The Formation of Vertical Vortices in the Atmospheric Convective Boundary Layer

University Of Oklahoma Norman Campus, Norman OK

Investigators

Abstract

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. The research objectives include the elucidation of the necessary meteorological conditions required for the existence of vertical vortices and the dynamical mechanisms responsible for their formation. Flow regimes are classified into three categories: a) convection in the presence of mean horizontal wind shear; b) convection in mean vertical wind shear; and c) convection in the absence of ambient wind or wind shears. Vertical vortex formation for category a) is straightforward and will not be considered under this research. The methodological approach to regimes b) and c) will be primarily through the use of a Large Eddy Simulation numerical model (LES) with some complementary theoretical work. Prior research results show that for category c), vertical vortices form readily at the vertices of cellular convective patterns. The current research will expand to also consider category b) in order to explore the parameter space of conditions that permit vertical vortex formation in the convective boundary layer. Prior investigators have suggested that strong mean winds can be inhibiting to the formation of dust devils and this contention will be investigated in the current research. The primary emphasis, however, will remain on category c), which is the most dynamically intriguing. In this case vertical vorticity centers that may be generated by convective circulations themselves and, subsequently tilted into the vertical, must exist initially above the surface. Therefore, the questions of how vertical vorticity extrema become co-located with updrafts and how a vortex becomes connected with the ground surface will be examined. Higher resolution numerical simulations and analyses will be performed to uncover the possible dynamical mechanisms. The results may be applicable to other small-scale vortices, including steam devils, fire-whirls and possibly tornadoes. This project may promote the understanding of the interactions and/or relationship between a vertical vortex and larger scale convective circulations. In turn the parameterization of the planetary boundary layer and the effects of these vortical flows may be improved in large-eddy simulation models. The results regarding vertical transports by such boundary layer vortices may have impact on pollution studies, hazardous waste management and possibly the deep convective initiation problem.