Snow-cover models are used in many applications in today’s snow and ice research. Descriptions of changes in size and shape are a major problem in modelling the snow cover. Empirical models for kinetic growth under temperature gradients have been developed, as well as more complicated models based upon microstructure. In this work a simple; physically based model is derived which depends on one adjustable geometric factor only Snow texture is described as a body-centred cubic lattice containing source and sink grains. The latter grow as plates due to water-vapour transport in the layer as well as between the layers. The model was implemented in a research version of the one-dimensional snow-cover model SNOWPACK. Model outputs are compared to experiments done in the cold laboratory where sieved snow is subjected to temperature gradients. Disaggregated snow samples are analyzed by digital image processing, by sieving and by visual characterization. In order to determine grain-size as objectively as possible, these various methods are evaluated for compatibility. The new model simulates very well kinetic grain growth for densities of 100–200 kg m−3 and temperature gradients up to –200 Km−1. The model will be incorporated in the operational version of SNOWPACK.

The texture of snow has a great impact on the mechanical and thermal properties of a snowpack. The texture index (TI) is a new concept to quantify the texture of snow at the micro- and mesoscale, a scale at which the properties of snow layers are responsible for the stability of a snowpack. It is calculated by taking the ratio of mean grain-size (mm) of snow to density (kg m−3) of snow in a given layer. It combines a microstructural with a mesostructural parameter. An empirically derived TI is compared with a simulated TI for laboratory experimental snow and natural snow profiles. TheTI was calculated throughout the development of three temperature-gradient snow experiments from laboratory measurements of grain-size and density. In a one-dimensional finite-element snowpack model (SNOWPACK), the development of the mean grain-size and density of these snow samples was simulated. It can be shown that the measured TI can be predicted by the modelled TI. Furthermore, three natural snow profiles from the Weissfluhjoch (Switzerland) test-field are compared with their simulations in terms of TI. This comparison reveals shortcomings in the measurement of fundamental parameters. It also shows the dependency of a good physical model on quantitative process descriptions at the microscale TheTI can be estimated using the force signal of the SnowMicroPen, a new high-resolution snow micropenetrometer. It is suggested that more accurate classical measurements combined with penetrometer measurements in the immediate vicinity of the snow profile will allow better verification of SNOWPACK simulations.

Dielectric measurements of snow were carried out in the temperature range –15° to 0°C and in the frequency range 50 Hz to 5 MHz. The snow samples (about 400 kg m–3 density) used were stored snow (average particle size: 2 mm) and hoar-frost (particle size: <1 to 5 mm). The frequency characteristics of dielectric parameters showed a dielectric dispersion (Davidson-Cole type) around 30 kHz and a low-frequency dielectric dispersion (Cole-Cole circular law type). The a.c. conductivity showed a dielectric dispersion around 30 kHz and two characteristic constant values in the frequency ranges above 1 MHz and below 100 Hz (the high-frequency conductivity σ∞ and the low-frequency conductivity σLOW). The low-frequency conductivity σLOW showed a peak at about –2°C. This behavior has never been noted by previous researchers. The σLOW showed an activation energy of about 1 eV below –5°C. This means that the σLOW is mainly caused by a surface conduction. The activation energy increased with increasing temperature above –5°C. This means that the σLOW in this temperature range is affected by the quasi-liquid layer on ice surfaces. The σLOW above –2°C decreased with increasing temperature. The apparently curious behavior near the melting temperature is attributed to the numerous free ice surfaces within the porous snow. This conclusion was reached because our measurements without the free ice surfaces showed no such conductivity peaks for solid polycrystalline ice samples and for snow samples soaked with kerosene in the cooling process.

During two winters, typical meteorological conditions (temperature, humidity, wind, radiation) and snow-surface conditions (snow surface and snow temperatures) were measured to simulate the formation and ablation processes (mainly sublimation) of surface hoar on two level snow plots, situated at 2500 and 1500 m a.s.l. In order to verify the simulated deposition/ablation rates, the surface-hoar mass, thickness and occasionally density were also measured. The evaluation shows that, using the aerodynamic bulk method for the half-hourly simulation periods, >90% of the day/night periods could be rated as either hoar-formation or ablation periods. The simulated sublimation rates (deposition/ablation) deviate in the mean by not more than 10% from the measured amounts! However, some larger deviations are present, mainly during ablation periods with heavy melting. Finally a method is shown for transforming net deposited vapour amounts into surface-hoar layers of a corresponding height in order to produce “layers” which might be integrated into snow-cover simulation models.

