Our field site is in the Eldorado Valley basin just outside Las Vegas in the southern tip of Nevada:
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It is formed from ridges of metavolcanic rock surrounding a sandy gentle-floored alluvial plain, with a central flat dry lake bed of crusted silt. Boulder City (of Boulder/Hoover Dam fame) sits on the northeastern shoulder of the basin. |
In Eldorado valley is a dry lake bed (a playa), which is ideal for dust devil chasing because of the flat nature of the terrain. This allows for high speed driving, to get the instrumentation into the path of the vortices. |
Many diverse activities take place out on the playa, although the surrounding alluvial plain is largely untouched. High power model rocketry contests gather once or twice a year, model remote controlled and ultralight pilots enjoy the still morning air, local parachute clubs operate over the valley (although one jumper tragically landed into a dust devil), and a variety of vehicles can be seen driving the lakebed surface. We go there to chase whirlwinds...
Steve Metzger began his Ph.D. fieldwork with the Desert Research Institute (DRI) by investigating the desert surfaces that either produced dust devils or did not. Undergraduate field assistant Mark McDaniel (now a Doctoral candidate at DRI) helped to systematically log hundreds of dust devils (location, time of day, duration, and dimensions) and assisted with an array of experiments performed on 25 sites across the valley. The study indicated that loose sand and dust are mutually essential for dust devil formation, and although common, if either are missing any vortices will fail to form dust columns. Roughness elements such as high-density rock fields near the range fronts or dried grass stubble, however, can prevent wind shear from directly scouring and eroding loose soil. Interestingly, the creosote bush and sage brush acted to shelter the formation of hot air along the ground from ambient wind thereby becoming profuse dust devil "breeding grounds" (as in the boulder fields on Sabancaya volcano, Peru - Metzger, 2000).
The goal of the 1996 season was to directly sample dust devil wind fields and, for the first time, measure their sediment loads. Field assistant Fred Milwer, a Las Vegas schoolteacher and retired USAF Meteorologist, continued the dust devil logging program while LANCE (the Laterally Arrayed Net Capacity Experiment) conducted the active sampling, often at high speed. The LANCE mobile instrument rig chased and tagged 20 dust devils, recording maximum rotation speeds greater than 25 m/s (55 mph) and core pressures 1.5 % below ambient. On average, the Eldorado Valley dust devils are lofting over 9 metric tonnes of soil each summer day.
The principal focus of the 1998 season was to utilise the improved DRI wind tunnel on "type locality" surfaces. The wind tunnel had been upgraded to monitor the response of a surface to erosion over time, count the sand saltation activity, and capture samples of the material swept from the surface. In addition, refinements enhanced the tunnel's precision, calibration, and ability to operate in a wide range of ambient wind conditions.
The 2000 season was largely concerned with testing a variety of innovative instruments that could detect dust devils remotely or image their airborne sediment. Participants included Jeff Houser & Bill Farrell (NASA Goddard), Steve Fuerstenau & Harold Kirkham (JPL), Steve Metzger, and Dennis (Discovery channel cameraman for Planet Storm). We detected vortex UHF and VHF electric fields, and recorded maximum PM10 dust loading (0.5 to 10 micron particle size) in excess of 150 mg/m^3 (3000x ambient dust levels). In the left image, the small red circle is surrounding a pick-up truck about to enter the dust column.
COTBOTT ("Chuck Off The Back Of The Truck....")
In 2002, COTBOTT was deployed - it was a mobile platform carried on the back of a pickup, that was driven in front of the dust devil path, and then deployed. This array was the first integrated attempt to profile vortex behaviour from the ground up (as opposed to just measuring at a single height)
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COTBOTT in the desert: |
COTBOTT has the following sensors: |
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In June 2002 in Eldorado Valley over 1 week, COTBOTT encountered over 40 dust devils: |
Below is a result from COTBOTT, from 26th June 2002 at 13:26. The numbers in the legends show the height of each sensor above the ground. Particle diameters are measured in metres, wind speed in metres per second, temperature in Centrigrade, pressure in hepta-pascals, and electric field in volts per metre. (Click on the image for a high res version.) |
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To give an idea of what its like to be in a dust devil, heres the team bravely encountering a very dusty big dust devil: |
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DASHER ("Dynamic Atmosphere-Surface Hardpan Environment Rig")
In 2003, DASHER was tested in action - it was an improved variant of COTBOTT, and was permanently fixed to the front of the pickup. this allowed repeated encounters with the same dust devil, to track its evolution. During August 2003 over 1 week, DASHER encountered 3 dust devils, mainly due to the poor weather conditions that meant that not many dust devils were forming. However, the results so far are look very interesting, due to the large number of instruments that recorded each dust devil simultaneously.
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DASHER deployed: |
DASHER sensors: |
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With DASHER, we concentrated on lower 60cm or so, and hence mounted more temperature sensors lower down to get a more detailed profile near to surface, whilst also using the full height of the mast to get a better profile in wind speed. |
A sample DASHER result, showing some of the data recorded during an encounter on the 18th July 03, at 13:19.
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