resolution investigation of snow particle characteristics within a measurement area of up to 50 mm x 50 mm. Snow particle size and number characteristics, and their variation with height in the saltation layer, were analysed. The following observations and findings were made for the saltation layer: (1) the particle number decreases exponentially with height, (2) the mean particle diameter is fairly constant, with a very slight tendency to decrease with height, (3) the maximum particle diameter decreases linearly with height, and (4) the snow particle size distribution can be adequately described by gamma probability density functions. The shape and scale parameters of the gamma distribution were found to vary systematically, though only slightly, with height over ground and between experiments with different snowpack characteristics.

A widely used, yet thus far unproven, fluid dynamical hypothesis originally presented by P. R. Owen 50 years ago, states that the surface shear stress induced by a fluid on the ground during equilibrium sediment saltation is constant and independent of the magnitude of the fluid velocity and consequently the particle mass flux. This hypothesis is one of the key elements in almost all current model descriptions of sediment erosion. We measured the surface shear stress in a drifting-sand wind tunnel and found Owen's hypothesis beingmerely an approximation of the real situation. A significant decrease of the fluid stress with increasing wind velocities was measured for low to intermediate particle mass fluxes. For high particle mass fluxes, Owen's hypothesis essentially holds, although a slight increase of the fluid stress was measured.

A study combining field observations and wind tunnel measurements of drifting snow is presented. In field measurements we found fluctuations of the snow mass flux on short time scales. From observations by eye and model results these may be ascribed to mass transport in coherent flow structures. The wind tunnel measurements do not show this type of flow structures, due to the restriction of the dimensions of the flow. However, we found increased fluctuations of mass flux of drifting snow with progressing erosion, which could be explained by the creation of surfaces ripples, which can break and release a large amount of snow particles in a short time.

Owens second hypothesis states that the surface shear stress induced by the fluid on the stationary sediment on the ground in a particle saltation layer equals the threshold for particle entrainment. Despite the fact that most numerical models describing sediment entrainment and particle mass fluxes in turbulent boundary layer flows make use of Owens second hypothesis, no direct experimental validation of this assumption has been presented to date. We present direct measurements of the fluid shear stress on the ground in sand saltation layers to validate this hypothesis. The shear stress was measured for different wind velocities with various particle concentrations in the saltation layer using Irwin sensors. Additionally, particle concentrations in the saltation layers were estimated from shadowgraphic images taken with a high speed camera.

Fluorescent Particle Tracking Velocimetry (FPTV) measurements of the pore-scale water flow through the pore space of a wet-snow sample are presented to demonstrate the applicability of this measurement technique for snow. For the experiments, ice-cooled water seeded with micron sized fluorescent tracer particles is either sprinkled on top of a snow sample to investigate saturated and unsaturated gravity-driven flow or supplied from a reservoir below the snow sample to generate upward flow driven by capillary forces. The snow sample is illuminated with a laser light sheet and the fluorescent light of the particles transported with the water in the pore space is recorded with a high-speed camera equipped with an optical filter. Tracking algorithms are applied to the images to obtain flow paths and flow velocities. A flow loop found in a pore space for the case of saturated gravity flow together with the tortuosity of the particle trajectories indicate the three-dimensionality of the water flow in wet snow. The average vertical flow velocities in the pore spaces were 11.2 mm s^-1 for the downward saturated gravity flow and 9.6 mm s^-1 for the upward flow that is driven by capillary forces for the limited cases presented as examples of the measurement technique. In the case of unsaturated gravity-driven flow, the average and the maximum flow velocities were found to be 30 times smaller than for the saturated gravity flow. Velocity histograms show that the fraction of the total water flowing against the main flow direction was about 3-5%, and that the horizontal velocities average to zero for both the saturated gravity-driven and the capillary flow.

LIF-PTV-measurements of the micro-scale water flow through the pore space of a wet snow sample driven by either gravitational or capillary forces are presented. For the measurements, fluorescent micron-sized particles in the water are illuminated with a laser light sheet and tracked with a high-speed camera. The results show the existence of a potential preferential flow path and a loop flow in a pore space in case of a gravity driven flow. Generally, the water flow is found to be highly 3-dimensional. The average flow velocities in the pore spaces are 11.3 mm/s for the gravity driven flow and 9.6 mm/s for the upward flow driven by capillary forces. Flow acceleration and deceleration was stronger for the gravity driven flow with particle decelerations stronger than accelerations in both cases.

The spatial peak surface shear stress τ^"_s on the ground beneath vegetation canopies is responsible for the onset of particle entrainment and its precise and accurate prediction is essential when modelling soil, snow or sand erosion. This study investigates shear-stress partitioning, i.e. the fraction of the total fluid stress on the entire canopy that acts directly on the surface, for live vegetation canopies (plant species: Lolium perenne) using measurements in a controlled wind-tunnel environment. Rigid, non-porous wooden blocks instead of the plants were additionally tested for the purpose of comparison since previous wind-tunnel studies used exclusively artificial plant imitations for their experiments on shear-stress partitioning. The drag partitioning model presented by Raupach (Boundary-Layer Meteorol 60:375-395, 1992) and Raupach et al. (J Geophys Res 98:3023-3029, 1993), which allows the prediction of the total shear stress τ on the entire canopy as well as the peak (τ^"_s/τ^1/2) and the average (τ^'_s/τ)^1/2 shear-stress ratios, is tested against measurements to determine the model parameters and the model's ability to account for shape differences of various roughness elements. It was found that the constant c, needed to determine the total stress τ and which was unspecified to date, can be assumed a value of about c = 0.27. Values for the model parameter m, which accounts for the difference between the spatial surface average τ^'_s and the peak τ^"_s shear stress, are difficult to determine because m is a function of the roughness density, the wind velocity and the roughness element shape. A new definition for a parameter a is suggested as a substitute for m. This a parameter is found to be more closely universal and solely a function of the roughness element shape. It is able to predict the peak surface shear stress accurately. Finally, a method is presented to determine the new a parameter for different kinds of roughness elements.