Various field observations of surface-hoar formation were carried out with measurements of vapor sublimation rate and meteorological conditions from 1994 to 1996 in a mountainous area of northern Hokkaido,Japan. The sublimation rate seemed to increase with wind speed when the relative humidity was high, whereas the sublimation rate changed from positive to negative as the wind speed increased when the relative humidity was low (60–70%RH). These results were explained with a numerical model (called the Simple model) for the heat balance at the snow surface; the results of the model showed that there is a specific wind speed which maximizes the sublimation rate when the relative humidity is low, and indicated that the sublimation rate becomes large under humid conditions (>90%RH) and if wind speeds are increasing in the range 0.5–3.5 m s–1. The heat balance at the snow surface reproduced by another snow model, Crocus, agreed well with the observation results. However, Crocus overestimated the sublimation rate by about 1.3 × 10–6 kg m–2 s–1 when surface hoar formed, and underestimated snow surface temperature by several degrees.

Heavy rain on snow often leads to disastrous damages in torrent watersheds. In January 1998 a project was started to investigate “runoff and infiltration characteristics of different alpine soil/vegetation units under snow cover”. One aim was to determine the runoff rates from snow-covered, differently cultivated soil/vegetation units (pastures, dwarf-shrub stands, forests), especially under conditions of sparse snow cover and frozen soil. Differences in runoff formation between artificially snowed skiing areas and plots with natural snow cover were also of interest. Heavy rain (intensity 100 mm h–1) was simulated on four plots by using a transportable spray irrigation installation. The investigations showed runoff coefficients of 0.4–0.7. The lowest runoff values were found where the soil under snow cover was not frozen, but even on these stands the runoff coefficient exceeded 0.4. Snow depth and runoff delay are strongly correlated (R2 = 0.8). An increment in snow of 10 cm is followed by a runoff delay of 3.6 min. Divergences from this coherence are due to the composition of the snowpack, especially in case of artificial snow. This result is encouraged by runoff simulations performed for the irrigated plots by use of a hydrological model.

This paper reports on the measurement of the viscosity coefficient of very light artificial snow with H. density of < 100 kg m−3. The temperature dependency on the viscosity coefficients of the snow was also obtained, and these results were applied to the problem of snow creep. General comparison between experimental and computed results shows good agreement, but differences between the two creep flows along the slope indicate that low-density snow near the freezing point should be considered as an orthotopic material.

In order to determine the viscoelastic properties of snow, torsional shear measurements were performed in a cold laboratory with a stress-controlled rheometer. Small samples (60 mm in diameter and about 7 mm thick of natural snow collected from the nearby study plot were loaded in simple shear with monotonically increasing stress (stress ramp) and with sinusoidally varying stress (oscillation). The dynamic measuring method allows the deformation process to be separated into a time-independent part (elastic) and a time-dependent part (viscous). The applied torque is sufficiently small to prevent destructive deformation, generally permitting the true viscoelastic properties of a sample to be obtained over a large range of frequency and temperature. The limit strain for linear viscoelastic deformation was found to be very small (0.5−5 × 10−4). Experiments performed beyond the linear range imply important textural changes (damage, breaking of bonds). The large strain reached during stress-ramp experiments showed that the ongoing damage process must be balanced by a healing (sintering) process. The usefulness of a rheometer was proven. It is a precise method for measuring with high reproducibility the rheological parameters of snow, and data gained with it improve our understanding of the deformation process under shear loading.

Water vapor in snow is responsible for two main processes connected to almost all studies where snow cover is involved: the snow-density change with time and snow recrystallization. Both processes are the result of a balance between evaporation and condensation on individual snow-crystal surfaces. However, such micro-scale mass balance has rarely been considered as a component of “macro-” heat and mass transfer in snow cover. The present work is an attempt to find a way of combining these two mass-exchange processes, as occurs in Nature. Density change and snow recrystallization rates are analyzed based on recently published temperature field observations around individual snow crystals, combined with experimental data on temperature distributions and recrystallization rates in snow under applied temperature gradients.

