During September and October 1997, in the framework of a stay at the Shinjo Branch of Snow and Ice Studies, we used a Cryospheric Environment Simulator (Higashiura and others, 1997) and simulated drifting snow to test four mechanical traps. First we present the intercomparison of the four mechanical gauges, then we compare the gauges with the snow-particle counter (SPG). Comparing the four different traps tested, we have observed that the box type (snow collector) is generally more efficient than the net-type collectors. These results confirm the tendency observed in field experiments (Font and others, 1998b). Using the SPG to calibrate the mechanical gauges, we observed that the net-type traps underestimate transport in low-transport conditions, but as transport increases the underestimation tends to zero. Comparing the snow collector with the SPG, we observed good agreement between the two gauges.

Sub-grid variability of the snow cover is an important issue with regard to catchment runoff or mesoscale meteorological modeling. Here, an evaluation is presented of spatial snow measurements conducted on 5 days in winter 1998/99 and 2 days in winter 1999/2000 in a 0.7 km2 Swiss pre-Alpine catchment. Snow-depth data were analyzed with two different linear regression models, one including altitude, terrain slope, terrain aspect and canopy density, and one using altitude and simple land-use indicators. For the single measurement date/the first model was somewhat superior to the indicator model. The error term of the regression models showed only weak spatial dependence. Finally, a time-space linear regression model describing both the temporal development and the spatial distribution of the snow cover was fitted with the measurements of the first winter and tested with measurements of the second winter. The validation showed a satisfactory match between measurements and models in late December, but a slight overestimation of the measurements by the models in early April. In view of the models’ability to reproduce the snow-depth patterns satisfactorily at this rather detailed scale, it was concluded that such regression models might be a suitable tool to treat sub-grid variability of snow depth in larger-scale models.

The application of nearest-neighbor algorithms to the task of regional avalanche forecasting in Switzerland is presented in this paper. The database used for the development of the model consists of snow and weather data from 60 manual weather stations and conventionally estimated avalanche-hazard levels. All these data are collected by the Swiss Federal Institute for Snow and Avalanche Research on a daily basis during winter. Data between 1987 and 1996 (10 winters) are used for our study. For the manual weather stations a nearest-neighbor model has been developed: NXD-VG calculates the 10 nearest-neighbor days by using a Euclidean weighted distance metric. A regional avalanche-hazard map is calculated by interpolating the results of NXD-VG between the stations. The avalanche forecasters can access the results of the model calculation directly because they are integrated into a program for bulletin construction. The model was validated using three complementary approaches. First, the database is cross-validated for all the winters available to estimate an unbiased prediction error of the models for two selected stations. Second, selected situations of the database are recalculated. Third, the model output is compared daily to the official forecasts published during winter 1999/2000.

Avalanche forecasters can better anticipate avalanche extremes if they understand the relationships between those extremes and atmospheric circulation patterns. We investigated the relationship between extreme avalanche days and atmospheric circulation patterns at four sites in the western United States: Bridger Bowl, Montana; Jackson Hole, Wyoming; Alta, Utah; and Taos, New Mexico. For each site, we calculated a daily avalanche hazard index based on the number and size of avalanches, and we defined abnormal avalanche events as the top 10% of days with recorded avalanche activity. We assessed the influence of different variables on avalanche extremes, and found that high snow water equivalent and high snowfall correspond most closely to days of high avalanche hazard. Composite-anomaly maps of 500 hPa heights during those avalanche extremes clearly illustrate that spatial patterns of anomalous troughing prevail, though the exact position of the troughing varies between sites. These patterns can be explained by the topography of the western United States, and the low-elevation pathways for moisture that exist to the west of each of the sites. The methods developed for this research can be applied to other sites with long-term climate and avalanche databases to further our understanding of the spatial distribution of atmospheric patterns associated with extreme avalanche days.

During winter 1999 three large avalanche events were triggered by explosives at SLF’s avalanche test site, Vallée de la Sionne, canton Valais, Switzerland. One important goal of these large-scale field experiments was to measure the release and deposition volumes of avalanches by photogrammetric methods. In this paper, the photogrammetric measurements of all three avalanches are summarized. For one avalanche event it was possible to realize the whole measuring procedure as planned, and to obtain volume measurements before and after the avalanche triggering In the other two avalanche events, the photographs before the triggering of the avalanche failed. Nevertheless the photographs taken after the avalanche provide valuable information on the fracture depth at the fracture line. The mean fracture depth of the largest avalanche was about 2.10 m, varying between 1 and 3.5 m over a width of > 1000 m. The total volume of the deposition of all three avalanche events was about 1300 000 m3. The deposits are distributed over a length of > 1000 m with depths up to 30 m. The difference between the released and deposited volumes proved that avalanches entrain a large amount of snow along the avalanche track. Furthermore, the snow distribution in the deposition zone provides important information about the behaviour of a dense flowing avalanche in the runout zone.

