54,847 research outputs found
Micrometeorological processes driving snow ablation in an Alpine catchment
No full textMountain snow covers typically become patchy over the course of a melting season. The snow pattern during melt is mainly governed by the end of winter snow depth distribution and the local energy balance. The objective of this study is to investigate micrometeorological processes driving snow ablation in an Alpine catchment. For this purpose we combine a meteorological model (ARPS) with a fully distributed energy balance model (Alpine3D). Turbulent fluxes above melting snow are further investigated by using data from eddy-correlation systems. We compare modelled snow ablation to measured ablation rates as obtained from a series of Terrestrial Laser Scanning campaigns covering a complete ablation season. The measured ablation rates indicate that the advection of sensible heat causes locally increased ablation rates at the upwind edges of the snow patches. The effect, however, appears to be active over rather short distances except for very strong wind conditions. Neglecting this effect, the model is able to capture the mean ablation rates for early ablation periods but strongly overestimates snow ablation once the fraction of snow coverage is below a critical value. While radiation dominates snow ablation early in the season, the turbulent flux contribution becomes important late in the season. Simulation results indicate that the air temperatures appear to overestimate the local air temperature above snow patches once the snow coverage is below a critical value. Measured turbulent fluxes support these findings by suggesting a stable internal boundary layer close to the snow surface causing a strong decrease of the sensible heat flux towards the snow cover. Thus, the existence of a stable internal boundary layer above a patchy snow cover exerts a dominant control on the timing and magnitude of snow ablation for patchy snow covers.<br/
Evaluation of snow cover and depth simulated by a land-surface model using detailed regional snow observations from Austria
No full textAn evaluation is undertaken of the accuracy with which the Joint UK Land Environment Simulator (JULES) can simulate snow cover and depth when driven using data from the Hadley Centre Regional Climate Model. The JULES model provides the facility to diagnose the thermal and hydrological state of the land surface and soil given time-varying inputs of air temperature, wind speed, humidity, shortwave and long-wave radiation, and precipitation. The observed dataset used in this study consists of daily snow depths measurements at 601 climate stations with more than 15 years of observations in the period from January 1976 to December 2000. In this study, the JULES model was driven using two datasets at 25 km horizontal resolution: one produced using the UK Met Office Hadley Centre regional climate model (RCM), HadRM3-P (RCM), the other in which RCM precipitation and air temperature data were replaced with observed values (RCM+PT). The results indicate good agreement between the land-surface model simulations and observations of snow cover at climate stations. The median snow cover accuracy indices for all 601 stations were 89% and 91% for the RCM and the combined RCM+PT driving datasets, respectively, with only a small inter-annual variation. In contrast, the differences between modeled and measured snow depth were much larger. The median values of mean snow depth bias were similar, −0.4 cm for the RCM and −1.2 cm for the RCM+PT, however, the RCM simulation was found to overestimate the observed snow depth at more than 25% of climate stations. The extent to which the results from RCM-driven simulations match observed data is strongly related to the accuracy of the RCM precipitation. The large overestimation has significant impact on the snow mass simulation and the assessment of extreme values in the mountains. We note that even if snow cover can be simulated with a high degree of accuracy, this should not imply a similarly high degree of accuracy in the simulation of snow depth. Model performance was poorest in regions of significant topographic heterogeneity and our findings suggest that the most promising additional model developments should be directed towards computationally-efficient representation of sub-grid topography
SNOWMIP2: An evaluation of forest snow process simulation
No full textThe Northern Hemisphere has large areas that are forested and seasonally snow covered. Compared with open areas, forest canopies strongly influence interactions between the atmosphere and snow on the ground by sheltering the snow from wind and solar radiation and by intercepting falling snow, and these influences have important consequences for the meteorology, hydrology and ecology of forests. Many of the land surface models used in meteorological and hydrological forecasting now include representations of canopy snow processes, but these have not been widely tested in comparison with observations. Phase 2 of the Snow Model Intercomparison Project (SnowMIP2) was therefore designed as an intercomparison of surface mass and energy balance simulations for snow in forested areas. Model forcing and calibration data for sites with paired forested and open plots were supplied to modelling groups. Participants in 11 countries contributed outputs from 33 models, and results are published here for sites in Canada, the USA and Switzerland. On average, the models perform fairly well in simulating snow accumulation and ablation, although there is a wide inter-model spread and a tendency to underestimate differences in snow mass between open and forested areas. Most models capture the large differences in surface albedos and temperatures between forest canopies and open snow well. There is, however, a strong tendency for models to underestimate soil temperatures under snow, particularly for forest sites, and this would have large consequences for simulations of runoff and biological processes in the soil
