Showing papers on "Snowpack published in 1990"

Modeling of Meltwater Infiltration in Subfreezing Snow

Abstract: A mathematical model which incorporates the processes that influence water flow and heat transfer in subfreezing snow was developed. Among the aspects of snow included are density and grain-size heterogeneities, capillary-pressure gradients, meltwater refreezing, time dependent hydraulic and thermal parameters, and heat conduction. From this conceptual mathematical model a numerical model of two-dimensional meltwater infiltration was developed. Results from various test cases show which data are most important to measure accurately in the field, in order to determine how the snowpack will respond to an introduction of meltwater. These simulations also show the importance of the orientation of the various layers which make up the snowpack and how randomly distributed heterogeneities can produce two-dimensional flow of meltwater under unsaturated conditions. Finally, it is demonstrated that various assumptions related to density and porosity variations, dimensionality of flow, capillary effects, etc., which have been made by past investigators for ideal situations may not be valid under many circumstances, and several suggestions are made for improving predictions of meltwater behavior. Sensitivity analysis showed that the model is most sensitive to changes in bulk density, residual saturation of wet snow and meltwater supply rates, whereas changes in snow temperature and mean grain size hadmore » less marked effect.« less

Snowpack Storage of Pollutants, Release during Melting, and Impact on Receiving Waters

Abstract: Information on the snowpack content of major ions, trace metals, and organic contaminants (pesticides and total PCBs) has been reviewed and discussed. Although several limitations exist, regional snowpack surveys have been successfully used to delineate spatial trends in acidic deposition. In contrast to the annual anionic predominance of SO 4 2- in atmospheric deposition, NO 3 - is often of a similar magnitude or even greater than SO 4 2- in the snowpack in locations affected by acidic deposition. Trace metal concentrations are generally greater than tenfold higher at European and North American regional locations than in Arctic or Antarctic “background” sites. The dry deposited component of the total snowpack pollutant load is generally less significant (1% to 45%) than the wet-deposited component, although there is much variability among chemical parameters and locations. There is conflicting evidence on the premelt stability of snowpacks; stability is clearly governed by many factors, and the occurrence of unfrozen underlying soils may be very important. The net radiative energy flux is primarily responsible for melting. Delivery of meltwater is often greatly complicated by the snowpack mesostructure; ice layers and the development of within-pack pipe flow can make modeling of this process very difficult. Rain-on-snow events can be very important both hydrologically and chemically. Fractionation of the pollutants during normal snowpack metamorphosis gives early meltwater ion and metal concentrations that are five- to ten-fold greater than those in the parent snow. Major ions are lost from the snowpack at differing rates during melting, a process known as preferential elution. Springtime reductions in pH, acid-neutralizing capacity, and base cations observed in surface waters occur due to both simple dilution and the differential release of snowpack pollutants. In contrast, lake and stream concentrations of Al and NO 3 - typically increase, although the behavior of the latter is variable from location to location. Concentrations of SO 4 2- remain comparatively constant, an observation attributed to rapid excahnge of this ion in the soil environment. In lakes, the effect of snowmelt is generally limited to a near surface layer, 1 to 3 m thick. The spring melt event may hold grave consequences for several species of aquatic biota; most reported fishkill events have occurred in Scandinavia.

Effects of Forests Near Timberline on Avalanche Formation

Abstract: In high alpine forests, extreme dry slab avalanches with fracture heights between 0.8 and 1.5m, corresponding to slope angles from 35 to 45in the starting zone, and mean return periods of 30 to 300y, may start in openings with downslope lengths of as little as 30m and widths of 15m. Larch stands near timberline have an open structure with tree distances often exceeding 15m. Larch stands only slightly affect the formation of the snow cover. Additional support of the slab is limited to distortions of the stress field by the not very numerous stems. Therefore formation of extreme avalanches is not significantly hindered. Dense spruce stands, particularly if multiply layered, effectively prevent formation of extreme slab avalanches. If the avalanche flow is not braked on a distance downslope from the fracture line of 30 to 6Om, depending on slope angle and fracture height, standard trees with diameters ~ 0.3m will be broken. The minimum extents of weak and super-weak layers for slab formation ( 25 and 5m, respectively), as well as the ranges of typical supports to the new snow slab scale with slab thickness. Dense spruce stands significantly affect snow distribution and radiation balance, and therefore the formation of weak layers.

