Download Formation and Distribution of Snowcover

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.
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; the process of metamorphism and
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ablation which 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.
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.
2) Mesoscale or local scale: characteristic linear
distances of 102 to 103 m in which redistribution
along meso-relief features may occur because of
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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.
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.
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
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transfer of heat and moisture, radiative exchange
and air movement.
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.
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.
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.
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.
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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.
Although initiated by wind action this timedensification 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.
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).
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Deposition (mass convergence) from a fullyladen air stream occurs where the wind velocity
decreases (along the edges of forests and cities).
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.
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.
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Very slight perturbations in the airflow, such as
produced by tufts of grass, ploughed soil, or
fences, may induce drift formation.
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).
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
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change of a snow crystal, while it is being
transported, resulting from its exchange of
vapour with the surrounding air (“blowing snow
sublimation”).
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.
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.
One property of the snowcover surface which
directly affects the solar energy absorbed by the
snow is its albedo (Table 5.4).
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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.
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.
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
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elevation, slope, aspect, roughness and the optical
and thermal properties of the underlying
materials.
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.
Slope:
Mathematically, the orographic precipitation rate
is predominantly related to terrain slope and
windflow rather than elevation.
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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.
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.
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
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terrain variables
distribution.
affecting
the
snowcover
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 snowfallproducing air masses; the frequency of snowfall;
and the energy exchange processes influencing
snowmelt and ablation.
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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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.
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 nonforest (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).
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.
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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.
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.
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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.
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.
Table 5.7, taken from Steppuhn (1976) shows the
snowcover depth statistics by landscape type for
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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.
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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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.
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.
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Metamorphism can also result from compaction
caused by the pressure of overlying layers of
snow.
This process is responsible for transforming snow
into glacial ice whose crystals sometimes attain
sizes of the order of 10 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).
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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