The shear frame allows testing of thin weak snowpack layers that are often critical for slab avalanche release. A shear metal frame with an area of 0.01–0.05 m2 is used to grip the snow a few mm above a buried weak snowpack layer. Using a force gauge, the frame is pulled until a fracture occurs in the weak layer within 1 s. The strength is calculated from the maximum force divided by the area of the frame. Finite-element studies show that the shear stress in the weak layer is concentrated below the cross-members that subdivide the frame and where the weak layer is notched at the front and back of the frame. Placing the bottom of the frame in the weak layer increases the stress concentrations, and results in significantly lower strength measurements than placing the bottom of the frame a few mm above the weak layer. Based on over 800 sets of 7–12 tests in western Canada, coefficients of variation average 14% and 18% from level study plots and avalanche start zones, respectively. Consequently,sets of 12 tests typically yield a precision of the mean of 10% with 95% confidence, which is sufficient for monitoring of strength change of weak layers over time in study plots. With consistent technique, there is no significant difference in mean strength measurements obtained by different experienced shear frame operators using the same approximate loading rate and technique for placing the frame. Although fracture surfaces are usually planar, only one of eleven shapes of non-planar fracture surfaces showed significantly different strength compared to planar fracture surfaces. For weak layers thick enough for density measurements, the shear strength is plotted against density and grain form. From these data, empirical equations are determined to estimate the shear strength of weak snowpack layers.

The various mechanical properties of dry coherent snow are determined by the same structural peculiarities of this medium, that can be described using the model of regular packed grains connected by rigid bonds. Analytic expressions for the many important snow mechanical parameters density elastic moduli, stress wave velocities, strength, etc.) are derived using the model by introducing three non-dimensional principal structural factors: texture friability, bond rigidity and coordination number. Analytic expressions that relate tensile strength to P and S stress wave velocities are proposed and used to examine the interrelations between the internal structure and tensile strength for snow-ice formations in a wide density range. The theoretically derived results and available experimental data are well correlated and form the basis for the development of non-destructive testing methods to evaluate the strength characteristics of snow, using seismic and acoustic measurements.

For the first time, three-dimensional (3-D) high-resolution images of snow were obtained using X-ray absorption tomography. Images with a spatial resolution of 10 μm were taken on four different cylindrical snow samples (9 mm high, 9 mm diameter). About 1000 two-dimensional X-ray absorption images were recorded at angular positions of the object around an axis spanning 180°. An appropriate algorithm was then used for these data to reconstruct a 3-D image. In the case of snow, experimental problems have been solved to prepare the samples and prevent both melting and metamorphism of snow during the experiments. This tomographic method provided 3-D data files from which images of 6003 voxels were extracted Several physical parameters of snow microstructure can be processed from these data. Porosity P and discrete local (3-D) curvature C of the grain/pore interface were computed for the four snow samples. Representative elementary volume (REV, in the sense of porous media) is a relevant index to the significance of the sample size with respect to a given parameter. From each image, the values of P and C are compared for subsamples of different size, as an attempt to assess the REVs for porosity and curvature. Results show that the observed volume of snow is statistically significant to achieve the porosity and the curvature distribution.

The purpose of this study is to identify characterize and quantify local, regional and remote effects of snow cover on western U. S. climate and water resources. An ensemble of predictability and sensitivity studies was made with the U.S. National Center for Atmospheric Research (NCAR) Community Climate Model, version 3 (CCM3) to investigate the relative roles of snow-cover anomalies and initial atmospheric states in the subsequent accumulation and ablation seasons. The suite of model experiments focuses on the direct effect of snow on regional climate anomalies and ultimately will be used to examine the lagged effect of anomalous snow cover on the climate. The set of ensemble simulations presented here looks at the climate-system response to anomalously high and low snow cover at the start of the ablation season over the western U.S.A. These current results suggest that the initial state of snow cover is more important than the initial state of the atmosphere or of sea-surface temperatures because of direct thermal effects on the surface and subsequent indirect, dynamical effects on the atmospheric circulation.

The water balance of the mountainous Durance river catchment, French Alps, is simulated from 1981 to 1994 with a soil-vegetation-atmosphere transfer (SVAT) model. Particular attention is paid to the snow-cover evolution using a detailed model of the snowpack evolution. The results are validated by comparison of the simulated discharges calculated by the SVAT with daily observations at three gauging stations located in the watershed. Three different spatial resolutions are used (1, 8 and 46 km) in order to evaluate the impact on the surface-water-budget results. Comparison with the finest resolution indicates the need for sub-grid-scale parameterization for the model with larger resolution.

The continental regions of China are extensively covered by snow during winter. In this paper, the seasonal snow cover in the Tien Shan and Altay mountains is observed and analyzed, based on the characteristics of the dry-cold snow cover accumulating under the continental climatic conditions in northwest China. Compared with the humid-warm snow cover, the dry-cold snow cover is characterized mainly by small density, low water content, large temperature gradient, thick depth hoar, etc., and its metamorphism is dominated by the thermal exchange and the pressure of the overlying snow layers. According to the observed data, the solar radiation flux on the snow surface is dominated by a negative balance in the arid areas in China during the snow season; the albedo on the new-fallen-snow surface is up to 96%, and the transmission depth of shortwave radiation in dry-cold snow cover is 28 cm. During the snowmelt season in spring, the thickness of the depth hoar can occupy 80% of the whole snow cover.