Application of a snow cover energy and mass balance model in a balsam fir forest

Abstract: The objective of this study was to adjust the parameters of the point energy and mass balance model of a snow cover developed by E. Anderson in 1976 when applied to a balsam fir forest. This physically based model was used to simulate the snow cover energy and mass balance during spring at Lac Laflamme (47°N, 71°W). The calibration of the model was done with the physical properties of snow and hourly outflow observed at a snow lysimeter in 1985 and 1986; the validation was performed with the 1987 data. For the three seasons simulated, the model yielded accurate predictions, particularly of hourly and daily outflows. As expected, the forest canopy limits latent and sensible heat transfers to the snow cover because of its effect on wind speed. The prediction of outflow was almost insensitive to variations to the roughness parameter and to the critical Richardson number but was moderately sensitive to most parameters related to liquid water retention and transmission. Lack of fit between predicted and observed outflows, densities and temperatures at various levels in the snowpack occurred when ice layers or ice lenses were suspected to be present in the snow.

Experimental study of microwave transmission in snowpack

Abstract: Microwave transmission loss in a natural snowpack was measured to provide fundamental data for snow subsurface radar. The snowpack consisted of many horizontal stratified layers of relatively large density and water inclusion. Field strength measurements were carried out in the vertical direction across horizontal snow layers in the frequency range 0.6-7.0 GHz. The transmission loss and the extinction coefficients were obtained as a function of frequency. >

NASA Goddard Space Flight Center, Greenbelt, Maryland

Abstract: A model originally developed to explain the spectral albedo and emissivity of terrestrial snow is extended to the case of carbon dioxide snow on Mars. The variation of albedo and emissivity with wavelength is caused by the spectral variation of the absorption coefficient of solid CO2. The most important variables controlling the radiative properties are grain size and contamination by dust or water. Solar zenith angle and snowpack thickness are of less importance. The observation that red albedo is higher than blue albedo in the Martian south polar cap indicates that the snow is contaminated with red dust. The interband absorption coefficient of CO2 ice in the thermal infrared is 2-3 orders of magnitude smaller than that of H20 ice, due to the absence of hydrogen bonding in CO2. This allows CO2 snow emissivity to be sensitive to grain size, emission angle, and impurities, in contrast to water snow which is nearly a blackbody under all conditions. The emissivity of CO2 snow varies substantially with wavelength, so energy budget modeling should be done in spectral detail. The addition of a thin layer of water frost over CO2 snow dramaticaliy raises the thermal emissivity but causes little change to the spectrally averaged albedo unless the underlying CO2 snow is dirty. Remote sensing of CO2 grain size, H20 content, and dust content may be possible. However, the design of a remote-sensing strategy awaits more accurate laboratory determination of the optical constants of CO2 ice.

Temporal accumulation and ablation patterns of the seasonal snowpack in forests representing varying stages of growth

Abstract: The extent of snow accumulation and timing of spring runoff is affected by many variables, including canopy density and basal area. Prior research on snow distribution patterns in clearcut forests and undisturbed forests, shows that smaller clearcuts result in a depositional increase of the seasonal snowpack and an accelerated average seasonal ablation rate. What has not previously been clarified is the extent of snow accumulation and the timing of ablation in forests representing intermediate stages of growth. To answer this data gap, four plots, each with a different forest structure, were chosen for study within a Iogdepole pine, subalpine fir forest of Montana. Plot A is within a small meadow. Plot B is a young regrowth forest with an approximate 6% canopy density, and a basal area of 2.2 m2/ha. Plot C is an older regrowth forest with a 56% canopy density and a basal area of 17 m2/ha. Plot D is a mixed species forest with a 85% canopy density and a basal area of 37 m2/ha. At each plot, snow depth, density and snow water equivalence data was collected throughout the snow accumulation and ablation seasons of 1989 and 1990. Weather data was obtained from the Lick Creek SNOTEL site maintained by the U.S. Soil Conservation Service and located within 0.5 km of each plot. Results of this study during the 1989 snow season suggest: 1. An inverse relationship exists between canopy density and maximum snow accumulation (correlation coefficient = -0.54; p-value < 104), and 2. A strong inverse relationship exists between canopy density and snow ablation rate (correlation coefficient = 0.87; p-value < 10"4). Results from the 1990 snow season imply that a poor inverse relationship exists between canopy density and maximum accumulation (correlation coefficient = -0.14; p-value = 0.4008), while the 1990 ablation patterns support the 1989 data (correlation coefficient = -0.73; p-value < 10*4). The results from this research may assist watershed managers in estimating the extent of snow accumulation and the timing of runoff in forests with a history of reduced canopy due to vegetative disturbances such as logging, fire and disease.

