Transcript 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.
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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 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.
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Source: Gray and Male (1981)
Source: 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)
• 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”).
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Source: Déry and Taylor (1996)
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Source: Jones et al. (2001)
Photographic evidence of blowing snow
in the Cariboo Mountains
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30 June 2007
AWS
16 September 2008
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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)
• 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: Slaymaker and Kelly (2007)
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Source: Slaymaker and Kelly (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 snowfallproducing 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)
Source: 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.
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Source: 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: Gray 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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