Physics
Scientific paper
Dec 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004agufm.p31b0992b&link_type=abstract
American Geophysical Union, Fall Meeting 2004, abstract #P31B-0992
Physics
5415 Erosion And Weathering, 5445 Meteorology (3346), 3307 Boundary Layer Processes, 1625 Geomorphology And Weathering (1824, 1886), 0343 Planetary Atmospheres (5405, 5407, 5409, 5704, 5705, 5707)
Scientific paper
The presence of roughness elements such as isolated boulders or sparse vegetation, common to many arid and semi-arid environments, protect the underlying finer-grained sediments from wind erosion, principally by absorbing momentum from the wind. As a result, wind shear stress is partitioned between the isolated roughness elements and the intervening surface. As part of a larger wind erosion study, a series of wind profile measurement experiments were conducted over a relatively flat un-vegetated, crusted field with an area of approximately 10,000 meters square, located at the Jornada Experimental Range, USDA ARS, Las Cruces, NM. Near the upwind margin of the field a triangular shaped area 2000 meters square (with the triangle base located downwind and perpendicular to the dominant wind direction) was covered with up to 1,9250, commercially available, 5-gallon buckets in regularly spaced staggered arrays. The buckets were used to produce four different roughness densities to investigate changes in wind flow patterns and momentum over the roughness configurations. Wind profiles were measured on two 9 m towers located at the upwind and downwind margins of the triangular arrays along the central axis. Wind speed was measured with 8 anemometers spaced logarithmically from 0.5 to 9 m. Wind direction was measured at the top of each tower with a wind vane. Wind speed and direction data were recorded with a data logger at 1s intervals with an averaging time of 10 minutes. Wind profiles at the leading edge of the arrays approaching at angles less than or equal to 10 degrees from normal to the front of the roughness array are associated with the wind flow over the flat, crusted surfaces and are well described by the Prandtl-von Karman log-linear relationship. For the same free stream wind speeds, the profiles at the downwind edge of the arrays are also characteristically log-linear when a displacement height (d) is included in the Prandtl-von Karman equation. A strong linear relationship (R2= 0.97) was found between the ratio of d to inter-element spacing (l) and the roughness density. As well, for similar free stream wind speeds, shear velocity increased markedly between the upwind and down wind towers, the difference of which was dependent on roughness density. Average momentum losses across the arrays (determined from the momentum deficit law) increased linearly with roughness density, indicating the increasing absorption of momentum by the roughness elements as the number of elements per unit area increased.
Bryant J. M.
Gillies J. A.
Nickling William G.
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