Showing papers on "Snowpack published in 1998"

The Influence of the Spatial Distribution of Snow on Basin-Averaged Snowmelt

Abstract: Spatial variability in snow accumulation and melt owing to topographic effects on solar radiation, snow drifting, air temperature and precipitation is important in determining the timing of snowmelt releases. Precipitation and temperature effects related to topography affect snowpack variability at large scales and are generally included in models of hydrology in mountainous terrain. The effects of spatial variability in drifting and solar input are generally included only in distributed models at small scales. Previous research has demonstrated that snowpack patterns are not well reproduced when topography and drifting are ignored, implying that larger scale representations that ignore drifting could be in error. Detailed measurements of the spatial distribution of snow water equivalence within a small, intensively studied, 26-ha watershed were used to validate a spatially distributed snowmelt model. These observations and model output were then compared to basin-averaged snowmelt rates from a single-point representation of the basin, a two-region representation that captures some of the variability in drifting and aspect and a model with distributed terrain but uniform drift. The model comparisons demonstrate that the lumped, single-point representation and distributed terrain with uniform drift both yielded poor simulations of the basin-averaged surface water input rate. The two-point representation was a slight improvement, but the late season melt required for the observed stream-flow was not simulated because the deepest drifts were not represented. These results imply that representing the effects of subgrid variability of snow drifting is equally or more important than representing subgrid variability in solar radiation.

Estimating the spatial distribution of snow in mountain basins using remote sensing and energy balance modeling

Abstract: We present a modeling approach that couples information about snow cover duration from remote sensing with a distributed energy balance model to calculate the spatial distribution of snow water equivalence (SWE) in a 1.2 km 2 mountain basin at the peak of the accumulation season. In situ measurements of incident solar radiation, incident longwave radiation, air temperature, relative humidity, and wind speed were distributed around the basin on the basis of topography. Snow surface albedo was assumed to be spatially constant and to decrease with time. Distributed snow surface temperature was estimated as a function of modeled air temperature. We computed the energy balance for each pixel at hourly intervals using the estimated radiative fluxes and bulk-aerodynamic turbulent-energy flux algorithms from a snowpack energy and mass balance model. Fractional snow cover within each pixel was estimated from three multispectral images (Landsat thematic mapper), one at peak accumulation and two during snowmelt, using decision trees and a spectral mixture model; from these we computed snow cover duration at subpixel resolution. The total cumulative energy for snowmelt at each remote sensing date was weighted by the fraction of each pixel's area that lost its snow cover by that date to determine an initial SWE for each pixel. We tested the modeling approach in the well-studied Emerald Lake basin in the southern Sierra Nevada. With no parameter fitting the modeled spatial pattern of SWE and the mean basin SWE agreed with intensive field survey data. As the modeling approach requires only a remote sensing time series and an ability to estimate the energy balance over the model domain, it should prove useful for computing SWE distributions at peak accumulation over larger areas, where extensive field measurements of SWE are not practical.

Nitrogen and Carbon Soil Dynamics in Response to Climate Change in a High-elevation Ecosystem in the Rocky Mountains, U.S.A.

Abstract: We have implemented a long-term snow-fence experiment at the Niwot Ridge Long-Term Ecological Research (NWT) site in the Colorado Front Range of the Rocky Mountains, U.S.A., to assess the effects of climate change on alpine ecology and biogeochemical cycles. The responses of carbon (C) and nitrogen (N) dynamics in high-elevation mountains to changes in climate are investigated by manipulating the length and duration of snow cover with the 2.6 x 60 m snow fence, providing a proxy for climate change. Results from the first year of operation in 1994 showed that the period of continuous snow cover was increased by 90 d. The deeper and earlier snowpack behind the fence insulated soils from winter air temperatures, resulting in a 90C increase in annual minimum temperature at the soil surface. The extended period of snow cover resulted in subnivial microbial activity playing a major role in annual C and N cycling. The amount of C mineralized under the snow as measured by CO2 production was 22 g m-2 in 1993 and 35 g m-2 in 1994, accounting for 20% of annual net primary aboveground production before construction of the snow fence in 1993 and 31% after the snow fence was constructed in 1994. In a similar fashion, maximum subnivial N20 flux increased 3-fold behind the snow fence, from 75 ?xg N m-2 d-' in 1993 to 250 ?xg N m-2 d-' in 1994. The amount of N lost from denitrification was greater than the annual atmospheric input of N in snowfall. Surface litter decomposition studies show that there was a significant increase in the litter mass loss under deep and early snow, with no significant change under medium and little snow conditions. Changes in climate that result in differences in snow duration, depth, and extent may therefore produce large changes in the C and N soil dynamics of alpine ecosystems.