Soil erosion by snow gliding - a first quantification attempt in a subalpine area in Switzerland
No full textSnow processes might be one important driver of soil erosion in Alpine grasslands and thus the unknown variable when erosion modelling is attempted. The aim of this study is to assess the importance of snow gliding as a soil erosion agent for four different land use/land cover types in a subalpine area in Switzerland. We used three different approaches to estimate soil erosion rates: sediment yield measurements in snow glide depositions, the fallout radionuclide Cs-137 and modelling with the Revised Universal Soil Loss Equation (RUSLE). RUSLE permits the evaluation of soil loss by water erosion, the Cs-137 method integrates soil loss due to all erosion agents involved, and the measurement of snow glide deposition sediment yield can be directly related to snow-glide-induced erosion. Further, cumulative snow glide distance was measured for the sites in the winter of 2009/2010 and modelled for the surrounding area and long-term average winter precipitation (1959-2010) with the spatial snow glide model (SSGM). Measured snow glide distance confirmed the presence of snow gliding and ranged from 2 to 189 cm, with lower values on the north-facing slopes. We observed a reduction of snow glide distance with increasing surface roughness of the vegetation, which is an important information with respect to conservation planning and expected and ongoing land use changes in the Alps. Snow glide erosion estimated from the snow glide depositions was highly variable with values ranging from 0.03 to 22.9 t ha(-1) yr(-1) in the winter of 2012/2013. For sites affected by snow glide deposition, a mean erosion rate of 8.4 t ha(-1) yr(-1) was found. The difference in long-term erosion rates determined with RUSLE and Cs-137 confirms the constant influence of snow-glide-induced erosion, since a large difference (lower proportion of water erosion compared to total net erosion) was observed for sites with high snow glide rates and vice versa. Moreover, the difference between RUSLE and Cs-137 erosion rates was related to the measured snow glide distance (R-2 = 0.64; p > 0.005) and to the snow deposition sediment yields (R-2 = 0.39; p = 0.13). The SSGM reproduced the relative difference of the measured snow glide values under different land uses and land cover types. The resulting map highlighted the relevance of snow gliding for large parts of the investigated area. Based on these results, we conclude that snow gliding appears to be a crucial and non-negligible process impacting soil erosion patterns and magnitude in subalpine areas with similar topographic and climatic conditions
Sierra Nevada Spain Snow Surface Data at the Ski Area 2024
No full textWe collected snow surface data on 2024-04-19 at the Sierra Nevada (Spain) ski area focusing on a groomed ski slope and an adjacent, natural (ungroomed) area. We used a 60-cm long snow roughness board and an iPad with the 3-D Scanner app to create two and three dimensional representations of the surface, respectively. The resolution of the snow roughness board and iPad scanning are about 0.2 mm and 1.5 cm. The roughness board is orange with a black edge. At each location, the snow roughness board were inserted into the snow so that the interface of the snow surface was highlighted by the orange of the roughness board. This interface was photographed and the raw snow roughness board are those images. The iPad scanning covered a 2-m wide swath over a 15-meter length. The data are in ASCII format as relative location (X,Y,Z) and colour (R,G,B)
Modeling Wet-Snow Shedding from Current-Carrying Conductors
No full textThe initiation of wet-snow shedding from currentcarrying conductors was studied experimentally and theoretically. A suspended cable with cylindrical snow accretion was considered, and some of the snow properties at the end of sleeve were measured and calculated until snow shedding. The current in the cable appears to be a heat source which accelerates snow melting, similarly to air temperatures above freezing, wind and heat radiation. All of these effects were taken into account to study how they contribute to the snow-shedding process. The properties observed were the liquid water content, density, and profile of snow at the end of a snow sleeve. As the snow warms, if the liquid water content and density increase to high enough levels, adhesion to the cable is weakened so that the end of the snow sleeve turns downward and then falls off. The experimental procedure involved forming a wet-snow sleeve on a suspended cable with negligible sag, frequently measuring snow properties of interest under controlled ambient conditions, and observing the deformation of the snow-sleeve until shedding occurred. The theoretical model applies the heat balance of the snow sleeve to calculate the effects of the heat sources mentioned, and simulates water percolation in the cross-section at the end of the snow sleeve from the top half downward. The model provides the rate of increase of liquid water content and density of snow in the end section, and predicts the deflection of the same section together with the time when this section is detached from the cable and snow sheds. The theoretical results were compared to the experimental observations, and satisfactory coincidence was observed in most of the cases examined
