Snow I: Formation, distribution, measurement, metamorphism,

avalanches

Why is snow important?

• Snow is a critical water resource in many parts of the world including Colorado

• It provides recreational opportunities, which equates to $

• It is an excellent insulator• Through its high albedo and latent

heat exchanges, it modulates climate from local to global scales

• It can present hazard (avalanches, transportation)

Photo R. Armstrong

Historic NWS collection

http://www.its.caltech.edu/~atomic/snowcrystals/photos/photos.htm

Snow formationSnow formation requires: (1) relative humidity ≥ 100%, (2) T < 0oC, (3)

condensation nuclei (CCN), (4) supercooled droplets

The basic steps:H20 Vapor + CCN + Supercooled droplets + T < 0oC

Nucleation (spontaneous below about -40oC)

Ice crystals

-----------------------------------Snow------------------------------------

Riming vapor diffusion aggregation(graupel) (flakes) (flakes)

1. Vapor diffusion

Arapahoe Basin – May 2005

Bergeron-Findeisen theory applies: If ice and supercooled water droplets exist together in a cloud, the ice crystals grow at the expense of water droplets, the reason being that if the cloud is saturated with respect to water, it is supersaturated with respect to ice, the net result being a transfer of water vapor from the supercooled (liquid) droplets to the ice crystals.

http://www.geminibv.nl/tidbits/dauwpunt-druk-en-temperatuur?set_language=en

http://www.gi.alaska.edu/alison/ALISON_Science_Snow.html

The process of vapor diffusion is most efficient at around -12 to -15oC , corresponding to the temperature of largest difference between water and ice saturation vapor pressures.Different combinations of temperature andsupersaturation determine the type of snowflake that forms

2. Aggregation• Growth by aggregation of snowflakes (snowflakes clump together to make

bigger aggregations)

3. Riming• Growth by collision and accretion of existing ice crystals with supercooled

droplets. Contact between the supercooled droplets and the ice crystals causes the supercooled water to freeze on the crystal. Ice crystals that exhibit frozen droplets on their surface are said to be rimed.

• Riming forms graupel (ice pellets) and hail• Most snowflakes are partially rimed

Graupel pellets

http://en.wikipedia.org/wiki/Graupe

http://en.wikipedia.org/wiki/Hail

Hail

Average number of weeks of snow cover over the Northern Hemisphere, basedon the NSIDC blended weekly product [from Serreze and Barry, 2005]. Snowcovers a large part of the Northern Hemisphere in winter; snow cover durationshows a general increase to the north reflecting a primary control bytemperature.

Average seasonal snow cover duration

Snow cover formation and disappearance

Dingman 2002 Figure 5-12

Average dates of snow cover formation (left) and disappearance(right) over North America are controlled by a combination oflatitude, elevation and precipitation.

Maximum snow depth (mm) over Eurasia compiled from Russiansources [courtesy of H. Ye, California State University, Los Angeles,CA, from Serreze and Barry, 2005].

Snow depth

Snow measurement

• Satellite: Optical and passive microwave instruments can measure snow cover extent (a yes/no designation - a pixel is either snow covered or snow free), but measurements are difficult under forest cover. Satellite retrievals of snowpack depth/water equivalent don’t work very well.

• Gauges and snowboards: Used to measure the depth of snowfall and (with knowledge of density), snow water equivalent. The Wyoming snow gauge was described earlier. A snowboard is generally a piece of painted plywood, depth is measured using a ruler.

• SNOTEL and Snowcourses: These networks were already described under the topic of precipitation measurement. There are over 600 SNOTEL and 1000 snowcourse sites across the western U.S. These provide measurements of snow water equivalent of the snowpack, which is key for seasonal streamflowforecasting. Snow courses are about 1000 feet long in meadows protected from the wind, tube samples are taken of depth and mass of the sample, from which snow water equivalent is determined. SNOTEL sites are automated, and directly measure water equivalent by the weight of snow atop snow pillows.

Snow water equivalent (SWE)For water management, SWE is the key snow-related hydrologic variable

SWE= h(ρs/ρw),

h = snow depth (m)ρs = snow density (approximately 1000 kg m-3) ρs = liquid water density (varies from typically 100-500 kg m-3)

• Snow depth, density and SWE depend many factors, including snowfall, temperature history, and wind redistribution.

• Variation in h are generally more important than variations ρs in determining SWE

• SNOTEL and snowcourse sites are not randomly sited, they are location at sites where SWE and streamflow are strongly correlated!

Examples: March 1 SWE time series from SNOTEL sites near Trinidad, CO

Courtesy Drew Slater

April 1 SWE for the U.S. West

Serreze et al., 1999, WRR

The figure at right shows 1 April SWE as recorded at SNOTEL sites based on data through the 1995/1996 snow season. SWE on 1 April typically represents peak seasonal SWE and is hence valuable for seasonal streamflowforecasting. SWE values tend to be highest in the Pacific NW due to high precipitation rates (orogragphic uplift and cooling of moisture-laden Pacific airmasses). Values tend to be lowest over the montane areas of the Southwest, in large part due to high temperatures. Typically SWE increases with elevation but this pattern tends to break down above the treeline. There has been much work to predict the spatial distribution of SWE in relation to elevation, slope and the surface energy balance.