Terminology and predominant processes associated with the formation of weak layers of near-surface faceted crystals in the mountain snowpack

Abstract: Although recent observations indicate that weak layers of near-surface faceted crystals are widely associated with snow avalanches, little research has addressed these layers. Further, current rese...

Radiometric and structural measurements of snow samples

Abstract: The interaction of microwaves with the natural snow cover strongly depends on the complex structure of the snowpack. In order to quantify this dependency, dedicated experiments were performed with homogeneous slabs of dry, natural snow samples measured over a frequency range from 11 to 94 GHz. A new method introduced by Matzler and Wegmuller [1995] and Weise [1996a] for determining the scattering and absorption behavior of test samples was applied and further developed by application of a multiple scattering model. Homogeneous samples of dry snow were (1) investigated using a set of portable, linearly polarized Dicke radiometers at frequencies of 11, 21, 35, 48 and 94 GHz, (2) characterized by temperature, grain size and shape, density and permittivity, and (3) structurally analyzed by digitized snow sections in order to obtain statistical information of the snow structure i.e. the autocorrelation function. During the winters 1994/1995 and 1995/1996 additional measurements of snow samples were made to extend the variability of the investigated snow types. Up to now, 20 samples, representing alpine snow in winter (that is, without melt metamorphism) have been collected during three winter campaigns. Here, we present the method and the radiative transfer model and show how it can be inverted to obtain scattering and absorption coefficients. A first assessment of the snow sample data is also presented. The results show good agreement between the measured and the theoretical absorption coefficient. The scattering coefficient turns out to be a strong function of frequency and correlation length as expected from Rayleigh scattering. However, distinct differences can be noted.

The potential of times series of C-Band SAR data to monitor dry and shallow snow cover

Abstract: A study was conducted to assess the potential of C-band synthetic aperture radar (SAR) data to determine the snow water equivalent (SWE). A multitemporal (three winters) SAR data set was obtained using the Convair-580 from the Canada Centre for Remote Sensing (CCRS) over a watershed in the Appalachian Mountains in Southern Quebec, Canada. The SAR data were relatively calibrated using extended targets (coniferous stands). Extensive ground measurements were done simultaneously to each of the seven flights, in order to measure the snow cover characteristics (depth, density, SWE, liquid water content, temperature, and dielectric profiles) as well as the soil characteristics (moisture, temperature). To estimate the SWE of a given snowpack, a model which links the scattering coefficient to the physical parameters of the snow cover and the underlying soil has been developed. The model is based on the ratio of the scattering coefficient of a field covered by snow to the scattering coefficient of a field without snow. The analysis has revealed that volume scattering from a shallow dry snow cover (SWE<20 cm) is undetectable. The backscattering power is dominated by soil surface scattering, the latter varying with the decrease of liquid water content in the surface layer with decreasing soil temperature below 0/spl deg/C. Then, the scattering ratio decreases proportionally to the dielectric constant of the soil in winter. Furthermore, a unique relationship for three acquisition dates has been found between the thermal resistance, R, of the snow pack and the backscattering power ratio. Then, the spatial distribution of the power ratio should depict the spatial distribution of R, given spatially uniform climatological conditions over the study area. Since linear relationships between SWE and R have been observed, it should be possible to estimate the SWE of shallow dry snow cover with C-band SAR data using few ground truthing data in an open area when the soil is frozen.

