Measurements of a Dusty Density Current in the Western Sonoran Desert

Abstract

Here we present observations of a dust storm that occurred on 22 February 2020 in the northwestern Sonoran Desert. In‐situ and remotely sensed measurements and output from numerical simulations suggest that evaporative cooling from cold frontal orographic precipitation spilling over an upwind mountain range generated a density current, with dust uplift occurring as the density current traveled over the emissive desert surface. Because the density current was laden with dust, time series of vertical profiles of aerosol backscatter and extinction from a ceilometer located 25 km downwind of the initial dust emission event show a well‐developed density current structure, including an overturning frontal head with a vertical extent of 1.2 km. Ceilometer measurements and soundings suggest a density current body depth of 400–500 m exhibiting a two‐layer structure that consisted of a positively sheared and dusty lower‐level, and a negatively sheared and pristine upper level. Kelvin‐Helmholtz instability at the top of the density current cold pool generated quasi‐regular oscillations in the height of the dust and pristine‐sky interfacial layer. Ridges and troughs in the height of this interfacial layer were coupled to maxima and minima in surface wind speed and near surface dust concentrations, respectively, with peak dust concentrations located directly under the interfacial layer ridges. These results corroborate several findings from model studies of dust emission and transport by density currents, and suggest that the internal circulation of a density current modifies the timing of dust emission and the patterns of dust concentration within the current body.

Characteristics of Dust Storms Generated by Trapped Waves in the Lee of Mountains

“…Temperatures and vector winds at 850 hPa imply low-level northwesterly cold air advection behind the fronts and westerly flow directed at the coastline and over the Salton Sea. The synoptic situations for these cases are similar to that described for dust outbreaks occurring on February 22, 2020 (Evan et al 2022c) and March 14, 2018 (Evan 2019).…”

“…8d). These features of the March 15 dust outbreak are similar to those for a dust outbreak that occurred on February 22, 2021, which was generated by spillover precipitation and evaporative cooling over the desert to the west of the research site (Evan et al 2022c). Here we noted no spillover precipitation for the March 15 case and thus any density-current like features are due to cold post-frontal downslope flow (Karyampudi et al 1995;Koch et al 1991).…”

“…Inversions upwind and near the heights of the ridge of the Peninsular Mountains were noted for the March 15 2021 and February 15 2022 cases (Fig. 10), as well as in the cases examined in Evan et al (2022c) and Evan (2019). As such, trapped waves are likely a common feature of strong cross barrier flow and dust outbreaks in the region.…”

“…The Vaisala processing software for the CL51 measurements, BLView, produces retrievals of vertical profiles of extinction and optical depth ⌧ from the backscatter profiles for the clear-sky atmosphere below 5 km. Although details regarding the retrieval process used in BLView are not publicly available, we are able to approximately reproduce the extinction profile retrievals using the methods described in (Fernald 1984), as discussed in Evan et al (2022c). We calibrate the 910 nm aerosol optical depth (AOD) retrieved from the CL51, which is obtained by integrating the retrieved extinction profiles in the vertical dimension to an equivalent 500 nm value by comparing values of 500 nm AOD from AERONET to the 910 nm AOD retrieved from the CL51, following the methods in Evan et al (2022a).…”

“…Previous studies have identified several processes associated with orographically-forced flow that result in high winds and dust lofting, including gap flow (Evan et al 2016;Jiang et al 2009;Todd et al 2008), downslope winds due to orographic precipitaction and latent cooling of air (Knippertz et al 2007;Evan et al 2022c), generic Foehn events (Gläser et al 2012;Evan 2019), and lee-side rotor circulations (GrubiöiÊ and Billings 2007;Pokharel et al 2017). Here we focus on the role of trapped lee-waves in generating dust outbreaks.…”