Re: Wind and Dust

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Posted by Arkose on January 07, 2003 at 13:53:10:

In Reply to: Wind and Dust posted by Harold Ericsson on January 07, 2003 at 10:55:01:

Monitoring dust storms and mapping landscape vulnerability
to wind erosion using satellite and ground-based digital images

by Pat S. Chavez, Jr., David J. Mackinnon,
Richard L. Reynolds, and Miguel G. Velasco

...Results and conclusions

Satellite and ground-based digital images have been successfully used to detect and monitor several dust storms in the Soda Dry Lake area of the central Mojave Desert. The temporal resolutions of the GOES imaging system and the ground-based digital camera station are ideal for detecting and monitoring dust storms. However, owing to the poor spatial and spectral resolution of the GOES images, only very large dust storms can be detected. The poor temporal resolutions of the higher spatial and/or spectral resolution satellite-imaging systems limits the usefulness of these systems for detecting and monitoring dust storms in the Mojave Desert. Change detection methods used on GOES images covering the Mojave Desert, and other image data when available, should be applicable to other arid regions of the world.

Using information derived from the satellite images, ground-based digital photographs, and wind data, we can say the following about the vulnerability of the Soda Dry Lake area during the relatively dry surface conditions in spring 2002:

An average sustained wind speed of approximately 8 m/s at 5 m above the surface was the threshold speed to initiate minor local dust emission.
An average sustained wind speed of approximately 13 m/s or greater was the threshold speed necessary to generate major regional dust storms, with the valley floor often filled with dust causing whiteout conditions.
When the average sustained wind speeds were above approximately 13 m/s for several hours and then slowly decreased, dust emission in the 8 to 9 m/s range was larger than when wind speed built to this range and then decreased after several hours (i.e., the wind speed did not get much higher than 8 m/s). This is consistent with high winds breaking the surface crust, leading to higher vulnerability at the subsequently lower wind speed. ....


Controls on wind erosion
wind velocity
E = V3rho, where E is erosivity , V is velocity and rho is air density
thus the erosivity of wind is an exponential function of wind velocity, i.e. if the wind velocity doubles, the wind is 8X more erodible or, if it triples, the wind is 27 times more erodible
that is why we observe massive wind erosion (dust) with a significant increase in wind speed

surface cover
an extremely important factor since there is no wind erosion on a vegetated surface
wind velocity decreases exponentially near the ground and is theoretically zero on a natural (i.e. rough) surface; thus erosivity (V3 is dramatically reduced)
on a windy day, put your nose next to the ground and you will discover there is no wind; small birds and insects take advantage of this on windy days
the zone of little or no wind is called the laminar sublayer (or the boundary layer), the rougher the surface (e.g. taller the vegetation) the deeper the layer of laminar air flow (i.e. no turbulence to entrain and suspend sediment)
thus there is no wind in the interior of a closed forest

grain size
threshold erosional velocity is related to the square root of particle size
thus when the threshold velocities for various particle size plot as a straight line when the particle size axis is on a square root scale
the threshold velocities are slightly lower for sand when impact among grains (saltation and creep) is taken into account
the fluid threshold velocities (wind shear) plot as two straight lines that slope down to converge at a minimum threshold velocity for coarse silt and fine sand (i.e. these are the most easily eroded grains)
with smaller particle sizes grains tend to cohere when wet and resist erosion
larger grains resist erosion by virtue of their greater size (mass) ...


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