Snowdrift is one of the many manifestations of two-phase flows in which the inherently stably stratified drift-density profile acts to destroy turbulence. This can be quantified by using an appropriate “particle” Richardson number, equivalent to thermally stratified flow. This Richardson number is proportional to the mean fall velocity of the particles (averaged over the particle-size spectrum) and the drift density and therefore depends strongly on height above the surface. It exhibits a maximum close to the surface, where drift densities are largest. It then decreases to minimum values at intermediate heights, above which Richardson numbers increase with height. The particle Richardson number and the associated decrease in turbulent exchange coefficient depend strongly on wind speed, particle-eddy exchange coefficient and mean particle radius It is found that particle-buoyancy effects in snowdrift are non-negligible for large wind speeds and large particle-eddy exchange coefficients.

Wind transport of snow can cause an additional snow load on leeward slopes, which often has a considerable influence on avalanche danger. For a quantitative assessment of this process, a model is proposed which calculates the snow transport over a two-dimensional mountain ridge, based on input measurements of wind speed and precipitation. Since the topography is idealized, the model is focused on the snow mass that is transported over the ridge, and no statements are made about the exact snow distribution over the slopes. Three transport modes are distinguished: snow transport in saltation, snow transport in Suspension, and preferential deposition of precipitation. Suspension is modelled with a one-dimensional diffusion equation, and for the saltation layer a newly developed model, based on the microscale physical processes, is implemented. The effect of speed-up of the wind over the ridge is included by assuming an analytical wind profile with a maximum wind speed at a few meters above the ridge Advective effects are taken into account in a parameterization of the turbulent shear stress profile. The model is compared with measurements taken at the experimental snowdrift site Gaudergrat in the Parsenn area, Switzerland, and good agreement is obtained between calculated and measured results.

Snowdrifting processes and the wind-velocity profiles around a collector and a blower snow fence were investigated in a cold wind tunnel. The purpose was to ascertain the effect of wind direction on drift control by snow fences. Three different cases were studied for both types of snow fence, and the resultant snowdrifts were compared. In the first case, the snow fence was perpendicular to the wind direction. In the second and third cases, it was tilted by 30° and 45°. When the collector snow fence was tilted, the amounts of snowdrift were much less than when the fence was perpendicular to the wind direction, because the area with low wind velocity was reduced to half behind the tilted fence. On the other hand, the blowing effect of the blower snow fence increased when it was set up at an angle to the wind direction. It is necessary to investigate the position where the blown snow is deposited by the tilted blower snow fence.

The SAFRAN/Crocus/MÉPRA software is used to assess the climatology of the avalanche hazard and its sensitivity to climate change. A natural avalanche-hazard index based on MEPRA analysis is defined and validated against natural avalanche observations (triggered avalanches are not taken into account). A 15 year climatology then allows a comparison of avalanche hazard in the different French massifs. Finally a simple climate scenario (with a general increase of precipitation and temperature) shows that avalanche hazard may decrease slightly in winter (mainly February) and more significantly in May/June. The relative proportion of wet-snow avalanches increases.

Vertical profiles of snowdrift density and concurrent meteorological data were measured over Antarctic snow and blue ice during a 1 month field experiment in austral summer 1997/98. It is found that drift densities and transport rates over blue ice are significantly smaller than over snow, which can be attributed mainly to the limited availability of snow particles at the blue-ice surface. Hence, over blue ice, near-surface drift densities and vertical gradients in drift density are relatively small. Over blue ice, snowdrift can occur in very weak winds, presumably because of the smoothness of the surface. Over snow, in contrast, drift occurs only in much stronger winds, such that the shear stress applied by the flow can overcome the cohesive interparticle bonds.

The Érosion torrentielle, neige et avalanche (Etna) unit of CEMAGREF and the Centre d’Etudes de la Neige of Météo-France have been working on snowdrift for 10 years. A numerical model was developed at CEMAGREF to simulate snowdrift (Naaim and others, 1998). To validate this model on in situ data, a high-altitude experimental site was developed, located at 2700 m a.s.l. at the Lac Blanc Pass near the Alpe d’Huez ski resort. It is a nearly flat area and faces winds primarily from north and south. After describing the experimental site, we present the processed data of winter 1998/99. First, we analyze the data from CEMAGREF’s acoustic snowdrift sensor. It is sensitive to snow depth and snow-particle type, so additional calibration is necessary. Nevertheless, it allowed us to study non- stationary aspects of drifting snow. An analysis of gust factors for wind and drifting snow indicates that strong wind-gust factors exist in the mountains, and that drifting snow is more important during a regular and strong wind episode than during high wind-gust periods. Therefore, the numerical model presented here uses only the recorded mean wind speed. The model, which attempts to reproduce several days of storm, takes into account the modification of input parameters (e.g wind speed) as a function of time. The comparison between numerical results and measurements for a given meteorological event shows good agreement.

