SNOW HYDROLOGY (GEOG 4321): SNOW METAMORPHISM

Instuctor: Mark Williams

Readings

Avalanche Handbook, pp 45-60.

Colbeck, Thermodynamics of Snow Metamorphism, J Glaciology, 1980.

Physics of Skiing, Appendix.

OVERVIEW

Definition

• term borrowed from geology, with same meaning;
• crystalline changes that take place due to the effects of temperature and pressure

Why of Interest

Causes changes in snow properties, including:

• sintering
• structural strength of snowpack
• permeability of snowpack
• reflectivity of the snow surface
• thermal conductivity of the snowpack
• snow density

These changes in snow properties affect:

• avalanche stability
• avalanche release
• melt-water runoff
• radiant-energy penetration into the snowpack
• release of solutes from the snowpack

Why Snow Metamorphsim Occurs

• Crystalline solid close to its melting temperature
• Thermodynamically unstable
  • large surface area to volume ratio
  • results in high surface free energy
  • thermodynamic equilibrium occurs when surface to volume ratio is minimized, eg a sphere or rounded grain.

INITIAL CHANGES

Examples

• Initial loss of points.
• Concavities filling, points disappering.
• Snowflakes now becoming rounded snow grains.
• Rounded snow grains with sintering between snow grains.

Snow crystals begin to change form immediately because (Avalanche Handbook, pages 46-48):

• Less supersaturation in snowpack than atmosphere
• Large surface to volume ratio: eg stellars
• Therefore high surface free energy
• Thermodynamically unstable
• Snow crystals with the highest surface to volume ratios (eg dendrites) are the most unstable and change form most quickly.
• Radius of curvature effects can be very large because of branched form of many new snow crystals.
• As branches disappear, there is a general DECREASE in the average particle size.
• However, after the branches disappear, size begins to increase.
• Larger particles grow at the expense of smaller particles because
  • Sublimation is more efficient from small particles because of the higher radius of curvature and hence higher vapor pressure, and
  • Water vapor then moves efficiently away from smaller particles and towards larger particles, which have lower vapor pressures because of smaller radius of curvature pressure effects.
  • Condensation or redeposition is more efficient over larger particles because of lower vapor pressures.
  • Thus, in a snowpack with a mixture of particle sizes, the average particle size INCREASES over time, after the initial decrease in size of particles with high surface to volume ratios.

EQUITEMPERATURE METAMORPHISM

Definition

Rounded grains that bond together into simple chains.

Example

Synonyms

• Destructive metamorphism.
• Equitemperature metamorphism.
• Equilibrium metamorphism.
• Radius-dependent metamorphism.
• Radius of curvature metamorphism.

Parameters

• Temperature gradient in snowpack is less than 10 deg C per meter.
• Vapor diffusion direction governed by radius of curvature of snow grains, such that there is a loss of mass from points on individual snow grains (convexities) and gain in mass in hollows (concavities).

Process

• Saturation vapor pressure is greater over points and lowest at hollows.
• The reason is that, at saturation, more vapor pressure can be supported over a convex surface than a flat surface, and more water vapor can be supported over a flat surface than a concave surface.
• For a given temperature, the saturation vapor pressure is greater over a point (convexity) than over a hollow (concavity).
• Water vapor then moves from a region of high vapor density (points) to a region of low vapor density (hollows).
• As water moves away from a point towards a hollow, the air in the pore space above the point becomes undersaturated (RH < 100%). Water then sublimes from the point into the air, causing a mass loss from the point. The water vapor above the point again reaches saturation, then moves towards lower vapor density above hollows. A positive feedback loop is then setup, such that ice constantly sublimes to water vapor above points, and the water vapor moves to areas with lower vapor density, such as hollows.
• At hollows, the vapor density becomes supersaturated because of the contribution of water vapor from points. Water vapor is then deposited into hollows as ice, filling the hollows. As water vapor is deposited in hollows, the vapor density decreases, increasing the vapor density gradient and causing more water vapor to move from points to hollows, increasing the amount of supersaturation. Another positive feedback loop is established, causing increased deposition of ice from the vapor phase to hollows.
• We have established a positive feedback loop, such that there is a loss of mass from points to the vapor phase, movement of water vapor to areas of lower vapor density over hollows, and deposition of water vapor to ice in the hollows.
• Sintering or bonding of two snow grains occurs by this process, because whenever two round shapes touch, a hollow or concavity is formed.

Additional information from Professor Paul Brooks' at the University of Arizona (former student in this class) here .

Results

• Reduction in the surface to volume ratio of snow grains.
• Reduction in surface free energy of snow grains.
• Snow grains fill the pore space in the snowpack.
• Density of the snowpack increases.
• Rounding of snow grains occurs.
• Bonding or sintering of snow grains occurs.
• The strength of the snowpack increases.
• The rate of ET metamorphism increases with increasing temperature.

TEMPERATURE GRADIENT METAMORPHISM

Definition

Angular grains with poor sintering

Synonyms:

• constructive metamorphism
• temperature-gradient metamorphism
• kinetic growth
• depth hoar
• sugar snow

Parameters:

• Coupled, simultaneous heat and mass transfer
• temperature gradient > 10 degrees C
• snow density < 350 kg/m^3
• radius of curvature effects not important

Process

• Strong temperature gradient over a short distance (10 ⁰ C m ^-1 )
• Causes a strong vapor pressure or vapor density gradient
• Water vapor then moves from the area of high water vapor density (warmer area of snowpack) to lower water vapor density (colder area of snowpack).
• Supersaturation becomes relatively high as the water vapor moves into a colder region of the snowpack, which has a lower saturation vapor pressure because of colder temperatures.
• This relatively large amount of supersaturation results in immediate deposition whenever the supersaturated air contacts a nucleating surface.
• This deposition occurs before water vapor can move from convexities of concavities.
• Generally, deposition occurs as facets in a plane.
• These facets can be from one molecule to several thousand molecules thick.
• The angle of deposition is approximately 120 ^O, leading to a stepped, then scrolled, and finally cup-shaped appearance.
• The cup grows TOWARDS the water vapor source, or TOWARDS the warm part of the snowpack.
• Here's some examples:
  • Depth Hoar 1
  • Depth Hoar 2
• Hence, the hollow part of TG snow grains is open towards the water vapor source, and
• TG snow grains become larger away from their initial point of origin and towards the water vapor source.
• The relocation of mass caused by temperature gradients in snow is usually on the scale of individual snow grains, eg a hand-to-hand process.

Results

• Density remains relatively constant, usually less than 350 kg/m3.
• Necks between snow grains remain the same size.
• Crystal size is usually greater than 3 millimeters.
• Decrease in the structural strength of the snowpack.
• TG grains grow towards the vapor source.
• Grains are usually vertically oriented.
• The vertical orientation resists compaction by gravity, further resisting an increase in density.
• The large, vertical snow grains act as levers with little attachment or bonding to neighboring snow grains.
• Mechanical strength is thus low.
• Almost always forms a weak layer in the snowpack.
• Forms at the bottom of the snowpack, on the underside of ice lenses, and near rocks and trees.

SUMMARY

Back to Course Content