Snow ablation modelling in a mature aspen stand of the boreal forest

Abstract: Snow ablation modelling at the stand scale must account for the variability in snow cover and the large variations of components of energy transfer at the forest floor. Our previous work successfully predicted snow ablation in a mature jack pine stand by using a one-dimensional snow process model and models predicting radiation below forest canopies. This work represents a second test of our basic modelling scenario by predicting snow ablation in a leafless, deciduous aspen stand and verifying the results with field data. New modifications to the snow model accounted for decreased albedo owing to radiation penetration through optically thin snowpacks. A provisional equation estimates litter fall on the snowpack, thereby reducing the areal averaged albedo. We showed that subcanopy radiation measurements can be used with a canopy model to estimate a branch area index for defoliated aspen as an analogue to the foliage area index used for conifers. Modelled incoming solar and long-wave radiation showed a strong correlation with measurements, with r 2 a 0.96 and 0.91 for solar and long-wave radiation, respectively. Model results demonstrate that net radiation overwhelms turbulent exchanges as the most significant driving force for snowmelt in aspen forests. Predicted snow ablation in the aspen stand compared very favourably with available data on snow depth. #1998 John Wiley & Sons, Ltd.

Physically based modeling of atmosphere‐to‐snow‐to‐firn transfer of H2O2 at South Pole

Abstract: Quantitative interpretation of ice core chemical records requires a detailed understanding of the transfer processes that relate atmospheric concentrations to those in the snow, firn, and ice. A unique, 2 year set of year-round surface snow samples at South Pole and snow pits, with associated accumulation histories, were used to test a physically based model for atmosphere-to-firn transfer of H2O2. The model, which extends our previous transfer modeling at South Pole into the snowpack, is based on the advection-dispersion equation and spherical diffusion within representative snow grains. Required physical characteristics of the snowpack, such as snow temperature and ventilation, were estimated independently using established physical models. The surface snow samples and related model simulations show that there is a repeatable annual cycle in H2O2 in the surface snow at South Pole. It peaks in early spring, and surface snow concentration is primarily determined by atmospheric concentration and temperature, with some dependence on grain size. The snow pits and associated model simulations point out the importance of accumulation timing and annual accumulation rate in understanding the deposition and preservation of H2O2 and δ18O at South Pole. Long-term snowpack simulations suggest that the firn continues to lose H2O2 to the atmosphere for at least 10–12 years (∼3 m) after burial at current South Pole temperatures and accumulation rates.

Climate, limiting factors and environmental change in high-altitude forests of Western North America

Abstract: The relationships between forest dynamics and climate are predictable for high-altitude forest ecosystems in western North America and other mountainous regions. The duration of snowpack interacts with spring and summer temperature to determine when a snowfree soil surface and sufficiently high soil temperatures for physiological activity occur. Regeneration of tree seedlings varies spatially and temporally as mediated by the duration of the snowpack, which affects the length of the growing season on high-precipitation sites and the soil moisture supply on low-precipitation sites. Regeneration is favoured by climatic conditions that produce a mesic soil moisture regime rather than extremes and by summer temperatures that are sufficiently high to facilitate carbon gain in seedlings. Relatively short-term climatic trends can have major impacts on regeneration patterns, particularly after disturbances. Tree growth in high-snowfall environments (under a marine climate and near the treeline) is generally limited more by precipitation than by temperature, with growth being negatively correlated with snowpack depth. There are many sources of spatial and temporal variation in growth response to climate, most of which are not included in modeling efforts at large spatial scales. Growth response varies between species and within species, depending on subregional climate (high vs. low precipitation in the same mountain range), altitude (treeline vs. lower elevation), aspect (north vs. south) and genotype. The effects of climatic variation on high-altitude forests are distinct from effects in low-altitude ecosystems, and models based on low-altitude forests are not necessarily applicable at higher altitudes. The potential for vegetational inertia—long lag times in response to environmental variation—needs to be considered when evaluating the response of high-altitude forests to climatic change.

Effects of winter weather conditions on soil freezing in southern michigan

Abstract: We examined climatic and (modelled) soil-temperature data from five winters in southern Michigan to ascertain the spatial variability in soil-freezing and freeze-thaw cycles at 5 cm. The five winters chosen for study (1951–1952, 1952–1953, 1953–1954, 1976–1977, and 1979–1980) represent the extremes of weather (e.g., cold and snowy, warm and dry) for the 1951–1980 period. We chose this study area because it lies on an ecotone between the cold, snowy climates of southern Canada and the warmer climates of the Ohio Valley where persistent snowpacks are rare, and because virtually no data on soil freezing exist for this area. Soil freezing in winter in southern Michigan is more dependent on snowpack persistence and thickness, especially in mid-winter, than on air temperatures. Here, even in warm winters, soils freeze to 5 cm, provided that snowpacks are thin or absent. Conversely, in even the coldest winters, soils rarely freeze where deep snows accumulate. Thus, freezing is least frequent, and in some years n...

