Download Formation and Distribution of Snowcover

ENSC 454/654: Snow and Ice
Week 3 – Part 2:
Snowcover formation and distribution
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Snow Class
Distributions
Source: Sturm
et al. (1995)
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Snowcover formation and distribution
• Snowcover forms 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 (such as
woody debris, algae, pollutants, etc).
• Its structure and dimensions are complex
and highly variable both in space and time.
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Snowcover variability depends on many
different factors:
1) the variability of the “parent” weather
(in particular, winds, temperature and
moisture of the air during precipitation and
immediately after deposition);
2) the nature and frequency of the parent
storms;
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3) the weather conditions during periods
between storms
This is 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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4) the processes of metamorphism and
ablation can alter the physical properties of
the snowcover so that it hardly resembles
the freshly-fallen snow;
5) surface topography, physiography and
vegetation also affect snow conditions.
Influenced by both accumulation and
ablation, snowcover is therefore the
product of complex factors that affect its
accumulation and ablation.
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Spatial scales of snowcover
variability
1) Macroscale (regional scale)
2) Mesoscale (local scale)
3) Microscale
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Macroscale Variabilty
• Areas up to 106 km2 with characteristic
linear distances of 104 to 105 m depending
on latitude, elevation and orography.
• Here the dynamic meteorological effects
such as standing atmospheric waves, the
directional flow of wind around barriers
and lake effects are important.
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Mesoscale Variability
• Characteristic linear distances of 102 to 103 m.
• Here 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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Microscale Variability
• Characteristic distances of 10 to 102 m.
• Major differences occur at these scales
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 ablation are
controlled primarily by atmospheric
conditions and land surface “state”.
• The governing atmospheric processes are
precipitation, deposition, condensation,
turbulent transfer of heat and moisture,
radiative exchange and air movement.
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Environmental Controls on Snow
Atmospheric
 Air Temperature
 Precipitation
 Winds
 Humidity
 Radiation
 Turbulent Fluxes
(Sensible/Latent)
Land Surface
 Physiography
 Latitude/Longitude
 Elevation
 Slope
 Aspect
 Vegetation
 Soil Temperature
 Roughness
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Temperature
• The temperature at the time of snowfall,
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 snow depth
increases (often linearly) with 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 often 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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Winds
• 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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• Wind moves snow crystals, changing their
physical shape and properties, and
redepositing them either into drifts 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 (see Table 5.1).
• Wind transports loose snow causing
erosion of the snowcover, packing it into
windslabs and crust, and forming drifts
and banks.
17
18
Source: Gray and Male (1981)
19
Source: http://www.avalanche.org/~uac/encyclopedia/wind_slab.htm
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• A loose snowcover composed of dry
crystals, 1-2 mm in diameter, is readily
picked up even by light winds (~ 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
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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.
• Snow mass transport rates in the highly
exposed Arctic Coast and tundra regions
are substantially greater than those in
more sheltered regions (Table 5.2).
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24
Source: Gray and Male (1981)
Source: Déry et al. (2010)
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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 patches 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 < 36 km
h-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”).
28
Source: Déry and Taylor (1996)
29
Source: Liston and
Sturm (1998)
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31
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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• A property of the snowcover surface that
directly affects the solar energy absorbed
by the snow is its albedo (Table 5.4).
• The spatial changes in albedo of a
snowcover relate 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
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of mass as melt or water vapour.
38
Source: Gray and Male (1981)
Physiography
• Landform and the juxtaposition of surfaces with
different thermal and roughness properties are
major factors governing snowcover properties.
• Winter snowcover reaches the greatest depths
in snowbelt areas to the lee of open water areas,
and on windward slopes through orographic
enhancement of precipitation.
• 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 that 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.
41
Source: Slaymaker and Kelly (2007)
42
Source: Slaymaker and Kelly (2007)
43
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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48
Source: Déry et al. (2004)
Source: Déry et al. (2004)
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Source: Déry et al. (2004)
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