The Dynamics and Energetics of Small-Scale Atmospheric Vortices

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9727150 Lilly The boundary layer vortex, which includes the relatively well-known dust devil, generally takes the form of a benign columnar atmospheric vortex. It is less well understood than its more dramatic cousin, the tornado, but recent evidence suggests that it may be more common than originally thought and may be important as a controlling factor of boundary layer structure and interactions. An investigation is proposed of the dynamics of the initiation and maintenance of boundary layer vortices, in particular, the dust devil vortex. This research will be primarily a theoretical and numerical simulation study, but may include limited acquisition and analysis of observational data, especially from mobile Doppler radar. As with their larger scale relatives, the principal scientific question regarding dust devils is their source of rotation. A single answer probably doesn't exist. An ambient source of vertical vorticity may be found, in some cases, as an environmental feature, or it may be generated locally. In the absence of horizontal shear, the vertical vorticity in a dust devil might form from tilting of horizontal vorticity, either that of the mean shear in the surface layer or again by local generation. In absence of other environmental sources, pre-existing convective cells and their mutual interactions have been evidenced as sources of both vertical and horizontal vorticity. The study will consider all these possibilities, but the primary focus is on the last, due partly to its mostly unexplained appearance in several laboratory experiments and field projects. By inhibiting dissipation and diffusion, highly helical dust devils may transport near-surface air higher into the atmosphere than can non-rotating thermals. Thus they may be important or even dominant in establishing the height of the mixed layer, controlling or modifying heat, momentum and particulate transports. These energetic and transport properties of rotating ther mals will also be considered in this study. The objectives of the proposed research are summarized as the following: 1. To seek and compare mechanisms by which convective thermals acquire rotation and become quasi-vertical columnar vortices in either the presence or absence of horizontal or vertical wind shear; 2. To examine the energetics associated with the maintenance of such vortices; 3. To evaluate and compare the vertical transports of heat, momentum and particulates by rotating and non-rotating boundary layer thermals From this study, some necessary conditions for the formation and maintenance of boundary layer vortices will be determined. Results may shed some light on the formation mechanisms of tornadoes. Knowledge gained of the role that boundary layer vortices play in transports of heat, momentum, chemicals and particulates may have impact on /pollution studies, hazardous waste management, and the prediction of initiation of deep convection. ***