Near-Surface Faceted Crystals Formed by Diurnal Recrystallization: A Case Study of Weak Layer Formation in the Mountain Snowpack and Its Contribution to Snow Avalanches

Abstract: In the winter of 1995-96 we investigated the temperature and vapor pressure gradient conditions associated with the formation of faceted crystals that develop in the upper levels of the snowpack du...

Using the temporal variability of satellite radar altimetric observations to map surface properties of the Antarctic ice sheet

Abstract: The problem of measuring surface height and snowpack characteristics from satellite radar altimeter echoes is investigated. In this paper, we perform an analysis of the ERS1 altimeter dataset acquired during a 3 day repeat orbit. The analysis reveals that there are temporal variations in shapes of the radar altimeter echo and that these variations are linked to meteorological phenomena. The time- and space-scales over which these variations apply are a few to tens of days and a few hundred kilometres, respectively. This phenomenon, if not accounted for, can create error in the height measurement. A numerical echo model is used to recover snowpack characteristics by taking advantage of the temporal variations of the radar echoes. A map of penetration depth of the radar waves in the Ku band over the Antarctic continent is obtained and suggests that grain-size produces the dominant effect on radar extinction in the snowpack at this frequency. Finally, a procedure is proposed to correct the height measurement within the context of ice-sheet mass-balance survey.

A statistical model of spatially distributed snowmelt rates in a boreal forest basin

Abstract: Spatial variation in snowmelt rates in the boreal forest can be explained by diAerences in canopy density. Canopy density, represented as gap fractions (GF), controls both the amount of short-wave radiation reaching the snowpack surface and wind speed over the snow surface, which in turn regulates sensible and latent heat fluxes. Reductions in shortwave, sensible and latent heatfluxes outweigh any increased contributions from longwave radiation from the canopy. DiAerences in the total energy available for melt do not translate to equally proportional changes in melt rates under diAerent canopy densities. As available energy increases, the melt rate increases with decreasing canopy density and the form of the relationship can vary depending on climatic conditions. A good relationship between ground-based GF measurements and a canopy closure index derived from Landsat TM provides the spatial fabric for the distribution of snowmelt rates that show comparable patterns of snow ablation during years of very diAerent climatological conditions. This physically meaningful method of determining the spatial variability of snow ablation and subsequent meltwater delivery to the soil interface is particularly useful for providing insight to the heterogeneous active layer development in permafrost regions of the boreal forest and the implications for runoA processes. #1998 John Wiley & Sons, Ltd.

Measurement of density and wetness in snow using time-domain reflectometry

Abstract: Time-domain reflectometry (TDR) is widely used in soil physics to determine water content. Existing equipment and methods ran be adapted to measurements of snow wetness. The main advantages compared to other methods are flexibility in constructing sensors, minimal influence on snow cover during measurements and sensors can be multiplexed. We developed sensors suitable for continuous and non-continuous measurements of snow wetness and density, measured the apparent permittivity in different snow densities and snow types, and compared the measurements to existing mixing formulas for mixtures of snow and air. In dry snow, density was measured from 110 to 470 kg m−3. The residual error is 14 kg m −3 and the 95% confidence interval of our model is 3 kg m−3. To measure snow density and wetness continuously suitable sensors have been constructed. Their small size and high surface area to weight ratio minimizes their movement in the snowpack, except when they are exposed to intense solar radiation. Results show that changes in dry-snow density of less than 5 kgm−3 can be detected. Infiltration of even small amounts of water clearly shows up in the permittivity. At the surface of the snowpack, problems occur due to the formation of air pockets around the sensors during long-term measurements.

