Formation and Distribution of Snowcover

1
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.

2

• 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

3

• the processes of metamorphism and ablation can
  alter the physical characteristics of the
  snowcover so that it hardly resembles the
  freshly-fallen snow
• and surface topography, physiography and
  vegetative cover.
• Influenced by both accumulation and ablation,
  snowcover is the product of complex factors that
  affect accumulation and loss.

4

• 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.

5

• 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.

14
Source Gray and Male (1981)
15
Source http//www.avalanche.org/uac/encyclopedi
a/wind_slab.htm
16
Factors controlling the evolution and
distribution of the seasonal snowpack
Source Rouse (1993)
17

• 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.

19
Source Gray and Male (1981)
20

• 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 lt 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).

23
Source Déry and Taylor (1996)
24
Source Liston and Sturm (1998)
25
Source Jones et al. (2001)
26
AWS
16 September 2008
27
Photographic evidence of blowing snow in the
Cariboo Mountains
30 June 2007
28
SWE (mm)
29
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.

33
Source Gray and Male (1981)
34

• 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.

38
Source Slaymaker and Kelly (2007)
39
Source Slaymaker and Kelly (2007)
40
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.

42

• 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.

44

• 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.

45
Source Déry et al. (2004)
46
Source Déry et al. (2004)
47
Source Déry et al. (2004)
48
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.

54
Source Jones et al. (2001)
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57
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 Gray and Male (1981)
63
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. et al. 2010 Blowing snow fluxes in
  the Cariboo Mountains of British Columbia,
  Canada, Arctic, Antarctic and Alpine Research,
  42, 188-197.