Example SNOTEL Records: Lake Eldora, CO

Serreze et al., 2000

The records at left, for the 1982/1983 and 1988/1989 snow seasons, show cumulate daily SWE (top panels) and daily changes in SWE at Lake Eldora, west of Nederland. Key point from the bottom panels: Seasonal SWE accumulation is dominated by a relatively small number of large events. For SNOTEL sites in the Colorado region, on average the single largest snowfall event accounts for about 10% of the total annual snowfall (as SWE). In the Arizona/New Mexico region, the corresponding figure is 23%. Note also how rapid the snow melt season is compared to the accumulation season.

Winter snowpack evolution

• The density of fresh snowfall can be quite low (<50 kg m-3) although 100 kg m-3 is often assumed. Winds break up snowflakes, and pack them into denser layers; the lowest snowfall densities tend to occur under cold, calm conditions.

• As soon as snow accumulates, it begins a process a metamorphism that continues until melting is complete. The bulk density of the snowpack increases from typical mid-winter values of 200-300 kg m-3 to spring values of 400-500 kg m-3.

• Snowpack loss through sublimation (solid directly to vapor) can be considerable in dry areas with a strong solar radiation flux. At Niwot Ridge, Colorado, it is estimated that 15% of the snowpack is lost due to sublimation.

Metamorphism processes:1) Gravitational settling2) Destructive (equi-temperature) metamorphism; 3) Constructive metamorphism (sintering and temperature gradient metamorphism)4) Melt metamorphism

2. Equi-temperature (destructive) metamorphism

Vapor pressure is higher over sharp points of a snowflake and lower over the hollows of the flake. Water vapor preferentially sublimates from the spikes and is deposited in the hollows, making larger, more spherical snow grains with time. The highest rate of equi-temperature metamorphosis occurs as temperatures approach 0oC with a small vertical temperature gradient in the snowpack. The process does not occur below -40oC. The process is effective until densities reach about 250 kg m-3.

1. Gravitational settling

The settling rate, which acts to increase snowpack density, takes place at rates that increase with the weight of the snow layer and the temperature of the layer but decrease with increasing density of the layer. Typical rates of density increase from gravitational settling are 2-50 kg m-3 per day.

3. Constructive metamorphism

a. Sintering (bonding) of snow grains, in which water vapor molecules are deposited in concavities where two snow grains touch, gradually building a “neck” between adjacent grains.

b. Temperature gradient metamorphism - works over longer distances (next slide)

3b. Temperature gradient metamorphismCold atmosphere and snow surface (e.g., -20oC)

------------------------------------------------------------------snow surface

upward temperaturegradient

upward vaporgradient

Low vapor pressure

High vapor pressure------------------------------------------------------------------- ground

•If temperature decreases upwards in the snowpack, vapor pressure also decreases upwards

•Water vapor migrates upward, from high to low vapor pressure

• Snow at the base sublimates, water vapor is deposited higher up in the snowpack

•Results in larger and weaker snow crystals, and development of depth hoar at the base of the snowpack (a weak low density layer)

•Process requires a big vertical temperature gradient and a shallow snowpack (the deeper the snowpack, the smaller the temperature gradient dT/dh)

Depth hoar crystals

http://thesnowpit.com/main/index.php?option=com_content&task=view&id=22&Itemid=40

4. Melt MetamorphismThis occurs from two processes:

1) Liquid water formed by surface melt or introduced by rain percolates downward in the snowpack and re-freezes. This can produce ice layers and lenses in the snow. The latent heat release from freezing also warms the snowpack.

2) A rapid disappearance of small snow grains and the growth of larger grains that occurs in the presence of water. From this processes, a melting snowpack typically has an aggregation of rounded grains of 1-2 mm. This is sometimes referred to as corn snow.

http://dailyapple.blogspot.com/2007/11/apple-282-kinds-of-snow.html

Snow climates

1) Maritime - Sierra Nevada, Cascades• Heavy snow and mild temperatures• Rain on snow events are relative common• Avalanches occur during or immediately following storms• Equi-temperature metamorphism dominates in winter

2) Continental – Rockies• Less snow and lower temperatures• Temperature gradient metamorphism is common in autumn and early winter• Structural weaknesses (promoting avalanches) are more common than in maritime

regions, these include depth hoar, hard wind-packed layers and buried surface hoar

Avalanches

http://www.meted.ucar.edu/afwa/avalanche/media/graphics/avPaths_partsLabeled.jpg

An avalanche is a sudden, rapid flow of snow down a slope, triggered by either natural or human causes. They occur when the stress on the snow exceeds the shear, ductile, and tensile strength either within the snow pack or at the contact of the base of the snow pack with the ground or rock surface.

