Tornado-Surface Interaction

在过去二十年里，我们对龙卷风和龙卷风的了解显著增加，这在很大程度上是由于不同尺度上的广泛的多普勒雷达观测、移动的中层网和其他现场观测，以及风暴和龙卷风尺度上的数值模拟。然而，其中一个明显的教训是，物理学在大范围不同尺度上的关键重要性，从完整的风暴尺度到为龙卷风角落流动提供动力的几米深的流入层，使得对龙卷风发生和在给定风暴中的行为的可靠预测仍然是一项艰巨的任务。这使得在大范围收集更完整的龙卷风风暴数据集成为一个明显的优先事项(就像国家科学基金会支持的大型现场观测计划VORTEX-II正在尝试的那样)。然而，这种复杂性也表明，需要在更理想化的研究中隔离和理解问题的不同部分。观测和模拟研究表明，龙卷风在地面和高空附近的行为非常多样和复杂。这在很大程度上是因为龙卷风对进入龙卷风角落和核心气流的近地表流入的特性非常敏感。这项研究将集中在龙卷风与地面的相互作用及其对龙卷风加强和结构的影响。一个主要的新组成部分将是使用“浸没边界”技术，将非平凡的表面几何图形合并到现有的高分辨率大涡模拟(LES)龙卷风模型中。这将使我们能够首次通过模拟研究解决几个重要问题：地形特征(如小山、小脊、小山谷或建筑物)对近地表龙卷风动力学的影响；各种真实龙卷风和碎片场对简单建筑物结构的压力和碎片强迫；处理单个表面粗糙度元素而不是采用简单的表面粗糙度长度近似对龙卷风动力学的潜在重要性；以及龙卷风中孤立的大型物体(如理想化的车辆)的放样。此外，我们将继续对龙卷风和中气旋动力学的不同方面进行几项正在进行的理论和数值研究，包括：远离轴对称的涡旋的行为和分析；不同空间尺度上的涡旋相互作用；龙卷风近地表加强的机制；龙卷风碎片动力学；以及龙卷风破坏路径和表面标记的分析。这项研究的一个长期目标是更好地了解龙卷风的发生和行为，以改进龙卷风预测，增加公共安全。龙卷风和泥石流对建筑物的强迫作用的模拟，以及遇到建筑物对龙卷风行为的影响的模拟，应该会改善对城市环境中潜在龙卷风损害的估计，并帮助工程师设计能够承受可信龙卷风条件的结构。了解地形对近地表龙卷风行为和加强的影响也可能导致在某些环境中减少强烈龙卷风破坏的可能性的策略。为本项目开发的不同几何形状的颗粒湍流大涡模拟的改进，可能会在燃烧、化学处理或污染物扩散等领域得到更广泛的应用。教育的主要组成部分将是对一名博士生的深入培训。此外，鉴于公众对龙卷风的痴迷，该项目还将通过向大众媒体捐款，促进更广泛的公众的科学教育和兴趣。

Our understanding of tornadoes and tornadic storms has increased significantly in the past two decades due in large part to extensive Doppler radar observations on different scales, mobile mesonet and other in situ observations, and numerical simulation on both the storm and tornado scales. One of the apparent lessons, however, has been the critical importance of physics on a large range of different scales, from full storm scale down to the few-meter deep inflow layer feeding the tornado corner flow, leaving the reliable prediction of tornado occurrence and behavior within a given storm as still a daunting task. This has made the gathering of more complete tornadic storm data sets on a large range of scales (as is being attempted in VORTEX-II, a large field observational program supported by National Science Foundation) a clear priority. However, this complexity also suggests a need to isolate and understand different pieces of the problem in more idealized studies.Intellectual Merit. Observations and simulation studies have demonstrated a great variety and complexity of tornado behavior near the surface and aloft. Much of this arises from the sensitivity of tornadoes to the properties of the near-surface inflow that feeds into the tornado corner and core flows. This study will focus on tornado-surface interactions and their effects on tornado intensification and structure. A principal new component will be to employ "immersed boundary" techniques to incorporate non-trivial surface geometry into an existing high-resolution large-eddy simulation (LES) tornado model. This will allow us to address several important issues with simulation studies for the first time: the effects of topographical features (such as small hills, ridges, valleys, or buildings) on near-surface tornado dynamics; the pressure and debris forcing on simple building structures for a variety of realistic tornado wind and debris fields; the potential importance to tornado dynamics of treating individual surface roughness elements rather than employing a simple surface roughness length approximation; and the lofting of isolated large objects (such as idealized vehicles) within tornadoes. In addition we will continue several of our ongoing theoretical and numerical studies of different facets of tornado and mesocyclone dynamics including: behavior and analysis of vortices far from axisymmetry; the interaction of vortices on different spatial scales; mechanisms for near-surface intensification of tornadoes; tornado-debris dynamics; and the analysis of tornado damage tracks and surface markings.Broader Impacts. A long-term goal of the research is better understanding of tornado occurrence and behavior in order to improve tornado prediction and increase public safety. The simulation of the forcing of tornado winds and debris flows on buildings, and of the effects of encountering buildings on tornado behavior should improve estimates of potential tornado damage in urban environments and aid engineer's attempts to design structures to withstand credible tornado conditions. Understanding the effects of topography on near-surface tornado behavior and intensification may also lead to strategies for reducing the likelihood of strong tornado damage in some environments. The improvements in LES of particle laden turbulent flows in different geometries developed for this project may find broader applications in other fields such as combustion, chemical processing or pollutant dispersal. The main educational component will be the in depth training of one PhD student. In addition, given the public fascination with tornadoes, the project will also promote science education and interest among the broader public through contributions to popular media.