Measurements of a Dusty Density Current in the Western Sonoran Desert

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.