Inside the Belly of the Beast Atmos 3200/Geog 3280 Mountain Weather and Climate Wendy Wagner, Leigh Jones and C. David Whiteman

Mountain Snowpack Beauty or Beast? Ripp’n it in the Wasatch © Tim Lane

Recap - Snow Formation and Accumulation How do we grow snow? © Kenneth G. Libbrecht SnowCrystals.com © Kenneth G. Libbrecht SnowCrystals.com 1 - Vapor Deposition 2 - Accretion 3 - Aggregation What does crystal type depend on? 1- Temperature 2- Supersaturation

New Snow - SLR (Snow Liquid Ratio) Baxter et al Volume water 10 cm^3 Volume snow 100 cm^3 X 100 = 10%

Mountain Snowpack - Diversity

Creep and glide - Snow Deformation McClung and Schaerer (1993) © 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/m 3 Average snowpack ~0.20~20%~200kg/m 3 Very dense snowpack %400kg/m 3 Slalom race course %600kg/m 3 Pure ice %917kg/m 3 Water1.0100%1000kg/m 3

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 < 10ْ 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

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.

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 crusts At 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.

Snowpack Layering Bruce Tremper, Utah Avalanche Center

Exposing the Belly of the Beast

Avalanches… Bill Gallagher