To monitor the snow-cover distribution in relation to meteorological conditions on high mountainous areas in Japan, NIED constructed a snow-observation network which it has operated for approximately 10 years. The network consists of seven pairs of stations, each comprising a mountainous site and a low-lying flatland site, from Hokkaido district in the north to San-in in the southwest. Data obtained include snow depth, snow weight, air temperature and global solar radiation. This study presents recent fluctuation of snow cover on mountainous areas for several recent winters in Japan. Most winters were warmer than average, but winter 1995/96 was normal, and maximum snow depths were recorded in high-elevation areas. Seventy-seven avalanche accidents occurred in winter 1995/96. The relationship between meteorological and snow conditions in mountainous areas and flatland areas was analyzed.

Detailed data on autumn, winter and spring snow-cover conditions from remote areas are often difficult or very expensive to obtain. Therefore, an inexpensive method of digital photography was tested in high-Arctic Greenland. Automatic digital photography has provided daily data on snow distribution and snow depth for > 1 year from the Zackenberg area (74°30’ N) in northeast Greenland. A standard digital hand-held camera (Kodak Digital Science DC50) was equipped to become automatic; it is supplied with an “automatic finger” and an external power supply and built into a protecting box with additional solar panels on top, in order to secure continuous operation throughout the year. The daily photograph covers a 100 m transect through a seasonal snowpatch, and thus on an annual basis also yields information on snow-cover duration in the different vegetation zones of the snowpatch. The camera was installed in mid-August 1998. Photographs from the period mid-August 1998 to early September 1999 were collected and analyzed. All photos are taken at astronomic noon in order to use the daylight as long as possible into the winter season before the 24 h winter darkness begins in mid-November.

The digital photographs yielded the following information for the year 1998/99: The first winter snowfall occurred on 18 October; small-scale snow redistribution by snow-drifting started when the winter snow cover was about 5 cm thick. The continuous winter snow cover lasted for a minimum of 170 days in the most snow-deprived areas downwind of the snowpatch, whereas snow in the centre of the snowpatch stayed on the ground for > 325 days, turning the snowpatch perennial in summer 1999.

Meteorological data obtained close to the photographed snowpatch site, in combination with the snow-cover depth and distribution data derived from the photographs, show that at wind speeds (at 2 m height) of up to around 6 m s−1, only small-scale snow-drifting took place, when the snow cover was thin. Intense snowdrifting, however, took place at wind speeds of 8–13 m s−1, particularly when enough snow was available in the upwind area. The automatic continuous photography technique demonstrated here could be particularly useful in remote areas at high risk of avalanches during winter. It is an alternative to traditional snow monitoring achieved mainly by sonic sensors, snow pillows or manual measurements of snow depths Likewise, it can provide better areal information than most standard methods, which give only point measurements.

In this paper, we show that a detailed snow model (here, the Crocus model) may help to validate large-scale inferred meteorological datasets (e.g. from climate models or analyses) over the data-sparse ice sheets. Two series of snow simulations are carried out with two different meteorological datasets in input to the snow model. Both datasets are extracted from the European Center for Medium-range Weather Forecasts meteorological analyses and forecast archives. First, the microwave signatures of the surface of central Greenland from the Scanning Multichannel Microwave Radiometer (SMMR) are compared with the simulated density, grain-size and stratigraphy. The annual mean gradient ratio and polarization ratio, which are related to the emissivity of snow, are found to correlate spatially with these snow structural parameters. The sensitivity of the snow structure to differences in the two meteorological sets is then examined. It is found to be high for temperature and infrared radiation, precipitation and surface wind. The quantitative value of this result is limited by a still limited snow model validation over Greenland. Also, an optimal use of satellite data and a snow model for meteorological validation would require physically based translation of the simulated snow parameters into radiative properties, i. e radiation transfer modeling.

Results of wind-tunnel investigations of the propagation of air within snow showed that the process responsible for causing horizontal air flux in the pore space of snow was not necessarily related to specific surface topography. The reasons for this flux formation are: (i) the near-surface wind characteristics, such as the frequency of the pressure variation related to the turbulent character of the near-surface wind; (ii) the structure of the near-surface snow, responsible for the air exchange between the atmosphere and the pore space; and (iii) the temperature difference between the snow and the atmosphere, forming the air-density difference which augments or decreases the air exchange. This paper summarizes the experimentally observed effects that the wind and the air flux generated in the pore space had on snow. These included isothermal temperature distributions throughout the artificial snow cover, snow-density change and snow recrystallization.