1 Formation and Distribution of Snowcover Snowcover comprises the net accumulation of snow on the...

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Formation and Distribution of Snowcover

• Snowcover comprises the net accumulation of snow on the ground resulting from precipitation deposited as snowfall, ice pellets, hoar frost and glaze ice, and water from rainfall, much of which subsequently has frozen, and contaminants.

• Its structure and dimensions are complex and highly variable both in space and time.

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• This variability depends on many factors: • the variability of the “parent” weather (in

particular, atmospheric wind, temperature and moisture of the air during precipitation and immediately after deposition);

• the nature and frequency of the parent storms;• the weather conditions during periods between

storms when radiative exchanges may alter the structure, density and optical properties of the snow and wind action may promote scour and redeposition as well as modification of snow density and crystalline structure;

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• the processes of metamorphism and ablation can alter the physical characteristics of the snowcover so that it hardly resembles the freshly-fallen snow;

• surface topography, physiography and vegetative cover also affect snow conditions.

• Influenced by both accumulation and ablation, snowcover is the product of complex factors that affect accumulation and loss.

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• The areal variability of snowcover is commonly considered on three geometric scales:

• 1) Macroscale or regional scale: areas up to 106 km2 with characteristic linear distances of 104 to 105 m depending on latitude, elevation and orography, in which the dynamic meteorological effects such as standing waves, the directional flow of wind around barriers and lake effects are important.

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• 2) Mesoscale or local scale: characteristic linear distances of 102 to 103 m in which redistribution along meso-relief features may occur because of wind or avalanches and deposition and accumulation may be related to the elevation, slope and aspect of the terrain and to the canopy and crop density, tree species or crop type, height, extent and completeness of the vegetative cover.

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• 3) Microscale: characteristic distances of 10 to 102 m over which major differences occur and the accumulation patterns result from numerous interactions, but primarily between surface roughness and transport phenomena.

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Factors Controlling Snowcover Distribution and Characteristics

• Snow accumulation and loss are controlled primarily by atmospheric conditions and the “state” of the land surface.

• The governing atmospheric processes are precipitation, deposition, condensation, turbulent transfer of heat and moisture, radiative exchange and air movement.

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• The major land features to be considered are those which affect the atmospheric processes and the retention characteristics of the ground surface.

• a) Temperature:

• Snowcover is a residual product of snowfall and has characteristics quite different from those of the parent snowfall.

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• The temperature at the time of snowfall, however, controls the dryness, hardness and crystalline form of the new snow and thereby its erodability by wind.

• The importance of temperature is apparent on mountain slopes, where the increase in snowcover depth can be closely associated with the temperature decrease with increasing elevation.

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• Wet snow, which is heavy and generally not susceptible to movement by wind action, falls when air temperatures are near the melting point; this commonly occurs when air flows off large bodies of water.

• Within continental interiors where colder temperatures often prevail the snowfall is usually relatively dry and light.

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• b) Wind:

• The roughness of the land surface affects the structure of wind and hence its velocity distribution.

• Because of the frictional drag exerted on the air by the earth's surface, the wind flow near the ground is normally turbulent and snowcover patterns reflect a resulting turbulent structure.

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• Also, the wind moves snow crystals, changing their physical shape and properties, and redepositing them either into drifts or banks of greater density than the parent material.

• For example, Church (1941) found that fresh snow with densities of 36 and 56 kg m-3 increased in density to 176 kg m-3 within 24 hours after being subjected to wind action.

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• Although initiated by wind action this time-densification of snow is also influenced by condensation, melting, and other processes.

• Table 5.1 lists the densities of snowcover subjected to different levels of wind action.

• Wind transports loose snow causing erosion of the snowcover, packing it into windslab and crust, and forming drifts and banks.

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Source: Gray and Male (1981)

15Source: http://www.avalanche.org/~uac/encyclopedia/wind_slab.htm

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Factors controlling the evolution and distribution of the seasonal

snowpack

Source: Rouse (1993)

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• A loose or friable snowcover composed of dry crystals, 1-2 mm in diameter, is readily picked up even by light winds with speeds ~ 10 km h-1.

• Erosion (mass divergence) prevails at locations where the wind accelerates (at the crest of a ridge).

• Deposition (mass convergence) from a fully-laden air stream occurs where the wind velocity decreases (along the edges of forests and cities).

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• The rate of transport is greatest over flat, extensive open areas, free of obstructions to the airflow, and is least in areas such as cities and forests having great resistance to flow.

• Table 5.2, summarizes the mean winter transport flux rates for different physiographic and climatic regions.

• These data show that the transport rates in the highly exposed Arctic Coast and Tundra regions are substantially greater than those in more sheltered regions, such as the Rocky Mountains.

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Source: Gray and Male (1981)

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• Drifts are deepest where a long upstream fetch covered with loose snow has sustained strong winds from one direction.

