Field measurements of horizontal forward motion velocities of terrestrial dust devils: Towards a proxy for ambient winds on Mars and Earth

Abstract

a b s t r a c tDust 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.

Comment on ''Observing desert dust devils with a pressure logger" by Lorenz (2012) – insights on measured pressure fluctuations from large-eddy simulations

“…Our simple calculation yields an underestimate, but the aforementioned assumptions are quite strong; for instance, background wind would tend to stretch convective cells and modify their width. The same order of magnitude can be found on the Earth, where the convective PBL is less deep than on Mars (about 2-3 km in warm arid regions) and background wind perhaps less casually reaches speeds ≥ 10 m s −1 than on Mars (Balme et al, 2012). Those estimates are in line with the measured period of about 1000 s measured by Lorenz (2012), which makes it plausible that the recorded pressure fluctuations are caused by PBL convective cells.…”

Field Measurements of Terrestrial and Martian Dust Devils

“…As noted by Lorenz (2011), most visual surveys (e.g., Sinclair 1965Sinclair , 1969Snow and McClelland 1990) report that small dust devils are underreported by observers. Possible exceptions to this are the studies of , Pathare et al (2010) and Balme et al (2012), which both used small (1 km by 1 km or smaller) study areas. Several recent studies have focused on power law and other functional forms that best describe the dust devil size-frequency population (e.g., Kurgansky 2006;Lorenz 2009Lorenz , 2011Pathare et al 2010)-a topic which is described in more detail by Lorenz and Jackson (2016) in this issue.…”

A Barometric Survey of Dust-Devil Vortices on a Desert Playa

“…The field site is a desert playa in southern Nevada that has been the subject of previous surveys of dust-devil activity (e.g. Pathare et al, 2010;Balme et al, 2012;Metzger et al, 2012). The playa is adjacent to route 95 in a basin to the south-east of Las Vegas (see Fig.1).…”