Avalanche risk is considered as the probability of an avalanche event that could cause certain losses. A unified approach for avalanche-risk evaluation by statistical simulation is suggested. A chain of models, describing snow deposition, transformation, stability, avalanche dynamics and interaction with an obstacle, is used within such an approach. Each of the models evaluates a given situation in a deterministic manner, yielding a unique result value. Output data of each model can be the input for the next model in the chain. Uncertainty of input data is described in a probabilistic manner. Using the chain of the deterministic models and simulating the input data according to appropriate probability distributions with the Monte Carlo method, risk evaluations are obtained as the ratio of a number of certain types of outcome to the total number of tests. All kinds of information on weather, snow and avalanches can be used within this scheme. The simulation process can be started at any stage of the modelling. In this study it was started from the snow-cover stability simulation. Application of the statistical simulation in such a way gives an opportunity to reflect uncertainty of the initial data in the results obtained. The suggested scheme was used for producing a computer-assisted workplace for avalanche forecasting, “LAVINA”, which has been exploited at the Centre of Avalanche Safety of “Apatit”, Kirovsk, Russia, since the early 1990s. It is an integrated system that permits spatial and temporal estimations of snow stability and avalanche dynamics to be made. Assessments of the validity of the risk estimation made with LAVINA are presented.

For about ten winter seasons, the Centre d’Etudes de la Neige and CEMAGREF have performed intensive field measurement campaigns concerning various snow and weather parameters during blowing-snow events. The observation sites are located near the ski resort, Alpe d’Huez, Col du Lac Blanc, France (2700–2800 m a.s.L). Our first objective is to include the main effects of the wind in the operational avalanche-hazard forecast at the massif scale, taking into account the effects of local winds on our snow-cover estimation without simulating the phenomena exactly. Only the effects on snow accumulation or erosion and snow characteristics are measured, without any direct link with a precise field location except for the validation. The underlying numerical simulations have been tested on the instrumented site and include an assessment of the appropriate shear velocity, an evaluation of the occurrence of transport related to the snow characteristics provided by the software PROTEON and an estimation of the transport rates. These different computations are embedded in the new software, SYTRON, which is inserted in the SAFRAN/Crocus/MEPRA operational chain! Different validations of this new application are presented in terms of shear velocities, observed transport periods and snow depths. The encouraging results augur well for an operational implementation next winter

In the Pacific Northwest of North America, significant flooding can occur during mid-winter rain-on-snow events. Warm, wet Pacific storms caused significant floods in the Pacific Northwest in February 1996, January 1997 and January 1998. Rapid melting of the mountain snow cover substantially augmented discharge during these flood events. An energy-balance snowmelt model is used to simulate snowmelt processes during the January 1997 event over a small headwater basin within the Reynolds Creek Experimental Watershed located in the Owyhee Mountains of southwestern Idaho, U.S.A. This sub-basin is 34% forested (12% fir, 22% aspen and 66% mixed sagebrush (primarily mountain big sagebrush)). Data from paired open and forested experimental sites were used to drive the model. Model-forcing data were corrected for topographic and vegetation canopy effects. The event was preceded by cold, stormy conditions that developed a significant snow cover over the sub-basin. The snow cover at sites protected by forest cover was slightly reduced, while at open sites significant snowmelt occurred. The warm, moist, windy conditions during the flooding event produced substantially higher melt rates in exposed areas, where sensible- and latent-heat exchanges contributed 60–90% of the energy for snowmelt. Simulated snow-cover development and ablation during the model run closely matched measured conditions at the two experimental sites. This experiment shows the sensitivity of snowmelt processes to both climate and land cover, and illustrates how the forest canopy is coupled to the hydrologic cycle in mountainous areas.

The saltation-layer structure of drifting snow was investigated using the wind tunnel in a cold room. Experiments were conducted under various snow-temperature and wind-speed conditions over loose and hard snow covers. Snow was seeded at the upwind end of the wind tunnel. Mass-flux profiles of drifting snow were measured with a snow-particle counter. The theoretical expression for the mass flux of saltating sand (Kawamura, 1948) was fitted to the measured profiles, and two parameters in the theoretical expression, saltation height h0 and mass flux at the surface q0, were determined. The main results are as follows: (1) In the case of hard snow cover, snow particles are hardly ejected and drifting snow is maintained by the seeded snow. The value of h0 linearly increases with wind speed and decreases with snow temperature, and q0 decreases with wind speed and is in proportion to seeding rate. (2) In the case of loose snow cover, erosion occurs under high-wind conditions and the contribution of the ejected snow particles to drifting snow is remarkable. The h0 linearly increases with wind speed, but its value is smaller than the value over hard snow cover. Due to erosion, q0 increases with wind speed. Snowdrift transport rate and q0 do not change with seeding rate under low-wind conditions, because the drifting snow is saturated. Under high-wind conditions, however, both snowdrift transport rate and q0 slightly increase with seeding rate.