• The drifts are less pronounced when the winds change direction, especially at low speeds.

• Very slight perturbations in the airflow, such as produced by tufts of grass, ploughed soil, or fences, may induce drift formation.

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• In areas with no major change in land use, and where the wind distributions are repeated seasonally, the drifts tend to form in approximately the same shapes and locations from year-to-year.

• The largest drifts are caused by major wind storms such as blizzards which may have speeds exceeding 40 km h-1.

• Most snow is transported by saltation and turbulent diffusion (suspension).

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• Saltation is the dominant wind-transport process at low wind speeds (U10 < 10 m s-1) whereas suspension dominates mass transport rates at higher wind speeds.

• An important aspect to consider in the redistribution of snowcover by wind is the mass change of a snow crystal, while it is being transported, resulting from its exchange of vapour with the surrounding air (“blowing snow sublimation”).

23Source: Déry and Taylor (1996)

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25Source: Jones et al. (2001)

2630 June 2007

Photographic evidence of blowing snow in the Cariboo Mountains

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16 September 2008

AWS

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Interaction in a Forest Environment

• Maximum accumulations of snow often occur at the edges of a forest as a result of snow being blown in from adjacent areas, but depend very highly on the porosity of the stand borders.

• Within the stand accumulations may not be uniform, however, generally the snowcover distribution is more uniform within hardwoods than within coniferous forests.

• Further, most studies have reported that more snow is found within forest openings than within the stand.

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Energy and Moisture Transfer

• During the winter months energy and moisture transfers to and from the snowcover are significant in changing its state.

• Prior to the period of continuous snowmelt the radiative fluxes are dominant in determining changes in depth and density.

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• The underlying surface, the physical properties of the snowcover and trees, buildings, roads or other features, and activities which interrupt the snowcover or alter its optical properties, affect the net radiative flux to the snow.

• Such factors, therefore, influence how the snowcover is modified by the different radiative fluxes to change its erodability, mass and state.

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• One property of the snowcover surface which directly affects the solar energy absorbed by the snow is its albedo (Table 5.4).

• The spatial changes in albedo of a snowcover relate at least to the snow depth (“masking depth”), which is a regional characteristic.

• Heat and mass transfers from the air and ground lead to changes in the crystal structure within the snowcover and to loss of mass as melt or water vapour.

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Source: Gray and Male (1981)

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• The turbulent transfer of heat and moisture, which occurs with chinook winds, can lead to evaporation, melting, the formation of glaze, and general physical alterations within the snowcover.

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Physiography

• Landform and the juxtaposition of surfaces with different thermal and roughness properties are major factors governing snowcover characteristics.

• Winter snowcover reaches the greatest depths in snowbelt areas to the lee of open water areas, and on windward slopes which stimulate the precipitation process.

• Shallow depths occur on sheltered slopes, particularly those with sunny exposures and at lower elevations where melt losses are more probable.

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• The usual wind patterns and slides occurring in rugged terrain may result in extremely varied depths.

• The physiographic features which rationally and demonstrably relate to snowcover variations are elevation, slope, aspect, roughness and the optical and thermal properties of the underlying materials.

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Elevation

• Normally, in mountainous regions elevation is presumed to be the most important factor affecting snowcover distribution.

• Often a linear association between snow accumulation and elevation can be found within a given elevation interval at a specific location.

• The increases observed with elevation reflect the combined influence of slope and elevation on the efficiency of the precipitation mechanism.

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Source: S

laymaker and K

elly (2007)

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Source: S

laymaker and K

elly (2007)

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Slope

• Mathematically, the orographic precipitation rate is predominantly related to terrain slope and windflow rather than elevation.

• If the air is saturated, the rate at which precipitation is produced is directly proportional to the ascent rate of the air mass and, over upsloping terrain this rate is directly proportional to the product of the wind speed and the slope angle.

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• Even where orography is the principal lifting mechanism and snowfall may be expected to increase with elevation, the depth of accumulation or deposition may not exhibit this trend.

• Besides the many factors affecting distribution, winds of high speed and long duration at the higher elevations are more frequent causing transport and redistribution.

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• In areas topographically-similar to the Prairies, where snow is primarily due to frontal activity and the exposed snowcover is subjected to high wind shear forces, slope and aspect are important terrain variables affecting the snowcover distribution.

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Aspect

• The importance of aspect on accumulation is shown by the large differences between snowcover amounts found on windward and leeward slopes of coastal mountain ranges.

• In these regions the major influences of aspect contributing to these differences are assumed to be related to: the directional flow of snowfall-producing air masses; the frequency of snowfall; and the energy exchange processes influencing snowmelt and ablation.

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• Within the Prairie environment it is accepted that the influence of aspect on accumulation is outweighed by the snow transport phenomenon and to a lesser extent by local energy exchange.

