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，超级单体后侧翼下降气流（RFD）可能在龙卷风形成过程中起着关键作用。对验证龙卷风旋转起源实验（VORTEX）中观测到的Dimmitt龙卷风进行了多传感器分析，揭示了涡度模式和演变过程与先前研究人员首先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单体案例研究非常相似。特别是，首席研究人员先前的研究支持了这样一个概念模型，即当空气从后向前穿过RFD时，会产生涡度。由于降水负荷和/或水成物的融化/蒸发，这种气压临床产生的准水平涡度在RFD附近的浮力最小值周围形成环状。当环向前穿过RFD时，它们的前缘在RFD前方的垂直速度梯度中向上倾斜，形成拱形结构。它们随后在低层上升气流中被强烈拉伸，导致反向旋转涡旋对的增强（在大多数历史案例研究中显示，但没有详细说明）。在迪米特风暴中，反向旋转涡旋对（由涡旋线的拱形连接）中的气旋性成员因流向入流涡度的强烈低空倾斜而增强。该成员随后通过低空上升气流的伸展而增强为龙卷风。在这项研究中，首席研究员试图推进对超级单体RFD过程的理解。他们将集中讨论以下主题：近地反旋转涡旋对的发展如何受下气流强度、浮力和周围切变剖面的支配？假设某些剪切剖面导致一个或两个旋涡的增强，和/或部分或完全消除一个或两个旋涡。什么因素可以发展中性或正浮力（接近地面）RFDs？假设高空潜在冷空气流入后翼的速率以及水成物的分布和类型起主导作用。什么因素决定了临床压产生的RFD涡度被集中成龙卷风涡的可能性？最近，有证据表明，美国国家气象局在龙卷风预警技能上的显著提升，可以归功于来自旋涡实验的新科学知识。这些首席研究员之前的研究成果已被引用到实时预测产品中。这项研究可能产生的社会影响是，龙卷风预报和警报的持续可衡量的改进，导致财产、健康和生命损失的持续减少。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.