Snow-albedo feedback and the spring transition in a regional climate system model: Influence of land surface model

Abstract: Using the Arctic regional climate system model (ARCSYM), we investigate the spring seasonal transition and mechanisms controlling snowmelt over a domain covering the northern half of Alaska. Annual simulations for 1992 comparing the Biosphere-Atmosphere Transfer Scheme (BATS) and the land surface model scheme (LSM) show that the BATS experiment enters the spring transition with respect to the large-scale atmospheric regime approximately one month earlier than observed climate and the LSM experiment transitions a month later than observed, even though the air temperature in the LSM experiment is generally warmer than in the BATS experiment. A more detailed examination reveals that each simulation commences and completes the snowmelt period at about the same time but that the LSM snowmelt is more rapid than in the BATS experiment. Controlling the snowmelt is the initial snowpack depth and the surface energy budget, both of which involve a complex series of feedbacks between shortwave and longwave radiation, cloud, surface turbulent fluxes, and vegetation. The snowmelt over tundra regions dominates the more rapid snowmelt seen in the LSM simulation. It is determined that the most crucial differences between the BATS and the LSM schemes are the partitioning of net ground heat flux between patches of snow and bare ground and the formulation of snow albedo.

Modeling ionic solute transport in melting snow

Abstract: A numerical model that includes the effects of mass transfer between mobile and immobile liquid phases, advection, hydrodynamic dispersion, and melt-freeze episodes was developed to simulate ionic solute transport in melting snow. Model calibration using a tracer-infused laboratory snowpack experiment yielded a dispersivity of 0.05 cm and a mobile-immobile phase mass-transfer coefficient of 4 × 10−6 s−1, but these parameter values are tentative because of the artificial nature of the experiment. The modeled concentration of meltwater flowing out the bottom of the snowpack was sensitive to residual water saturation, flow rate, dispersivity, mass-transfer rate, and the initial distribution of solute within the pack, similar to experimental observations. The model was applied to a small watershed, and it was found that the ability of the model to accurately simulate solute movement depends on the validity of the assumption of one-dimensional flow and on the accuracy of modeling the snowpack energy balance.

Sublimation of snowpacks in subalpine conifer forests

Abstract: This effort to improve prediction of forest snowpack evaporation was aimed at understanding differences in winter snowpack among forested aspects. Theoretical arguments combined air temperature, humidity, wind speed, and radiation into a sublimation index. Monitoring weather factors and the mass of a 65-cm-diameter pan of snow set in a forest snowpack provided calibration of the index. Changes in snow surface structure caused a decrease with time in the empirical ratio of snowpack-to-index sublimation. This decrease was proportional to total sublimation since snowfall. We applied the model to compute daily snowpack sublimation on two adjacent forested slopes in central Colorado, U.S.A., during a 40-d accumulation period in March and April 1996. The estimate for both slopes combined was 0.52 mm/d, with sublimation from a south slope snowpack (0.61 mm/d) averaging 1.2 times that from an adjacent north-facing slope (0.43 mm/d). Extending the average difference of 0.18 mm/d through a winter season (150 d) wou...

Field observation and modelling of weak-layer evolution

Abstract: For operational snow-cover simulations, an adequate modelling of the evolution of buried weak layers is of crucial importance. Therefore, the processes governing snow metamorphism within weak layers before and after burial must be known in detail. At the study site of the Swiss Federal Institute for Snow and Avalanche Research, 2540 ma.s.l., a 2 cm thick weak layer of column-grown cup-shaped crystals formed beneath a thin crust in mid-January 1996. Exposed to near-surface processes for about 4 weeks, the layer was buried on 8 February and persisted in the snowpack until mid-April. Numerous temperature profiles and characterizations of both the weak and the adjacent layers were performed in situ. Snow-grain samples, as well as larger snow blocks, were taken to the cold laboratory for further analysis of the texture. The shear strength of the buried weak layer was also investigated by means of shear-frame tests. The field observations and measurements are compared with model simulations of snow temperature and stratigraphy. The comparison shows potential and problems in the modelling of weak-layer evolution.

Interannual, seasonal, and spatial patterns of meltwater and solute fluxes in a seasonal snowpack

Abstract: Meltwater discharge and electrical conductivity were measured in eight 1 × l m lysimeters, and snow accumulation and electrical conductivity of melted samples were measured in snow pits during four snowmelt seasons at Mammoth Mountain, California. The peak snow-water equivalent ranged from 0.57 to 2.92 m over the four melt seasons. Lysimeter discharges ranged from 20% to 205% of the mean flow; however, mean lysimeter flow was representative of snow ablation observed in snow pits. The electrical conductivity in snow pit samples and meltwater averaged 2–3 μS cm−1. Peak meltwater electrical conductivity ranged from 6 to 14 times the bulk premelt snowpack electrical conductivity. Snow depth did not affect the magnitude of the ionic pulse, and ion depletion as a function of snow ablation was similar from year to year despite interannual contrasts in melt rate and snow accumulation. Diel fluctuations in electrical conductivity were more pronounced in shallower snowpacks.

