P1.5
The formation of vertical vortices in the convective boundary layer
Katharine M. Kanak, School of Meteorology, University of Oklahoma, Norman, OK OK; and D. K. Lilly and J. T. Snow
An ongoing study of the dynamical formation mechanisms of atmospheric boundary layer vortices is being conducted. These vortices are typically manifest in the atmosphere as dust devils, although there is evidence that they may exist with some frequency in the absence of visible flow tracers. For example, MacPherson and Betts (1997) point out instrument observations of invisible boundary layer vertical vortices over the boreal forest. Most observational investigators have reported that dust devils form in environments characterized by low wind speeds although there are exceptions. The most intriguing unknown is the source of vorticity for the formation of such vortices in convection in the absence of mean winds. Vertical vortex formation in convection without mean winds has not often been documented in laboratory or numerical simulations. We have performed a Large Eddy Simulation (LES) of the convective boundary layer, 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. We suggest 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 centers with local updraft maxima. Results presented at the conference may also include new data from a vortex detection algorithm (as in McWilliams et al. 1999) that will help to quantify the vertical vortex events. Further results from LES's that explore the effects of ambient winds and wind shears of the formation of these vortices may also be presented.
Poster Session 1, Cloudy and Convective BLs
Monday, 15 July 2002, 2:00 PM-2:00 PM
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