Wheels and tracks in snow : validation study of the CRREL shallow snow mobility model

Abstract: In 1986, a mobility model was developed for predicting the traction and motion resistance of both wheeled and tracked vehicles on shallow snow, and a winter field season was dedicated to gathering mobility data for a diverse family of vehicles (including four on wheels and three tracked) to validate the model. The original version of the model, SSM1.0, used the Mohr-Coulomb shear failure equation from soil mechanics to predict gross traction. This required input of the snow strength parameters c and phi. Motion resistance is predicted by calculating the amount of work done by the tire in compacting snow and only requires snow depth and density values as input snow properties. Some effort was expended in determining an easy and reliable method of obtaining snow strength parameters. The model was originally designed to use an initial snow density-snow strength relationship established from past instrumented vehicle test results. Historically, shear annulus apparati have been used to obtain Mohr-Coulomb strength parameters. A comparison of snow strength obtained via these three methods (shear annulus, instrumented vehicle, calculated from initial density using the relationship in SSM1.0) for individual snow covers showed no agreement. SSM1.0 assumed that snow strength parameters for mobility prediction were a function of initial snow density; however, traction is developed in the compacted snow under the driving element, whose strength properties bore little relation to those of the initial snow. It appears that the shear strength of the compacted snow is essentially a constant for all of the vehicles and snow covers tested here.

Pattern of solute movement from snow into an upper Michigan stream

Abstract: Precipitation, snowpack, snowmelt, and streamwater samples were collected in a small gauged watershed draining into Lake Superior during winter 1987–88 to assess the importance of snowmelt pattern and meltwater pathways in the occurrence of solute pulses in streamwater The snowpack along the south shore of Lake Superior can contain 50% of annual precipitation inputs and 38% of annual ionic inputs including moderate levels of strong acids Throughout winter, thawed surface soils and small but steady snowpack moisture release promoted movement of snowpack solutes to surface mineral soils Preferential elution of K+, NH4+, and H+ from the snowpack occurred with the initial thaw Most ions exhibited pulses in snowmelt Transport of snowpack solutes to the stream during snowmelt was through near-surface soil macropores and overland flow For those ions with concentrations higher in the snowpack than in the premelt streamwater, K+, NH4+, and H+, the earliest snowmelt pulses had the greatest influence on stream

Multiyear trends in snowpack ion accumulation and loss, northern Michigan

Abstract: From December 1982 to May 1987 the quality and quantity of snowfall and snowpack were measured at four stations in a small (176 ha) gauged watershed adjacent to Lake Superior's south shore. Annual precipitation was less than and winter precipitation greater than observed east and south of the Lake Superior basin. Snowfall averaged 37% of annual precipitation but was quite variable. Snowfall significantly increased (from 21.1 to 35.7 cm, P < 0.001) with 150 m increase in elevation. Hydrogen and SO42− were the dominant ions in precipitation and snowpack while Ca2+ and HCO3− dominated stream water. No significant time trends in winter precipitation chemistry were found. Snowpack ionic retention rarely matched precipitation input. Generally, midwinter freeze-thaw periods resulted in elevated concentrations of solutes in snow meltwater and greatly reduced snowpack ion loads prior to peak stream water discharge. All ions exhibited pulses in snowmelt each year. Preferential elution from the snowpack of SO42− and K+ was observed. Stream water H+, NH4+ and most NO3− pulses were associated with midwinter thaws. Stream water SO42− pulses were rare. For those years exhibiting the most pronounced spring increase in stream H+, NO3− and SO42− concentration, the magnitude and rate of concentration change was as great in a year of small peak snowpack ion load as it was following a more rapid ion loss from a larger snowpack load. This suggests that the pattern of snowpack melt and especially the pathway the meltwater follows to the stream may be important factors in determining whether stream water chemical pulses occur.

Variability of Liquid Water Content in an Alpine Snowpack

Abstract: During the early stages of snowmelt runoff, snowpack liquid water content was sampled at five sites in an alpine basin in the southern Sierra Nevada. Typical values ranged from 1 to 3 percent by volume with some values from 5 to 8 percent. The high variability of the measured liquid water content suggests that objectives and sampling design must be carefully considered when attempting to monitor snowpack liquid water content.

Snowpack structure and avalanching, Craigieburn Range, New Zealand

Abstract: A study of the seasonal development of the snowpack at 1700 m elevation in the Craigiebum Range, New Zealand, has revealed three phases of snowpack development. This phased development is considered in relation to snow avalanching. Analysis of a short period of continuous snowpack temperature gradient data has shown that temperature gradients exceed those generally required for recrystallisation processes, as evidenced by the common occurrence of temperature gradient forms in the snowpack. Partially metamorphosed new snow, especially clusters of rimed needles, and snow in the early stages of temperature gradient metamorphism, are weak in shear and frequently appear in avalanche sliding layers. Weak snowpack stratigraphies resemble those found for Northern Hemisphere inland-continental and coastal-maritime snow climates. Three categories of potential sliding layers for avalanche release are suggested. These are: (1) weaknesses in new snow stratigraphy, (2) weaknesses at old snow surfaces, and (3) ...

