Collaborative Research: Concentrating Vorticity Near the Ground: Investigation of Supercell Rear-Flank Precipitation, Vorticity Generation, and Transport Processes

最近的研究表明，超级单体后侧下沉气流（RFD）在龙卷风形成过程中可能发挥关键作用。 对龙卷风旋转起源验证实验（VORTEX）期间观察到的迪米特龙卷风的多传感器分析揭示了涡度模式和演化与先前研究人员首次从理论上描述的过程一致。 此外，观察到的模式与文献中大多数超级细胞案例研究中的模式非常相似。 特别是，主要研究人员之前的研究支持了这样的概念模型：当空气从后到前穿过 RFD 时，斜压产生涡度。 由于降水载荷和/或水凝物的融化/蒸发，这种斜压产生的准水平涡度在 RFD 处存在的浮力最小值周围形成环。 当环向前穿过 RFD 时，它们的前导侧在 RFD 前方的垂直速度梯度中向上倾斜，从而形成拱形结构。 随后，它们在低层上升气流中被强烈拉伸，导致反向旋转涡对的增强（在大多数历史案例研究中已显示，但未详细说明）。 在迪米特风暴中，反向旋转涡旋对（由涡线拱形连接）的气旋成员因流向流入涡度的强烈低层倾斜而增强。 该成员随后通过低层上升气流的拉伸而增强为龙卷风。 在这项研究中，主要研究人员试图加深对超级单元 RFD 过程的理解。 他们将集中讨论以下主题：近地反向旋转涡对的发展如何受到下沉气流强度、浮力和环境剪切剖面的控制？ 据推测，某些剪切剖面会导致一个或两个涡流的增强，和/或一个或两个涡流的部分或完全消除。哪些因素允许形成中性或正浮力（靠近地面）RFD？ 据推测，高空潜在冷空气流入后翼的速率以及水凝物的分布和类型起主导作用。哪些因素控制斜压产生的 RFD 涡旋集中成龙卷涡旋的可能性？ 最近，事实证明，国家气象局龙卷风预警能力的显着提升可归因于 VORTEX 实验的新科学知识。 这些首席研究员之前的研究成果已在实时预测产品中被引用。 这项研究可能产生的社会影响是龙卷风预报和预警的持续可衡量的改善，从而持续减少财产、健康和生命的损失。 此外，首席研究员将继续强调与公众（网站、公开讲座、电视纪录片、电子邮件查询）以及预报员的直接联系。

Recent research has indicated the possibility of a critical role of the supercell rear flank downdraft (RFD) in the tornadogenesis process. A multi-sensor analysis of the Dimmitt tornadic storm observed during the Verification of Origins of Rotation in Tornadoes Experiment (VORTEX) has revealed vorticity patterns and evolution consistent with processes first described theoretically by prior researchers. Further, the observed patterns were very similar to those in most of the supercell case studies in the literature. In particular, the prior research of the Principal Investigators supported the conceptual model that vorticity is generated baroclinically as air passes through the RFD from rear to front. This baroclinically generated quasi-horizontal vorticity develops in rings about the buoyancy minimum present at the RFD owing to precipitation loading and/or melting/evaporation of hydrometeors. As the rings pass forward through the RFD, their leading sides are tilted upward in the gradient of vertical velocity present just ahead of the RFD, giving them an arch-like configuration. They are subsequently strongly stretched in the low-level updraft, leading to an intensification of a counter-rotating vortex pair (shown, but not elaborated upon, in most historical case studies). In the Dimmitt storm, the cyclonic member of the counter-rotating vortex pair (connected by arches of vortex lines) was augmented by intense low-level tilting of streamwise inflow vorticity. This member subsequently intensified into a tornado through stretching in the low-level updraft.In this research, the Principal Investigators seek to advance the understanding of supercell RFD processes. They will concentrate on the following topics: How is the development of the near-ground, counter-rotating vortex pair governed by downdraft strength, buoyancy, and the ambient shear profile? It is hypothesized that certain shear profiles lead to augmentation of one or both vortices, and/or partial or complete cancellation of one or both.What factors allow for the development of neutral or positively-buoyant (near the ground) RFDs? It is hypothesized that the rate of inflow of potentially cold air aloft into the rear flank, and the distributions and types of hydrometeors, play dominant roles.What factors govern the likelihood of baroclinically generated RFD vorticity being concentrated into a tornadic vortex? Recently, it has been demonstrated that a measurable jump in tornado warning skill by the National Weather Service can be attributed to new scientific knowledge coming from the VORTEX experiment. Prior research findings of these Principal Investigators have been cited in real-time forecast products. The likely societal impact of this research is a continued measurable improvement in tornado forecasts and warnings, leading to continued reductions in losses of property, health, and lives. Further, the Principal Investigators will continue to emphasize direct outreach to the public (web site, public lectures, television documentaries, email queries) as well as to forecasters.