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Source: Déry et al. (2004)

45Source: Déry et al. (2004)

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Source: Déry et al. (2004)

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Vegetative Cover

• Vegetation influences the surface roughness and wind velocity thereby affecting the erosional, transport and depositional characteristics of the surface.

• If the biomass extends above the snowcover it affects the energy exchange processes, the magnitudes of the energy terms and the position (height) of the most active exchange surface.

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• Also, a vegetative canopy affects the amount of snow reaching the ground.

• Most studies of the interaction between vegetation and snow accumulation can be divided into separate investigations of forest and non-forest (short vegetative cover) ecosystems.

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Forest

• A forest differs from other vegetative covers mainly in providing a large intercepting and radiating biomass above the snowcover surface.

• Generally more snow is consistently found in forest clearings than within the stand

• Kuz'min (1960) reports that the snowcover water equivalents in a fir forest WEPf and in a clearing WEPc can be related to tree density p (expressed as a fraction) as follows: WEPf = WEPc (1 - 0.37p).

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• In addition to affecting the wind velocity distribution and interception, which influence snow accumulation and distribution, a forest modifies the energy flux exchange processes which change snowcover erodability, mass and state.

• One of the greatest differences in the hydrological balance between forests and short vegetation lies in the interception of precipitation.

• A much greater fraction of precipitation is intercepted by a forest canopy because of the large surface area of foliage, the canopy structure of forests, and interactions with the boundary layer.

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• Precipitation is either intercepted by foliage or falls directly to the forest floor as throughfall.

• Intercepted precipitation can remain on the canopy, evaporate or sublimate, or fall to the forest floor.

• Conifers intercept more water (snow and rain) than hardwoods, since they maintain their leaves throughout the entire year.

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• The amount of intercepted snow depends on canopy density, whether the snow is wet or dry, the amount already on the canopy, and meteorological conditions.

• Large trees in the BC coastal forests intercept up to 50% of snowfall, whereas shorter trees within the interior tend to intercept less snowfall.

• This impacts the amount of snow reaching the ground and snowpack evolution in forested environments.

53Source: Jones et al. (2001)

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Source: Jones et al. (2001)

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Prairies and Steppes

• Terrain and wind are especially important in establishing snowcover patterns on the Prairies.

• Over the highly exposed, relatively flat or moderately-undulating terrain, the increased aerodynamic roughness resulting from meso- and microscale differences in vegetation may produce wide variations in accumulation patterns.

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• Accumulations are most pronounced where sustained strong winds from one direction act on a long upstream fetch of loose snow and less pronounced when winds frequently change direction, especially for low speeds.

• Forests, pastures, cultivated fields, ponds, etc., within the same climatic region tend to accumulate snow in recurring patterns unique to specific terrain features and land use.

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• Table 5.7, taken from Steppuhn (1976) shows the snowcover depth statistics by landscape type for west central Saskatchewan. Several aspects of the data are noteworthy:

• 1) The depth of snow collected by bushes is consistently higher than that collected on fallow, stubble or pasture, independent of the terrain features.

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• 2) A strong dependency exists between vegetation and terrain in relation to the comparative amounts of snow retained by fallow, stubble and pasture.

• 3) The number of observations required to obtain comparable values of the coefficient of variation varies widely with landscape type.

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Source: G

ray and Male (1981)

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Snowcover Structure and Metamorphism

• Snow stratification results from successive snowfalls over the winter and processes that transform the snow cover between snowfalls

• Snow metamorphism depends on temperature, temperature gradient, and liquid water content.

• The size, type, and bonding of snow crystals are responsible for pore size and permeability of the snowpack.

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• In low wind speed environments, fresh snowfall has low hardness and density (50 to 120 kg m-3).

• Temperature gradients induce water vapour pressure gradients, vapour diffusion from the warmest crystals, and consequent change in the shape of the crystals.

• Metamorphism can also result from compaction caused by the pressure of overlying layers of snow.

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• This process is responsible for transforming snow into glacial ice whose crystals sometimes attain sizes of the order of 1 cm.

• During its early stages, the refreezing of melt water can accelerate the densification process.

• Snow density often assumed to increase exponentially with time (e.g. Verseghy, 1991).

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• The flow of water is affected by impermeable layers, zones of preferential flow called flow fingers, and large meltwater drains.

• Meltwater drains are usually large and end at the base of the snowpack, whereas flow fingers occur between two snow layers only.

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For further reading…

• Déry, S. J., Crow, W. T., Stieglitz, M., and Wood, E. F. 2004: Modeling snow-cover heterogeneity over complex Arctic terrain for regional and global climate models, J. Hydrometeorol., 5(1), 33-48.

• Déry, S. J. and Yau, M. K. 2001: Simulation of an Arctic ground blizzard using a coupled blowing snow-atmosphere model, J. Hydrometeorol., 2, 579-598.

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