Field measurements of horizontal forward motion velocities of terrestrial dust devils: Towards a proxy for ambient winds on Mars and Earth |
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| Authors: | MR Balme A Pathare SM Metzger MC Towner SR Lewis A Spiga LK Fenton NO Renno HM Elliott FA Saca TI Michaels P Russell J Verdasca |
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| Institution: | 1. Planetary Science Institute, 1700 E Fort Lowell Rd., Suite 106, Tucson, AZ 85719, USA;2. Department of Physical Sciences, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK;3. Applied Geology Department, Curtin University of Technology, GPO Box U1987, Perth, Western Australia 6845, Australia;4. Laboratoire de Météorologie Dynamique, Université Pierre et Marie Curie, Paris, France;5. Carl Sagan Center at the SETI Institute, 189 Bernardo Ave., Suite 100, Mountain View, CA 94043, USA;6. Department of Atmospheric, Oceanic and Space Sciences, Space Research Building, University of Michigan, 2455 Hayward St., Ann Arbor, MI 48109-2143, USA;7. Southwest Research Institute, 1050 Walnut St., Suite 400, Boulder, CO 80302, USA;8. Center for Earth and Planetary Studies, Smithsonian Institution, PO Box 37012, National Air and Space Museum, MRC 315, Washington, DC 20013-7012, USA;9. Centro de Astrobiología (CSIC/INTA), Instituto Nacional de Técnica Aeroespacial, Ctra de Torrejón a Ajalvir, km 4, 28850 Torrejón de Ardoz, Madrid, Spain |
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| Abstract: | Dust devils – convective vortices made visible by the dust and debris they entrain – are common in arid environments and have been observed on Earth and Mars. Martian dust devils have been identified both in images taken at the surface and in remote sensing observations from orbiting spacecraft. Observations from landing craft and orbiting instruments have allowed the dust devil translational forward motion (ground velocity) to be calculated, but it is unclear how these velocities relate to the local ambient wind conditions, for (i) only model wind speeds are generally available for Mars, and (ii) on Earth only anecdotal evidence exists that compares dust devil ground velocity with ambient wind velocity. If dust devil ground velocity can be reliably correlated to the ambient wind regime, observations of dust devils could provide a proxy for wind speed and direction measurements on Mars. Hence, dust devil ground velocities could be used to probe the circulation of the martian boundary layer and help constrain climate models or assess the safety of future landing sites.We present results from a field study of terrestrial dust devils performed in the southwest USA in which we measured dust devil horizontal velocity as a function of ambient wind velocity. We acquired stereo images of more than a 100 active dust devils and recorded multiple size and position measurements for each dust devil. We used these data to calculate dust devil translational velocity. The dust devils were within a study area bounded by 10 m high meteorology towers such that dust devil speed and direction could be correlated with the local ambient wind speed and direction measurements.Daily (10:00–16:00 local time) and 2-h averaged dust devil ground speeds correlate well with ambient wind speeds averaged over the same period. Unsurprisingly, individual measurements of dust devil ground speed match instantaneous measurements of ambient wind speed more poorly; a 20-min smoothing window applied to the ambient wind speed data improves the correlation. In general, dust devils travel 10–20% faster than ambient wind speed measured at 10 m height, suggesting that their ground speeds are representative of the boundary layer winds a few tens of meters above ground level. Dust devil ground motion direction closely matches the measured ambient wind direction.The link between ambient winds and dust devil ground velocity demonstrated here suggests that a similar one should apply on Mars. Determining the details of the martian relationship between dust devil ground velocity and ambient wind velocity might require new in situ or modelling studies but, if completed successfully, would provide a quantitative means of measuring wind velocities on Mars that would otherwise be impossible to obtain. |
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