Snowmelt sensitivity to radiation in the urban environment

Abstract: Despite having the same snowmelt processes, snowpacks in urban environments experience a range of conditions different from those of rural areas. Melt is intensified at some sites due to greater radiative energy. Shading, however, can reduce radiation and melt at other sites. Changes to the radiation balance and snowpack processes have been investigated. A physical snowpack model was developed and tested against data from an impervious study plot in Sweden. Estimated surface runoff compared favourably with that measured. An urban radiation scheme captured the observed net allwave radiation well. Series of sensitivity analyses were made by perturbing the scheme to represent three urban locations: open ground and the southern (sunny) and north (shaded) sides of a hypothetical building. Cloudiness, albedo, wall temperature and sky view were altered to reproduce common urban conditions. Even without perturbation, the shaded and sunny sides of the building had different radiation fluxes—the south side...

Temperature history and accumulation timing for the snowpack at GISP2, central Greenland

Abstract: Previous research has documented a close association between high-resolution snow-pit profiles of hydrogen and oxygen stable-isotope ratios and multi-year Special Sensor Microwave/lmager (SSM/I) 37 GHz brightness temperature data in central Greenland Comparison of the SSM/I data to profiles obtained during the 1989-91 field seasons indicated that δ D and δ 18 O data from the near-surface snow at the Greenland summit are a reliable, high-resolution temperature proxy To test this new technique further, additional stable-isotope data were obtained from a 2 m snow pit constructed during late-June 1995 near the GISP2 site This new profile, supported by pit stratigraphy and chemistry data, confirms the utility of comparing stable-isotope records with SSM/I brightness temperatures The sub-annual variation of the δD record at the GISP2 site was determined using 15 match points, from approximately December 1991 through June 1995 and was guided in part by time-constrained hoar layers The close association of these temperature proxies supports the assertion that snow accumulation occurs frequently through the year and that the isotope record initially contains temperature information from many times of the year This is also independently confirmed by analysis of H 2 O 2 data The slope of the multi-year T vs δ correlation was evaluated along with the sub-annual variation in the amount, rate and timing of accumulation These new results are consistent with those from the previous study and they also demonstrate that the snow in this area initially contains temperature and chemical records with sub-annual resolution This encourages confident interpretation of the paleoclimatic signal variations in the GISP2 and GRIP deep cores

A Quantitative Approach to Evaluating the Effects of Snow Cover on Cold Airmass Temperatures across the U.S. Great Plains

Abstract: Over the past two decades a greater emphasis has been placed on the accuracy of the representation of snow cover–atmosphere interactions in weather and climate prediction models. Much of the attention centered upon snow cover is a result of concerns associated with anthropogenic and natural causes of potential changes in the global environment that may be intensified by the snow cover climatology. As a predictive tool, the importance of the interactions between snow cover and the overlying atmosphere is recognized in areas ranging from daily and seasonal surface air temperature forecasts, to anomalies in large-scale atmospheric circulation patterns. Within this study the effects of snow cover on surface air temperatures within cold air masses moving across the U.S. Great Plains in winter were investigated. Through the adaptation of a one-dimensional snowpack model, the thermal characteristics of the core of a cold air mass were derived from the equation governing the heat balance between the surf...

A case study of the synoptic patterns influencing midwinter snowmelt across the northern Great Plains

