The Development of Simulated Dust-Devil-Like Vortices

Abstract

Abstract In this study, the cause of rotation in simulated dust-devil-like vortices is investigated. The analysis uses a numerical simulation of an initially resting, dry, atmosphere, in which uniform surface heating leads to the development of a growing convective boundary layer (CBL). As soon as convective mixing sets in, regions of weak vertical vorticity develop at the lowest model level. Using forward trajectories, this vorticity is shown to originate from horizontal baroclinic production and simultaneous reorientation into the vertical within the descending branches of the convective cells. The requirement for vertical vorticity production in the downdraft cells is shown to be a nonaxisymmetric horizontal footprint of the downdraft regions. The resulting vertical vorticity is not initially associated with rotation. However, as the CBL matures, like-signed vortex patches merge, the vertical vorticity magnitude increases due to stretching, and deformation in the vortex patch decreases, leading to the development of vortices. The ultimate origin of the vortices is thus initially horizontal vorticity that has been produced baroclinically and that has subsequently been reoriented into the vertical in sinking air. Significance Statement Dust devils are concentrated vortices consisting of rapidly rising buoyant air, which may pose a risk to small aircraft and light structures on the ground. Although these vortices are a common occurrence in convective boundary layers, the origin of the vorticity within these vortices has not yet been fully established. The present study uses a numerical simulation of an evolving convective boundary layer and analyzes air parcel trajectories to identify the origin of vertical vorticity at the surface during dust-devil formation. The work contributes an answer to the long-standing question of what causes dust devils to spin.