Physical Properties of the Snowpack Richard Brandt – University of Washington Paul Smith’s College 21 Feb 2006

OVERVIEW Microphysical properties: Snow grain types Mass density Metamorphism Thermal properties Thermal Conduction Thermal Convection Optical properties Spectral absorption in pure ice Scattering vs absorption Spectral albedo Solar (allwave) albedo Internal spectral fluxes and heating rates

Other properties we don’t have time to discuss: Permeability Gas transfer Chemistry Ice surface chemistry Internal ice chemistry Mechanics Grain scale (micro) Continuum scale (macro)

Microphysical properties of snow Crystalline structure of ice:

Microphysical properties of snow Primary snow grain types: New snow crystal type depends on temperature and water vapor supply. Plate (new snow) Column (new snow) Dendrite (new snow) Images from: The Snowflake: Winter’s Secret Beauty -- by Kenneth Libbrecht and Patricia Rasmussen

Microphysical properties of snow Old (metamorphosed) snow crystal type depends on temperature and temperature gradients in the snowpack. Scroll or cup (TG metamorphism) Rounded ice grains (ET metamorphism) (Melt- Freeze Metamorphism) (Firnification)

Microphysical properties of snow Snow and ice density: Snow/ice typemetric tons/m 3 or grams/cm 3 Dry new snow Damp new snow Settled snow Depth hoar Wind packed Firn (0.83 = air bubble closure) Glacier ice Bubble free ice0.917

Microphysical properties of snow Metamorphism: As snow ages, the crystalline structure changes depending on the temperature and temperature gradient, the presence of liquid water, and the weight of the overlying snow: “Destructive” Metamorphism increases snow density and grain size ET (equitemperature) rounds the grains by vapor diffusion: FT (freeze-thaw) makes grains cluster and round Firnification compacts and rounds grains “Constructive” Metamorphism decreases snow density and increases grain size (also weakens snowpack and leads to avalanches) TG (temperature gradient) vapor migration makes depth hoar

Thermal properties of the snowpack Thermal conductivity k is a measure of a material’s ability to transfer heat. A high value of k is a good conductor while a low value of k is a good insulator. k has units of Wm -1 K -1 Fresh Snow 0.03 (better than fiberglass insulation!) Old Snow0.4 Ice2.1 Snow conductivity increases with density: k    2

Thermal properties of the snowpack Snow conductivity also increases with ET metamorphism. Snow will dampen temperature swings at the surface. For example, a 1 degree temperature change in 15 min at surface will only change the temperature at 20cm depth by 0.1 degree, and at 1m depth by 0.01 degree. Thermal Convection: In rare instances of fresh snow with a large temperature gradient, Thermal convection has been observed.

Optical properties of the snowpack Sunlight can penetrate the snowpack, although the depth of solar penetration is extremely dependent on the color of the light. While ice does not absorb much in the ultraviolet, blue, and green parts of the spectrum, it is highly absorbing in the red and near infrared. Absorption coefficient  is the fraction of light absorbed in 1m. 1/  is the ‘e-folding’ of light penetration. In snow, light is both scattered by grains and absorbed:

Optical properties of the snowpack Albedo is a measure of light that exits upward through the snowpack surface. Albedo = Upward Flux / Downward Flux Spectral Albedo depends on: Wavelength (highest in the visible) Grain size (inversely) Solar elevation (inversely) Snow depth (directly) Water Content (inversely)

Optical properties of the snowpack Solar or Allwave Albedo is the average albedo at all wavelengths. Allwave Albedo depend on the variables above for Spectral Albedo plus the cloud cover. Clouds absorb red and infra red, so the light that is transmitted through the cloud produces a higher average snow albedo than on a clear day. Allwave snow albedo Clear sky Cloudy Sky 0.45 to to 0.87

Optical properties of the snowpack Internal Spectral Fluxes Spectral transmission is the percent of a particular color of light at a certain depth in the snowpack, compared to the light at the surface. The deeper light travels in the snowpack, the more the red and infrared light is absorbed. At 1-2 m depth, all that is left is the blue light. Snow Heating rates: In dry snow over 90% of the solar heating is in top 1 cm. Light deeper in the pack has a high albedo and is scattered out. Dark objects, water, and ice lenses in the snow or at it’s base can absorb the light deep in the snowpack which can strongly increase heating rates.

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