This study investigates the accuracy of spatially resolved skin friction velocity u_τ measurements using a large number of Irwin sensors (Irwin, 1981). A fast and simple procedure to calibrate the sensors against a two-component hot-film anemometer is introduced which leads to a universal calibration function that can then be used for all sensors. Insignificantly lower accuracies are obtained using the universal calibration function instead of individual calibration functions for each sensor. The overall accuracy of the skin friction velocity measurements averages about ±5% for u_τ > 0.13 m s^-1 and decreases strongly to ±20% at lower u_τ. The main sources of errors are as follows: (i) deviations from flush mounting of the sensors with the wall surface, (ii) inhomogeneous boundary layers produced in wind tunnels, (iii) pressure transducer inaccuracies, (iv) Irwin sensor geometry variations and (v) Irwin sensor calibration errors. An excellent linear relationship between u_τ measured with the Irwin sensors and the free stream velocity as well as the good agreement of Irwin sensor with independent hot-film measurements validate the reliability of the measurement setup. An exemplary measurement of the skin friction velocity distribution around a single wall-mounted rectangular block is presented, discussed and compared to literature and can serve as a test case for simulations.

This study presents spatiotemporally-resolved measurements of surface shear-stress tau (s) in live plant canopies and rigid wooden cube arrays to identify the sheltering capability against sediment erosion of these different roughness elements. Live plants have highly irregular structures that can be extremely flexible and porous resulting in considerable changes to the drag and flow regimes relative to rigid imitations mainly used in other wind-tunnel studies. Mean velocity and kinematic Reynolds stress profiles show that well-developed natural boundary layers were generated above the 8 m long wind-tunnel test section covered with the roughness elements at four different roughness densities (λ = 0, 0.017, 0.08, 0.18). Speed-up around the cubes caused higher peak surface shear stress than in experiments with plants at all roughness densities, demonstrating the more effective sheltering ability of the plants. The sheltered areas in the lee of the plants are significantly narrower with higher surface shear stress than those found in the lee of the cubes, and are dependent on the wind speed due to the plants ability to streamline with the flow. This streamlining behaviour results in a decreasing sheltering effect at increasing wind speeds and in lower net turbulence production than in experiments with cubes. Turbulence intensity distributions suggest a suppression of horseshoe vortices in the plant case. Comparison of the surface shear-stress measurements with sediment erosion patterns shows that the fraction of time a threshold skin friction velocity is exceeded can be used to assess erosion of, and deposition on, that surface.

This study presents the results from a series of wind-tunnel experiments designed to investigate the aerodynamic roughness length z₀ of fresh snow under no-drift conditions. A two-component hot-film anemometer was employed to obtain vertical profiles of velocity statistics in a zero pressure gradient turbulent boundary layer for flow over naturally deposited snow surfaces. The roughness of these snow surfaces was measured by means of digital photography to capture characteristic length scales that can be related to z₀. Our results show that, under aerodynamically rough conditions, the mean value of the roughness length for fresh snow is z₀ = 0.24 mm with a standard deviation σ(z₀) = 0.05 mm. In this study, we show that variations in z₀ are associated with variations in the roughness geometry. The roughness measurements suggest that the estimated values of z₀ are consistent with the presence of irregular roughness structures that develop during snowfalls that mimic ballistic deposition processes.

An experiment setup for measuring surface shear stress distributions in different dense live plant canopies was introduced and first results were presented. A surface shear stress sensor calibration procedure is demonstrated which determines a universal calibration function that can be applied for multiple sensors without significant reduction in accuracy. This avoids the enormous work of calibrating every single sensor when using multiple sensors for the measurements. Accuracy tests verified that reliable surface shear stress data can be obtained with the employed calibration. Data processing and averaging procedures applied to the data are discussed. First analyses of the measured surface shear stress distributions show an overall trend of higher momentum absorption and increased sheltering effect for higher canopy densities as expected. The influence of streamlining of the plants is reflected by narrower sheltered areas and slightly increased surface shear stress at the sides of the plants at higher wind velocities.

Velocity profiles over fresh fallen snow surfaces have been measured in the SLF cold atmospheric boundary layer wind tunnel. Aerodynamic roughness lengths z0 have been estimated from log-law velocity profile fitting. In parallel, photographs of the snow surfaces have been taken and evaluated using digital image analysis giving snow surface contour line coordinates. Applying structure functions to the snow surface coordinates and statistical fitting analyses resulted in sets of surface characteristic length scales (random field approach). The synthesis of both approaches suggests a linear relationship between z₀ and one of the surface characteristic length scales.

Aerodynamic roughness lengths z₀ of snow surfaces have been determined from wind velocity measurements in a boundary layer wind tunnel. Complementary, surface characteristic length scales have been deduced from photographs of these snow surfaces by means of digital image processing and statistical analyses. Both sets of data are confronted in order to reveal possible correlations between the geometric properties of snow surfaces and the associated aerodynamic roughness lengths z₀. The current dataset generally shows increasing roughness lengths z₀ with increasing characteristic length scales.