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Inside the Belly of the Beast
Atmos 3200/Geog 3280Mountain Weather and Climate
Wendy Wagner, Leigh Jones and C. David Whiteman
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Mountain Snowpack
Beauty or Beast?
Ripp’n it in the Wasatch © Tim Lane© Tim Lane
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Recap - Snow Formation and Accumulation
How do we grow snow?
© Kenneth G. LibbrechtSnowCrystals.com
© Kenneth G. LibbrechtSnowCrystals.com
1 - Vapor Deposition
2 - Accretion3 - Aggregation
What does crystal type depend on?1- Temperature2- Supersaturation
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New Snow - SLR (Snow Liquid Ratio)
Baxter et al. 2005
Volume water 10 cm^3
Volume snow 100 cm^3X 100 = 10%
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Mountain Snowpack - Diversity
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Creep and glide - Snow Deformation
McClung and Schaerer (1993)
© F. Baker© F. Baker
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Accumulating a Snowpack
Snow pack layering– Character of fallen snow
initially defines the layering in the snowpack
– The snowpack can be transformed by wind action even after snowfall has stopped. Location of wind deposited snow depends on terrain.
Jim Steenburgh in 15’ snowpit Ben Lomond
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Snowpack Densities
SWE/depth ratio
Percent density
Snow density
Cold new snow
<.04 <4% <40kg/m3
Average snowpack
~0.20 ~20% ~200kg/m3
Very dense snowpack
0.40 40% 400kg/m3
Slalom race course
0.60 60% 600kg/m3
Pure ice 0.917 91.7% 917kg/m3
Water 1.0 100% 1000kg/m3
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Snowpack Physical Characteristics
Snowpack density (can vary for different layers)
Albedo – Solar reflectivity– Fresh snow >0.9– Wet snow 0.6
Snowpack temperature profile– In temperate zones, the snowpack-ground
interface is maintained throughout the winter very close to the melting point of 0°C.
– The snowpack temperature gradient is determined by the thickness of the snowpack and snow surface temperature
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What’s Inside the Belly of the Beast?
Center for Snow and Avalanche Studies
Snowpack Stratigraphy is a record of meteorology/climate.
Sequence of weak/strong layers and binding between layers– Wind strength and
direction– Number and intensity of
storms – rain– Solar and longwave
radiation– Temperature, melting,
humidity, pressure
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Snowpack Temperature Profile
Isothermal? Weak temperature gradient? Strong temperature gradient?
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Snowpack Metamorphism
Changes in the snowpack due to heat flow and pressure
Three types of snowpack metamorphism:– Equitemperature – rounded grains (strong snow)– Temperature-gradient – faceted grains (weak snow)– Melt-freeze – rain water or melt water, percolation,
undergoes diurnal melt and freeze cycles. (weak in melt phase, strong in frozen phase)
***The temperature gradient of a snow layer determines the type of metamorphism while the temperature determines the rate of metamorphism
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Snow Temperature Profile and Snow Metamorphism
Barchet 1978
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Equitemperature Metamorphism
Perla & Martinelli (1975)
Sintering - formation of bonds between snow grains
There is a tendency for water vapor to evaporate from convex surfaces and condense at concave surfaces where grains touch to form necks.
*** Weak temperature gradient < 10 ْ C / m
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Equitemperature Metamorphism
Snow grain metamorphism (at constant temperature) by curvature effects.The time in days in given in the lower right.
AKA:– Sintering– Rounding– Destructive metamorphism
Grain is decreasing its surface area to volume ratio => surface area/volume
Settlement cones
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Equitemperature Metamorphism
Growth rates of snow grains within a snowpack
Low grain growth rates– The colder the
temperature the slower the growth rate
– The weaker the temperature gradient the slower the growth rate
Produces more bonds per unit volume and greater strength in the snowpack
USDA Electron Microscopy Snow Unit
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Temperature Gradient Metamorphism
Perla & Martinelli (1975)
*** Strong temperature gradient
The critical temperature gradient to produce faceted forms in alpine snowpacks is > 10°C / m.
AKA:– Faceting– Kinetic growth– Constructive
metamorphism
Growth by vapor diffusion from areas of relatively high vapor pressures (temperatures) to low vapor pressures (temperatures).
Minimal grain bonding
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Temperature Gradient Metamorphism
www.avalanche.org
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Depth Hoar
USDA Electron Microscopy Snow Unit
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Radiation Recrystalization
(Morstad, Adams and McKittrick 2004)
Also called:Near surface faceting
Faceting near or on the surface of the snowpack due to a temperature gradient induced by absorbed solar radiation
Peak in snow temperature occurs 2-5cm below the snow surface
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Temperature Gradient Metamorphism
Growth rates of snow grains within a snowpack
High grain growth rates– The stronger the temperature gradient the higher
the growth rate– The warmer the temperature the higher the growth
rate– The larger the pore space size the higher the growth
rate
Grains produced under high growth rates (surface hoar, depth hoar, faceted snow, radiation recrystallization) form weak, unstable snow that is often responsible for serious avalanche conditions.
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Metamorphic Forms
LaChapelle (1962)
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Temperature Gradients – Different Climates
Maritime climate: usually, temperatures are mild and snowpacks are deep so that there are weak temperature gradients and warm temperatures in the snowpack.
Continental climate: thinner snowpacks and colder air temperatures produce large temperature gradients. This leads, more often, to faceted snow crystals in the snowpack and buried layers of instability.
Intermountain climate: deeper snowpack than continental climates so temperature gradients are lower, yet they still can exist and lead to snowpack instability.
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Surface Hoar
USFS (1968)
Eastern Sierra Avalanche Center
Faceted crystals formed by deposition onto the snowpack surface when water vapor pressure in the air exceeds the equilibrium vapor pressure over ice at the surface.
The crystals usually form when– a sufficient supply of water vapor is
present in the air– a high temperature gradient (inversion)
is present above the snow surface. ***Thus surface hoar usually forms on cold,
clear nights with calm or nearly calm conditions (common in continental climates).
Surface hoar may be inhibited in concave areas of the snow surface and under trees
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Surface Hoar
Surface hoar is extremely fragile and easily destroyed by sublimation, wind, melt-freeze cycles, and freezing rain.
When buried in the snowpack, a surface hoar layer is extremely efficient in propagating shear instabilities (fractures).
Surface hoar may gain strength by bond formation with adjacent layers, but thick layers may persist for months within the snowpack.
www.avalanche.org
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Melt-Freeze Metamorphism
Warm snowpacks have variable amounts of liquid water– Water-saturated snow (slush; water content > 15% by volume)– Very wet snow (8-15% by volume)– Wet snow (3-8%, water cannot be pressed out by gentle hand
squeezing, but a meniscus of water occurs between grains)– Moist (<3%; snow sticks together to make a snowball).
Strength of wet snow decreases with increasing water content.
Small particles have a lower melting temperature than larger ones, so they melt first. The heat of melting comes from larger particles, which undergo surface re-freezing (release of heat) and an increase in size; corn snow!!
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Melt-Freeze Metamorphism
Perla &Martinelli (1975)
Sun crustsAt low water contents, clusters of grains form near the snowpack surface where melting and freezing cycles can occur.
After night cooling this combination produces very strong crusts composed of frozen grain clusters that lose almost all their strength after midday heating.
www.avalanche.org
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Snowpack Layering
Bruce Tremper, Utah Avalanche Center
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Exposing the Belly of the Beast
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Avalanches…
Bill Gallagher