Meteorological and geological controls on dust devil activity: Initial results from a field study at Smith Creek Valley, Nevada, USA

We present initial results of an investigation into meteorological and geological controls on the formation of dust devils (i.e., dust-filled vortices formed in the daytime dry convective boundary layer). During a 2-week field campaign in June 2019 at Smith Creek Valley (SCV), Nevada, USA, we conducted automated time-lapse stereo imaging of dust devils (DDs), while monitoring local meteorological conditions with a broad suite of instruments. Counts of imaged dust devils from two near-cloudless days were compared with a standard suite of atmospheric measurements from a weather tower, eddy correlation flux measurements, and ceilometer backscatter returns. DDs forming in moderate winds (5–8.5 m/s) were more likely to be relatively wide and disorganized, with qualitatively low dust opacity, whereas those forming in weaker winds were more likely to be coherent, dusty, well-formed conical or cylindrical structures. The daily maximum DD counts at SCV occurred shortly after their onset in late morning (11:00–12:00 local time), coinciding with a surge in CBL growth that was likely delayed by the thermal properties of the playa. This late morning peak contrasts with previous studies conducted elsewhere that typically observed peak DD counts in the afternoon. As observed in previous field studies, DDs formed in highly convective conditions, when the heat flux ($H$) and friction velocity ($u_{\ast }$) were elevated and the convective ratio $w_{\ast }/u_{\ast }$ exceeded ∼ 4 (i.e., $-h/L$ exceeded ∼ 25). However, values of>4$w_{\ast }/u_{\ast }$ also occurred in mid-morning, prior to DD formation and CBL growth, suggesting that this metric is not the sole condition required for DD generation. Aside from the late morning maxima, DD counts fluctuated considerably throughout the afternoon at timescales of 0.5–2  h–correlating poorly with fluctuations in $H$ and $u_{\ast }$, and not at all with either $w_{\ast }/u_{\ast }$ or the Monin-Obukhov length ($L$). Several factors, such as local variations in surface thermal properties and meteorology, may be responsible for these short-term fluctuations.