While the are different types of avalanches, they all involve a trigger which causes the avalanche, a start zone from which the avalanche originates, a slide path (or track) along which the avalanche flows, a runoutzone where the avalanche comes to rest, and a debris deposit which is the accumulated mass of the avalanched snow once it has come to rest (wikipedia)

Temperature Gradient Metamorphism : Depth Hoar

http://emu.arsusda.gov/snowsite/Light_and_LT-SEM/default.html

Avalanches are facilitated by snowpack weaknesses: TG metamorphism, hoar, surface melt crusts

http://www.mrablog.com/picture-of-the-week-16/http://sawtoothmts.wordpress.com/2009/02/

http://www.avalanche.ca/cac/library/avalanche-image-galleries/Avalanches2009-2010

Surface hoar: Dew as ice crystals on the snow surface. On humid nights, long-wave radiational cooling of the

snow pack causes the temperature to drop below the dew point at the snow surface. This results in condensation

and the growth of ice crystals

When buried by subsequent storms it forms a weak layer in the snowpack.

Near surface faceted crystals form bi-directionally near the snow surface as a result of T-G metamorphism. Diurnal changes in the temperature (and thus vapor pressure) gradients give rise to the changes in crystal growth direction.

Day Night

Warm

Cold

Cold

Warm

Like depth and surface hoar, these faceted crystals can form persistent weak layers in a the snowpack.

T-G

T-G

The drier “Continental” West (i.e. Colorado) is more prone to avalanches due to a climate that favors T-G metamorphism of the snowpack.

Basic avalanche typesLoose snow (or point) avalanches:

Occur in freshly fallen snow that has a low density and are most common on steeper terrain. In fresh, loose snow the release is usually at a point and the avalanche then gradually widens down the slope as more snow is entrained, usually forming a teardrop appearance. Can move 75-100 mph.

Slab avalanche (“Wet” or “Dry” Slabs):

These are the most destructive type. Slab avalanches occur when there is a strong, cohesive snow layer (a slab), usually formed when falling snow is deposited by the wind on a lee slope, or when loose ground snow is transported elsewhere. When there is a failure in a weak layer, a fracture very rapidly propagates so that a large area, that can be hundreds of meters in extent and several meters thick, starts moving. Can move 50-75 mph.

Wet snow avalanche:

Occurs when the snow pack becomes saturated by water. These tend to also start and spread out from a point. If water content is high they may take the form of slush flows. The move 25 mph.

http://www.sierraavalanchecenter.org/phpBB2/viewtopic.php?f=10&t=642

http://www.sierradescents.com/skiing/harwood/2006/stockton-flats.html

Point Release Avalanches: Sierra Nevadas

Anatomy of a slab avalanche

http://www.math.utah.edu/~eyre/lectures/snow/anatomy_cross.html

Key elements:

•A bed surface (snow, rock, ground) that the avalanche runs on•A weak layer of snow (poorly bonded with weak shear strength)

that fails, initiating the avalanche•A slab (a cohesive layer) that moves during the avalanche

http://cagem.bayindirlik.gov.tr/turleri-en.htm

http://library.thinkquest.org/03oct/01027/avalanches.html

30-45 deg. at starting zone

http://www.math.utah.edu/~eyre/lectures/snow/anatomy_cross.html

30-45 deg. at starting zone

The Physics of Failure

An avalanche occurs if: shear stress > shear strength(downslope gravitational force) > (cohesion/friction)

Forces on a hillslope: Gravity is the primary driver of slope failures and avalanches. A component of the weight (or gravitational force) of the snowpack is acting parallel to the slope proportional to the sine of the slope angle, this is the shear stress. The other component of gravity acts perpendicular to the slope, compressing the snowpack, and increasing friction.

Fg: Force of gravity

Fgcosθ

Fgsinθ

θ

dept

h

Slab avalanches typically occur on slope angles of 30-45 degrees: at low slopes the shear stress is insufficient to cause failure, and at higher slopes snow is constantly shed and does not accumulate to sufficient depths.

θ

Factors such as wind loading, new snow, skiers, and explosives all increase the shear stress on the snow pack. If weak layers are present (low shear strength), then a slab avalanche may occur.

Dry Slab Avalanche:1) Shear failure of weak layer caused by loading (or snow metamorphism)2) Fracture of weak layer propagates outward (putting snow in tension)3) Tension cracks propagate vertically through the snow (eventual crown)4) Slab releases…during an avalanche the slab may entrain deeper layers of

snow, sometimes all the way to the ground.

θθ

1

2

2

3

4

Vail Pass, 1/29/2011

Slide stepped to ground in some locations

Source: CAIC

All Information: Colorado Avalanche Information Center, CAIC

This slab avalanche failed on a faceted layer that formed two months prior.

Note a nearly two week faceting period in November, followed by a steadily

increasing snow pack. Perfect conditions for instability. Early season weak layers can persist and result in

avalanches many months later.

Wet Slab Avalanche: Water either increases the load (rain) or decreases cohesion and friction due to melt.

Arapahoe Basin Ski Area (In-Bounds), May 20th, 2005, 1 skier killed

All Information: Colorado Avalanche Information Center, CAIC

All Information: Colorado Avalanche Information Center, CAIC

Melt-water percolates through the snowpack until it reaches a relatively impermeable layer (an ice layer/the ground). The water then lubricates the slide surface causing the overlying slab to fail.

Wet Slab Avalanche

Wet debris is very dense