Large-eddy simulation of dust devils in the atmospheric boundary layer

This study systematically analyzes the thermodynamical structure and formation mechanism (especially the source of vorticity) of dust devils in the atmospheric boundary layer considering an environment with and without horizontal mean background winds. For this purpose a high-resolution large-eddy simulation of a complete convective boundary layer is performed for the first time. With a model domain of 4 km x 4 km x 1.7 km and a grid spacing of 2 m (respectively 2049 x 2049 x 448 grid points) this simulation is finally able to resolve all relevant large-scale turbulent structures of the atmospheric boundary layer and the small-scale dust devils themselves simultaneously. The results indicate that dust devils develop at the edges of the large near-surface convective cells where strong horizontal convergence and local maxima of vertical velocity occur. These positions provide important information regarding the formation mechanism. In order to discover the source of the vorticity necessary for the dust devils to form, the three-dimensional vorticity equation was additionally calculated. With the help of an automatic identification of the dust devil centers it was possible to reconstruct the path of the dust devils. This tracking mechanism allowed us to calculate the averaged flow patterns of dust devils for the first time. The results show that although the simulated dust devils are less strong than the observed, they have similar characteristics. In addition, a three-dimensional visualization will be presented to improve the understanding of the dynamical and thermodynamical structure of dust devils. We also performed simulations with different horizontal background winds of 2.5 m/s and 5.0 m/s which clearly show that small background winds increase the strength of dust devils.