24,193 research outputs found
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
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
Living Snow Fences; TR-460, 2006
No full textBlowing snow can cause significant problems for mobility and safety during winter weather in three distinct ways. It may drift onto the road, thus requiring almost continuous plowing while the wind is blowing (which may occur when a given winter storm is over). Snow may drift onto wet pavement (perhaps caused by ice control chemicals) and dilute out the chemicals on the road, creating ice on the road. And sufficient blowing snow can cause a major deterioration in visibility on the road, a factor which has been shown to be significant in winter crashes.
The problem of blowing snow can be very effectively addressed by creating a snow storage device upwind of the road that requires protection from snow drifting. Typically, these storage devices are fences. Extensive design guidance exists for the required height and placement of such fences for a given annual snowfall and given local topography. However, the design information on the placement of living snow fences is less complete. The purpose of this report is to present the results of three seasons of study on using standing corn as snow fences. In addition, the experience of using switch grass as a snow storage medium is also presented. On the basis of these experimental data, a design guide has been developed that makes use of the somewhat unique snow storage characteristics of standing corn snow fences.
The results of the field tests on using standing corn showed that multiple rows of standing corn store snow rather differently than a traditional wooden snow fence. Specifically, while a traditional fence stores most of the snow downwind from the fence (and thus must be placed a significant distance upwind of the road to be protected, specifically at least 35 times the snow fence height) rows of standing corn store the majority of the snow within the rows. Results from the three winters of testing show that the standing corn snow fences can store as much snow within the rows of standing corn as a traditional fence of typical height for operation in Iowa (4 to 6 feet) can store. This finding is significant because it means that the snow fences can be placed at the edge of the farmer’s field closest to the road, and still be effective. This is typically much more convenient for the farmer and thus may mean that more farmers would be willing to participate in a program that uses standing corn than in traditional programs.
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On the basis of the experimental data, design guidance for the use of standing corn as a snow storage device in Iowa is given in the report. Specifically, it is recommended that if the fetch in a location to be protected is less than 5,000 feet, then 16 rows of standing corn should be used, at the edge of the field adjacent to the right of way. If the fetch is greater than 5,000 feet, then 24 rows of standing corn should be used. This is based on a row spacing of 22 inches. Further, it should be noted that these design recommendations are ONLY for the State of Iowa. Other states of course have different winter weather and without extensive further study, it cannot be said that these guidelines would be effective in other locations with other winter conditions
No. 57, Dorothy G. Snow, interview by Winnifred Margetts
No full textTranscript (45 pages) of interview(s) by Winnifred Margetts with Dorothy G. Snow, on May 1, 1985.This interview is no. 57 in the Everett L. Cooley Oral History Project, and tape no. 285. Accompanied by two obituaries for SnowSnow (b. 1901) talks about her pioneer heritage, family life and education, her teaching career at the University of Utah, and the influence of George Thomas, President of the University. Interviewer: Winnifred Margett
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/
Adhesion of Wet Snow to Different Cable Surfaces
No full textCohesion of snow and its adhesion to cable surfaces are the decisive factors for wet-snow shedding from power-line cables. Knowing the adhesive strength of snow is essential to predict when snow will shed and what consequences it will have on the elements of the transmission line. It also appears to be a basic input for simulating wet-snow shedding. Snow adhesion depends on several parameters, among which snow liquid water content and density, and cable surface geometry were examined experimentally. In particular, the adhesion of wet-snow samples to flat surfaces of different roughness, and to stranded cable surfaces was examined in this study. Two series of experiments were conducted to measure shear adhesive strength as well as tensile adhesive strength of snow. Shear adhesive strength was measured with a centrifuge adhesion test device where a snow sample was placed on a beam, which was then rotated with increasing angular frequency until detachment, the angular frequency at detachment being proportional with shear adhesive force and strength. The tensile adhesive tests were carried out with a material test machine on a semi-spherical snow sample. The sample was compressed slowly at a constant speed until it reached a predefined compressive force limit, and then a tensile load was applied until the detachment of snow. The main observations showed that adhesion was strongest for a critical value of liquid water content, that shear adhesive strength was greatest on stranded cable surfaces, and that tensile adhesive strength was weaker on stranded than on flat cable surfaces
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
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
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
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