The Formation of Vertical Vortices in the Atmospheric Convective Boundary Layer

边界层涡旋，包括相对知名的尘暴，通常采取良性柱状大气涡旋的形式。人们对它的了解不如它的近亲龙卷风，但最近的证据表明，它可能比最初想象的更普遍，并且可能是边界层结构和相互作用的重要控制因素。本文对边界层涡旋，特别是沙尘暴涡旋的形成和维持进行了动力学研究。研究目标包括阐明垂直涡旋存在的必要气象条件及其形成的动力机制。气流形式可分为三类：a)平均水平风切变存在时的对流；B)平均垂直风切变对流；c)没有环境风或风切变时的对流。a)类的垂直涡旋形成很简单，本研究不考虑。对b)和c)的方法方法将主要通过使用大涡模拟数值模型（LES）和一些补充的理论工作。先前的研究结果表明，对于c)类，垂直涡很容易在单体对流模式的顶点形成。为了探索对流边界层垂直涡形成条件的参数空间，目前的研究将扩展到b)类。先前的研究人员认为，强平均风可以抑制沙尘暴的形成，这一论点将在当前的研究中进行调查。然而，主要的重点仍然是c)类，这是最具动态吸引力的。在这种情况下，垂直涡度中心可能是由对流环流本身产生的，随后向垂直方向倾斜，最初必须存在于地表以上。因此，垂直涡度极值是如何与上升气流共存的，以及涡旋是如何与地面相连的，这些问题将被研究。将进行更高分辨率的数值模拟和分析，以揭示可能的动力机制。这些结果可能适用于其他小规模的涡旋，包括蒸汽旋风、火旋风和可能的龙卷风。该项目将促进对垂直涡旋与大尺度对流环流之间相互作用和关系的理解。反过来，行星边界层的参数化和这些涡旋流的影响可以在大涡模拟模式中得到改善。关于这种边界层涡旋垂直输送的研究结果可能会对污染研究、危险废物管理以及可能的深层对流起始问题产生影响。

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.