SNOW HYDROLOGY (GEOG 4321): BLOWING SNOW

Instuctor: Mark Williams
Telephone: 303/492-8830

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Basic Wind Field
Threshold Wind Speed
Modes of Snow Transport
Biogeochemistry

Snowfence and blowing snow, Niwot Ridge, Colorado.

Nice overview by Don Cline, required for class .

Basic Wind Structure

• Geostropic Winds:
  • Winds at altitudes greater than 1 kilometer
  • Pressure-gradient forces in equilibrium with Coriolis forces
  • Winds essentially governed by large-scale weather systems and independent of surface topography
• Earth's surface:
  • Friction between the earth's surface and atmosphere
  • Resuts in a "no-slip" condition such that wind speed must be zero.
• Boundary layer:
  • Boundary layer is the section of the earth's atmosphere between the earth's surface and geostrophic winds.
  • Wind speed in the boundary layer increases from zero at the earth's surface to its geostropic value at the top of the boundary layer.
  • The thickness of the boundary layer and the distribution of wind velocity with height above ground level depend on surface roughness.
  • Over rough terrain the boundary layer is thicker and the wind speed increases relatively slowly with height.
  • Over flat, open terrain the boundary layer is thinner and the wind speed increases relatively fast with height.
• Measuring wind speed:
  • Wind speed is usually measured at a height of 10 meters above terrain that is relatively flat and open.
  • Measured wind speed at airports and other meteorological sites will generally differ from wind speeds at other heights and/or over different terrain.
• Shear stress
  • Formal definition: downward transfer of momentum to the surface.
  • Descriptive definition: drag of the wind on the earth's surface.
  • Units: force per unit area (N m^-2).
  • Velocity gradients in the boundary layer imply the existence of shear stresses in the wind flow.
  • Shear stress is at a maximum at the earth's surface.
  • Shear stress decreases with increasing height above the ground.
  • Shear stress becomes zero in the geostrophic wind above the boundary layer.
  • The shear stress exerted by the wind on the snowcover surface causes the movemen of loose snow.

Threshold Wind Speed

Definition

Minimum wind speed at which snow is moved or redistributed by wind.
Complicated funtion of the physical conditions of the snow surface.

Empirical observations

1. Threshold wind speed increases with increasing temperature and humidity.
2. If the original deposition occurs with wind, the particles will be broken into small pieces and they will pack to a higher density to subsequently increase threshold wind speed.
3. Threshold wind speeds increase with time since snow deposition, as a result of sintering or bonding processes among the deposited snow grains.
   • The rate of increase in threshold wind speed decreases with time.
   • The increase in threshold wind speed is slower at colder temperatures.
4. Threshold wind speeds are much lower when there is a new source of snow particles, such as
   • new snowfall;
   • snow on trees; and
   • low snow strengthh layeer.

Modes of Snow Transport

Biogeochemistry

We have implemented a long-term snow fence experiment at the Niwot Ridge Long-Term Ecological Research (NWT) site in the Colorado Front Range of the Rocky Mountains in the USA to assess the effects of climate change on alpine ecology and biogeochemical cycles. The response of nitrogen (N) and carbon (C) dynamics in high-elevation mountains to changes in climate was investigated by manipulating the length and duration of snow cover with the 2.6x60\|m snowfence, providing a proxy for climate change. Results from the first year of operation in 1994 showed that the period of continuous snow cover was increased by 115 days relative to the control site and 90 days longer than at the snow fence site the year before construction (1993) [Brooks et al., 1995a]. We have implemented a long-term snow fence experiment at the Niwot Ridge Long-Term Ecological Research (NWT) site in the Colorado Front Range of the Rocky Mountains in the USA to assess the effects of climate change on alpine ecology and biogeochemical cycles. The response of nitrogen (N) and carbon (C) dynamics in high-elevation mountains to changes in climate was investigated by manipulating the length and duration of snow cover with the 2.6x60\|m snowfence, providing a proxy for climate change. Results from the first year of operation in 1994 showed that the period of continuous snow cover was increased by 115 days relative to the control site and 90 days longer than at the snow fence site the year before construction (1993) [Brooks et al., 1995a].

