Structure and formation of dust devil-like vortices in the atmospheric boundary layer: A high-resolution numerical study

The development of dust devil-like vortices in the atmospheric convective boundary layer (CBL) is studied using large-eddy simulation (LES). Special focus is placed on the analysis of the spatial structure of the vortices, the vorticity-generating mechanisms, and how the vortices depend on the larger-scale coherent near-surface flow pattern of the CBL. Vortex centers are automatically detected during the simulation, and a tracking method is developed, which allows us to determine the temporally averaged structures of selected vortices. Also, various vorticity budget terms are calculated. A reference study with high resolution (2 m) and large model domain (2000 x 2000 x 500 grid points) is carried out to account for the dependency of vortex generation on the larger-scale CBL flow pattern, i.e., the near-surface hexagonal cells. Vortices predominantly appear within the vertices of the cells. Their vorticity is maintained by a combination of divergence and twisting effects. Flow visualizations by tracers show that the vortices have an inverted cone-like shape, similar to observed dust devils. Simulated vortex characteristics like tangential velocity or vorticity are at the lower limit of observed values. Strength and number of vortices heavily depend on the background wind. A small background wind enhances vortices, but for a mean wind speed of 4.4 m s(-1), vortex generation is significantly reduced, mainly because the near-surface flow changes from a cellular to a more band-like pattern. A new mechanism is suggested, which relates the initial vortex generation to the cellular flow pattern.