Intercomparison of snow density measurements: bias, precision, and vertical resolution
No full textDensity is a fundamental property of porous media such as snow. A wide range of snow properties and physical processes are linked to density, but few studies have addressed the uncertainty in snow density measurements. No study has yet quantitatively considered the recent advances in snow measurement methods such as micro-computed tomography (uCT) in alpine snow. During the MicroSnow Davos 2014 workshop, different approaches to measure snow density were applied in a controlled laboratory environment and in the field. Overall, the agreement between uCT and gravimetric methods (density cutters) was 5 to 9 %, with a bias o
Statistical properties of fresh snow roughness
No full textWe present results from a series of experiments in which fresh snow roughness was measured by means of digital photography and analyzed using the random field approach. The aim of the paper is to investigate the scaling properties of fresh-snow-covered surfaces and to capture key roughness length scales which can characterize the surface geometry and the size of the snow crystals. Results from our experiments show the following: (1) fresh snow roughness exhibits two distinguished scaling regimes, one at scales comparable with the crystals size and another one at larger scales; (2) we confirm that the large scales are built up during snowfall and their scaling behavior is consistent with that of Ballistic Deposition (BD) processes; and (3) we suggest that the crossover length scale separating the two scaling regimes effectively defines a representative length scale of the aggregated snow crystals on the surface. The definition of this length scale is independent of the difficulties associated with measuring snow grain sizes by means of standard microscopic analysis of disaggregated crystals. Furthermore it can be obtained from a low-cost and quick experimental procedure. Results from this study provide a plausible justification for the wide scatter of aerodynamic roughness length values encountered in the literature for fresh snow. Moreover, they provide insight on the key roughness length scales which should be used for the modeling of this parameter
Experimental Simulation of Wet-Snow Shedding from Sagged Cables
No full textThe process of wet-snow shedding from overhead cables was simulated in cold-chamber experiments under different ambient conditions. The main objective of the study was to examine how cable sag influences the snow-shedding process. However, the effects of several other parameters were also considered, such as air temperature, solar radiation, snow-sleeve length, and periodic excitation of the cable. Periodic excitation was applied at the suspension point of the cable, leading to cable vibration which may simulate galloping. The two most important parameters related to wet snow adhesion to the cable, liquid water content and density, were measured as a time function along the entire snow sleeve until snow shedding occurred. The experimental observations were compared to data gathered in a recent study on snow shedding from cables with negligible sag. The main difference between negligible and large sag is that with the latter water migrates to the lower region of the sleeve in the middle of cable, and that droplets start dripping after the snow becomes locally saturated. The time when dripping began and the mass of dripping water were also measured. Forced cable vibration accelerated the shedding process; the relationship between the excitation amplitude and the time when shedding occurred were determined. Experiments also revealed that the maximum liquid water content of snow, which was reached when shedding occurred, depended on the initial snow density
On the saltation of fresh snow in a wind tunnel: profile characterization and single particle statistics
No full textWe present experimental results on the snow drift in a turbulent boundary layer over a flat fresh snow-covered surface. Vertical profiles of mass flux and of the distribution of particle diameters were obtained by means of a pair of Snow Particle Counters parallel with measurements of the stream-wise velocity profile. The aim of the paper is to discuss current parameterizations of the vertical mass flux profile for fresh snow and to investigate the range of timescales involved in a developing saltation layer occurring in a turbulent boundary layer. The novelty of the work consists of using an intact fresh snow cover as an erodible surface able to provide realistic snow crystals as drifting particles. Results show that (1) the parameters scaling the vertical mass flux profiles of fresh snow can significantly differ from those given in the literature for ice or compacted snow particles; (2) though drifting snow covers an extremely wide range of temporal scales, the mean time interval between saltating particles ??t ? is the key timescale of the saltation process; (3) ??t ? allows for the optimal reconstruction of the mass flux as a continuous signal and for neglecting the effects related to the heterogeneous distribution of particle size on the mass flux. Implications on the modeling of snow drift and on the processing of field observations are discusse
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