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

Abstract

Dust devils, and other columnar vortices, are associated with local surface pressure drops that can be observed in time-series data on both Earth and Mars. High cadence measurements are needed to resolve these small structures, and we report a month-long survey (June/July 2012) on a Nevada desert playa using microbarographs sampled multiple times per second. Candidate dust-devil signatures are classified, with detections being robust at about one per day for pressure drops exceeding 0.3 hPa (roughly a 5:1 signal-to-noise threshold, where the observed noise level corresponds reasonably well with the dynamic pressure associated with the estimate convective velocity scale). The vortex population is evaluated and compared with those observed on Mars : a broken power law or a more convex distribution describes the terrestrial data. A single station observes about three events per week (for normalized pressure drops of 0.06%), about three times fewer than Mars observations for the same normalized drop. We find evidence for clustering of vortex events in a pseudoperiodic manner with a 20-min period, consistent with the size of boundary-layer convection cells.

Field Measurements of Terrestrial and Martian Dust Devils

“…suggest a periodicity of around 45 minutes and Renno et al (2004) a periodicity of about 20 minutes. Lorenz and Lanagan's (2014) pressure-excursion data hint at a similar result.…”

“…Mobile sampling systems appear to give larger values: ∼ 1-10 mbar; fixed monitoring stations give values < 1.5 mbar Sinclair (1973); Metzger (1999); Lorenz and Lanagan (2014) Electric fields Field of ∼ 10-100 kV/m measured; early measurements hampered by field reaching measurement limit of instrument …”

“…However, many of these reports are based on observer surveys which are both qualitative and which are unlikely to have run throughout the day-so there is always a possibility of bias. Recent work by Lorenz and Lanagan (2014) using a continuous month-long survey of pressure excursions to detect dust devils showed that most dust devils occur between 10:00 and 16:00 local time. They do note that about 10 % of the day's dust devil events occur after 16:00 and there is a measurable tail of activity even after 18:00.…”

“…The systems are inexpensive enough (∼ $150) compared with the cost of deployment and retrieval that they can be considered somewhat expendable. These systems have allowed large numbers (hundreds to thousands) of unperturbed vortex encounters to be obtained without chase biases, finally yielding robust statistics on vortex populations from pressure drops (Lorenz and Lanagan 2014; and on dust loading . Furthermore, the small measurement stations can be deployed in spatial arrays to make simultaneous measurements that resolve the two-dimensional horizontal structure of dust devils (Lorenz et al 2015a(Lorenz et al , 2015b(Lorenz et al , 2015c.…”

History and Applications of Dust Devil Studies

“…15 The cumulative frequency that a spot will encounter a vortex of a given intensity, defined by a pressure drop. Dust devil vortex encounter frequencies in summer on Eldorado Playa in Nevada determined by a high-resolution pressure record in Lorenz and Lanagan (2014) In addition to prompt injury, dust devils may adversely affect human health via injection of pathogens, toxins or irritants into the air (see Morman and Plumlee 2014). A particular concern in the USA is 'Valley Fever' wherein airborne fungal spores from the soil may be inhaled and lead to infection by Coccidioidomycosis, usually with minor symptoms but occasionally pneumonia, joint pain and fatige (e.g.…”