Inside the Belly of the Beast

Atmos 3200/Geog 3280Mountain Weather and Climate

Wendy Wagner, Leigh Jones and C. David Whiteman

Mountain Snowpack

Beauty or Beast?

Ripp’n it in the Wasatch © Tim Lane© Tim Lane

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

New Snow - SLR (Snow Liquid Ratio)

Baxter et al. 2005

Volume water 10 cm^3

Volume snow 100 cm^3X 100 = 10%

Mountain Snowpack - Diversity

Creep and glide - Snow Deformation

McClung and Schaerer (1993)

© F. Baker© F. Baker

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

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

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

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

Snowpack Temperature Profile

Isothermal? Weak temperature gradient? Strong temperature gradient?

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

Snow Temperature Profile and Snow Metamorphism

Barchet 1978

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

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

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

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

Temperature Gradient Metamorphism

www.avalanche.org

Depth Hoar

USDA Electron Microscopy Snow Unit

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

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.

Metamorphic Forms

LaChapelle (1962)

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.

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

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

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!!

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

Snowpack Layering

Bruce Tremper, Utah Avalanche Center

Exposing the Belly of the Beast

Avalanches…

Bill Gallagher