Fuzzy factorial analysis of snow avalanches

Abstract: The mutual influence of 21 factors pertaining to terrain, weather, forest and snowpack have been discussed by 10 experts. The semantic (nil, weak, medium, high) evaluations are translated as membership degrees of fuzzy sets, and averaged between the experts by taking their fuzzy expectation value, yielding a 21 × 21 fuzzy matrix for direct interactions. Fuzzy successive multiplications and additions of the matrix give the indirect interactions. The activity and the passivity of the factors with respect to avalanches is represented by directed weighted graphs, and the average semantic values by a 4 × 4 matrix: the activity (passivity) is weak (strong) for tree damage; medium (nil) for altitude; medium (weak) for ground shape; medium (medium) for vegetation, soil, wind, microclimate, tree type, vertical distribution and mechanics of trees, snowpack distribution and snow gliding; strong (nil) for ground inclination and exposition, weather; strong (medium) for heat, precipitation, horizontal distribution of trees, snowpack constitution and stability.

A 2-d microscopic simulation of heat and mass transport in dry snow

Abstract: The problem of acid deposition and its effects on the environment is receiving increasing attention in North America and Europe. The interaction between seasonal snowcovers and deposited pollutants is of particular importance because a snowpack accumulates and stores pollutants which can ultimately be released in a rapid pulse with the first melt water in the springtime. As a direct result of an impurity pulse, water quality degrades with deleterious effects on the local environment and aquatic biological species. The timing and severity of an inpurity pulse is dependent upon the redistribution of pollutants in a snowpack which is attributed to a process known as temperature gradient metamorphism. This work investigates the influence of geometry, density and temperature on the coupled heat and mass transport in idealized, two dimensional ice lattice cells. Mass flux, concentration and temperature distributions, as well as effective diffusion coefficients and thermal conductivities are presented a...

Effects of Forest Cover on a Snowpack in the Sierra Nevada

Abstract: Adjacent snowpacks in a clearing and under a forest canopy were compared over a decade at a site near Lake Tahoe. This comparison provides a few indications of the differences in snowpack water equivalence (SWE) that can be expected in similar situations. By early January, SWE in the clearing was usually more than 10 cm greater than that in the forest. By the time of peak accumulation, the open snowpack exceeded the forest snowpack by an average of 20 cm. From the time of peak accumulation until snow cover disappearance, the rate of snowmelt in the clearing was 75 percent greater than that in the forest, on the average. Because of the slower rate of snowmelt in the forest, the forest site remained snow-covered for an average of 18 days after the disappearance of snow in the clearing.

The Width of Unconfined Slab Avalanches Based on Field Measurements of Slab Properties

Abstract: Using principles of fracture mechanics, a linear elastic model of an unconfined dry slab and simplifying assumptions, expressions are derived for the energy release rate and for the fracture resistance during the propagation of the rapid brittle fractures which release slab avalanches. The simplified energy release rate is constant as the fractures advance whereas the fracture resistance increases as the fractures spread across the slope. The arrest criterion equates the release rate to the resistance and gives an expression for the width of unconfined slab avalanches in terms of two empirical constants and three field measurements, namely the tensile strength of the slab, the slab density and the inclination of the starting zone. The empirical constants are evaluated by regressing the expression for the width of unconfined slab avalanches on data obtained from measurements made at 13 strictly unconfined slab avalanches during the winters of 1988 and 1989 in the Rocky Mountains of Western Canada. The significant correlation coefficient (0.81) suggests that the expression may provide a quantitative approach to forecasting the width of unconfined slab avalanches.

The spatial variability of snow strength on potential avalanche slopes

Abstract: (Note: The complete paper is being submitted to the Journal of Glaciology, and only an abstract is provided in these proceedings. Please write to the author to receive a copy of the complete paper.) Since snow avalanches are believed to release from zones of localized weakness, knowledge of snow strength patterns is important for determination of slope stability and for the application of effective avalanche control measures. In this study, the spatial variability of average snow resistance (an index of snow strength) and depth were measured and compared with terrain features on two inclined slopes in Montana during two winter field seasons. An instrument that indexes snow strength by measuring snow resistance was refined (Brown and Birkeland, 1990), allowing the strength of an entire snow slab to be characterized in a short time. The spatial pattern of trees -appears to affect the pattern of snow depth at the first site (Figures , . 1 and 2), where a significant linear relationship was found between snow depth and average snow resistance during both years of study (p-values < 1 X 10-6) (Figure 3). These results suggest that localized snow depth variations may be important in snow strength genesis. Although a linear relationship existed at that site, low r2 values for the two years (r 2 < 0.357) indicate additional factors may be critically relevant. A second site with greater cemplexity of terrain features and less localized wind drifting qid not show a linear relationship between depth and average resistance. /Complicated patterns of resistance at that site demonstrate that many factors contribute to snow resistance. In particular, the snow over rocks was found to have significantly weaker resistance than adjacent areas which were not over rocks (p-value < 1 X 10-6). Results may provide predictive information of weak zone locations in snow slabs, which would improve avalanche forecasting and control techniques.