A simple center-of-mass avalanche model that accounts for avalanche flow in forests is presented. The model applies the principle of conservation of energy to calculate the deceleration of avalanches caused by tree fracture, overturning and debris entrainment. The model relates the physical properties of forests (tree spacing, tree age, tree type, soil conditions) to avalanche flow. Modified dry-Coulomb and velocity-dependent friction parameters commonly used in avalanche runout calculations are derived. Example calculations demonstrate how the model can be applied to back-calculate observed avalanche events. The model quantitatively explains why large avalanches can destroy forests without significant deceleration. Furthermore, it shows why tree fracture consume/little of the avalanche’s energy. Finally, the model reveals how protective forests in avalanche tracks can be maintained over time to provide the best protective capacity against snow avalanches.

Snow surface temperature (Ts) plays an important role in the formation of surface hoar or near-surface faceted crystals The goal of this study was to obtain detailed information on Ts in different forest stands nelr the timberline. The investigations were conducted during clear nights and showed that the snow surface temperature is influenced very strongly by the forest canopy. While the air temperature was very similar on the different experimental sites, Ts was higher in the forest than in the open field; on the south-facing slope the difference between the forest and the open field was 3–4.5°C, and on the north-facing slope approximately 3–7°C. Taking into account that εair is 0.7 and εtree is 0.94, the incoming radiation (I ↓) for the different experimental sites was calculated by the equation of Brunt (the canopy density was estimated using photographs taken with an 8 mm fish-eye). To calculate Ts, air temperature and averaged values of the net radiation (because the net radiation (I) has only a small range of variation during clear nights) were used. The results show that the calculated values were higher than the measured values (by approximately 2°C). However, a better correlation was found by using lower values of the emissivity (εair0.67 and εtree0.91).

This paper contains statistical analyses of parameters to characterize starting zones of destructive avalanches which have resulted from clear-cut logging in British Columbia, Canada. Data from 76 avalanche sites in the Coast Mountains (western British Columbia) and the Columbia Mountains (eastern British Columbia) are analyzed. The parameters include a selection which characterize snow supply (related to potential avalanche frequency), avalanche magnitude and those which are known to affect avalanche formation including terrain features, vegetation density, vegetation height and ground surface roughness. The results provide the data framework for possibly preventing future disasters by altering logging plans.

Since 1993 the Avalanche Centre of Arabba has managed a test site to determine avalanche-dynamics parameters (Sommavilla and others, 1997; Sommavilla and Sovilla, 1998). The system is located on an avalanche track which is representative for the Dolomites, northern Italy It monitors avalanche pressures, speed, flow height and variations of the avalanche shape and extent. In winter 1997/98, together with the standard measurements, a series of new field measurements and observations of the snow cover on the avalanche path were conducted for the first time in order to accurately determine the avalanche mass balance. The information collected is typical for dense flow avalanches which have small dimension and develop mainly along a channelled path. In winter 1997/98, four events were studied. For each event, in several sections from the starting zone to the deposition zone, manual measurements were carried out in order to investigate mass entrainment and deposition processes. The mass evolution of the avalanche has been determined. It is shown that the avalanche mass increases by up to 720% with respect to the initial release mass. This entrainment process is related to the speed reached by the avalanche front. In addition, it has been determined that during the acceleration phase of the avalanche front the underlying snow cover is mostly eroded and there is no deposition of snow. In the deceleration phase, by contrast, erosion decreases progressively, reaching the value zero, while deposition becomes progressively larger. These results underscore the importance of the mass balance as a fundamental component in avalanche-dynamics research.

The distribution of slushflows in northern Europe is described. Schematic maps of the beginning and ending periods of slushflow risk are presented. The months with maximum slufhflow activity are also shown. Potential changes in the geographic and temporal distribution of slushflows due to global warming were analyzed for this region. Calculations for the year 2050 were done on the basis of climatic changes given by three global circulation models. The method of calculation was based on the general relationships between climatic parameters and the geographical and temporal distribution of slushflows. Schematic maps of potential distribution of slushflows show changes when calculated according to the most extreme of the three climatic-change scenarios.