The deeper and earlier snowpack behind the fence insulated soils from winter air temperatures, resulting in a 9deC increase in annual minimum temperature at the soil surface and a 12deC increase at a depth of 15 cm. Warmer soils allowed microbial activity, measured as @co2@ flux, to continue through much of the winter. Carbon dioxide production under the deeper, earlier snowpack after construction of the snowfence was 55% greater than production before construction of the fence. The loss of @co2@ from snow-covered soils increased from about 20% of above-ground primary production before fence construction to 31% after fence construction, with shallow snow sites losing as little as 2% of above-ground primary production as @co2@. Surface litter decomposition studies were conducted to test if increased snow depth and duration behind the snowfence increased the rate of decomposition and N mineralization relative to controls. Initial results show that there was a significant increase in the litter mass loss under deep and early snow, with no significant change under medium and little snow conditions.

Snowpack duration and depth also appear to control soil N dynamics [Brooks et al., 1995b]. Under deeper, earlier accumulating snowpacks, N mineralization was generally higher (1712-1960\|@mgNm2@) and with smaller spatial variation (CV 4-26%) compared to shallow and later accumulating snowpacks (511-1440\|@mgNm2@, CV 42-83%). In contrast, the lowest nitrification rates were found under deep/early snowpacks (8-18% of mineralized N) compared to larger rates under shallow/late snowpacks (16-58% of mineralized N). These results are consistent with faster rates of decomposition under deeper/earlier snow producing more mobile N. Nitrogen and C dynamics in high-elevation mountain soils are very dynamic and appear to be sensitive to small changes in climate.

Microbial activity and other biological processes under snow may be as important to N and C cycling in high-elevation ecosystems as that occurring during the growing season [Brooks et al., 1996]. Small changes in climate that affect the timing, duration, and depth of snow cover may cause large changes in the N and C dynamics of alpine ecosystems. These biogeochemical changes in soil may in turn cause changes in plant community structure and composition. Further, N export in surface waters of high-elevation catchments is generally considered to be from the storage and release of @no3@ in seasonal snow [eg Williams and Melack, 1991]. These results suggest that microbial activity and N cycling under snow may control @no3@ export in these surface waters, and that small changes in climate may produce large changes in the chemical content of surface waters.

Readings for those interested (NOT REQUIRED!)

Williams, M. W., P. D. Brooks, T. Seastedt, and S. Schmidt, Nitrogen and carbon soil dynamics in response to climate change in a high-elevation ecosystem in the Rocky Mountains, Arctic and Alpine Research, in review.
Suggested Reading:

Brooks, P. D., M. W. Williams, and S. K. Schmidt, Snowpack controls on soil nitrogen dynamics in the Colorado alpine, Biogeochemistry of Seasonally Snow Covered Basins, edited by K.A. Tonnessen, M. W. Williams, and M. Tranter, IAHS-AIHS Publication 228, Wallingford, UK, pp 283-292, 1995.

Williams, M. W., P. D. Brooks, A. Mosier, and K. A. Tonnessen, Mineral nitrogen transformations in and under seasonal snow in a high-elevation catchment in the Rocky Mountains, USA, Water Resources Research, V 32, N 10, pp 3175-3185, 1996.

Brooks, P. D., S. K. Schmidt, and M. W. Williams, Winter production of CO2 and N2O from alpine tundra; environmental controls and relationship to inter-system C and N fluxes, Oecologia, in press.

Brooks. P. D., M. W. Williams, D. Walker, and S. K. Schmidt, The Niwot Ridge snowfence experiment: biogeochemical responses to changes in the seasonal snowpack, Biogeochemistry of Seasonally Snow Covered Basins, ed by K. A. Tonnessen, M. W. Williams, and M. Tranter, IAHS-AIHS Publication 228, International Association of Hydrological Sciences, Wallingford, UK, pp 293-302, 1995.

Williams, M. W., P. D. Brooks, A. Mosier, and K. A. Tonnessen, Mineral nitrogen transformations in and under seasonal snow in a high-elevation catchment in the Rocky Mountains, USA, Water Resources Research, V 32, N 10, pp 3175-3185, 1996.

Brooks, P. D., M. W. Williams, and S. K. Schmidt, Microbial activity under alpine snowpacks, Niwot Ridge, Colorado, Biogeochemistry, V 32, p 93-113, 1996.

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