Abstract: Snow cover is found across extensive areas of the northern hemisphere during the winter and early spring seasons. Meltwater provided by this snow cover can be an important source of freshwater for agriculture. domestic uses and hydroelectric power. Rapid ablation of the snowpack, however, can also pose environmental hazards such as flooding. The ability to forecast meltwater quantities is dependent upon a knowledge of the factors influencing the snowmelt process. This paper employs a hybrid modelling and synoptic climatological approach to investigate the relationships between synoptic weather patterns, surface energy fluxes and midwinter snowmelt in the northern Great Plains. The first objective of this study is to identify distinct synoptic patterns that are associated with days where significant snow cover ablation occurred. The second objective is to evaluate the relationships between synoptic-scale weather patterns, snow surface energy transfers and snowmelt. A case study of 21 February 1975 is used to illustrate these relationships. Unlike the other synoptic-type studies, which rely on empirically derived energy flux data from single index sites, this study employs a physically based snowpack model to generate estimates of energy fluxes. The use of modelled fluxes instead of measured values allows for a more spatially extensive analysis as surface fluxes over the entire study region can be analysed in conjunction with the prevailing synoptic-scale weather patterns. Three major synoptic types, characterized by the presence of a midlatitude cyclone, are associated with large midwinter snowmelt episodes in the northern Great Plains. The case study illustrates how variations in temperature, humidity, cloud cover and wind speeds associated with such cyclonic storms can play a major role in affecting snow surface-atmosphere energy exchanges. As expected, elevated wind speeds and stronger temperature and humidity gradients significantly increased the transfers of sensible and latent heat between the snow surface and the atmosphere. Increased cloud cover near the low pressure centre reduced incoming solar radiation but through counter radiation also reduced the loss of long-wave radiation.

Meltwater fluxes, hillslope runoff and stream flow in an arctic permafrost basin

Abstract: This paper integrates studies of snow distribution, snowmelt percolation, hillslope runoff pathways and stream flow in the continuous permafrost region of the Canadian western Arctic. Observations and model results showed high variability in the relative flow velocities and travel time within each flow component. Vertical flow through the snowpack is initially very slow, with a delay of up to 2 weeks in drifts. Once snow is thoroughly wetted, percolation is rapid (hours). On hillslopes, the reverse is true, with lateral transfer being rapid (hours) early in the melt period, but decreasing (days to weeks) as the active layer develops. Hillslope runoff accumulates as a saturated layer at the base of the deep snowpack in the stream channel. Initially, discharge through the snow-choked channel occurs through this layer, but as the snow is removed by thermal and mechanical processes, the flow velocity increases.

A network of automatic weather and snow stations and supplementary model calculations providing snowpack information for avalanche warning

Abstract: The Swiss Federal Institute for Snow and Avalanche Research (SLF) began to construct a network of high Alpine automatic weather and snow measurement stations in the Summer of 1996. Presently 35 stations are in operation and another 25 stations will be on-line before the Summer of 1999. The stations measure wind, air temperature, relative humidity, snow depth, surface temperature, ground (soil) temperature, reflected short wave radiation and three temperatures within the snowpack. The measurements are transferred hourly to the SLF in Davos and the data are used to drive a finite-element based physical snowpack model. The model runs every hour and provides supplementary information regarding the state of the snowpack at the sites of the automatic stations. New snow amounts, settling rates, possible surface hoar formation, temperature and density profiles as well as the metamorphic development of the snowpack are all predicted by the model. The model is connected to a relational data base which stores the measurements as well as the model results. New visualization tools are available which allow a fast, easy and comprehensive access to the stored data. The model has been tested in quasi-operational mode during the Winter of 1997 / 98. The calculation is reliable in terms of the energy budget and the mass balance. The description of snow metamorphism is currently being improved. The model will be fully operational in the Winter of 1998/99 and will be used by local, regional and national avalanche forecasters.

Snowmelt‐generated runoff and soil erosion in Fife, Scotland

Abstract: The hydrology and contrasting erosional responses of two snowmelt events on arable farmland in Fife, Scotland, are compared. Snowmelt-generated runoff in January 1993 caused widespread soil erosion across eastern Scotland. Gullying was exemplified by three sites in Fife, where thaw of a drifted snowpack was augmented by rainfall to produce a larger erosive response than meteorological data alone would have predicted. Up to 127 m3 of soil was lost from individual gullies in fields sown to winter cereals. In February 1996 snowfall of comparable depth again covered the field area, but a more uniform snowpack, slower thaw, greater crop cover and lower rainfall during the thaw phase combined to lessen the impact of erosion. These case studies demonstrate the complexity of the erosion/runoff relationship for rain on snow events, in which erosional severity depends not just on snow depth but on snow distribution, thaw rate and the amount and timing of rainfall during the thaw phase. © 1998 John Wiley & Sons, Ltd.