The formation of vertical vortices in the convective boundary layer

A study of the dynamical formation mechanisms of atmospheric boundary-layer vortices is being conducted. These vortices are typically manifest as dust devils although there is evidence that they exist with some frequency in the absence of visible flow tracers. For example, in 1997 MacPherson and Betts pointed out instrument observations of invisible boundary-layer vertical vortices over the boreal forest. Several possible mechanisms for vortex formation under a variety of different dynamical regimes are described. Most observational investigators have reported that dust devils form in environments characterized by low wind speeds. The most intriguing unknown is the source of vorticity for the formation of vortices for convection in the absence (or near absence) of mean winds. Vertical vortex formation in convection without a mean shear has not often been documented in laboratory or numerical simulations. A Large-Eddy Simulation of the convective boundary layer is performed, without mean winds, with the purpose of examining vertical vortex-formation mechanisms. The current work emphasizes the conjectured larger (convective) scale vorticity-generating mechanisms. The results indicate that vertical vortices form at some, but not all, of the vertices of the simulated pattern of open convective cells. The columnar vortices may be essentially vertical or tilt with height, and may or may not extend to the surface. It is suggested that the vertical vorticity initially available to the Vortices comes from the tilting of horizontal vorticity associated with local unidirectional shears due to the convective-cell circulations, with convective-cell asymmetries being important to the process. Subsequent evolution apparently results in the co-location of vertical vorticity centres with local updraught maxima.