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)可能在龙卷风发生过程中起着关键作用。对龙卷风旋转起源验证实验(涡旋)中观测到的Dimmitt龙卷风的多传感器分析揭示了涡度模式和演变，这与以前的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，首席研究人员先前的研究支持了涡度的概念模型，即当空气从后到前通过RFD时，涡度是斜压产生的。这种斜压产生的准水平涡度在RFD存在的浮力最小值附近环状发展，这是由于降水加载和/或水流星的熔化/蒸发造成的。当环向前穿过RFD时，它们的前侧在RFD前面的垂直速度梯度中向上倾斜，使它们形成拱形结构。随后，它们在低层上升气流中被强烈拉伸，导致反向旋转涡旋对的加剧(在大多数历史案例研究中显示了，但没有详细说明)。在Dimmitt风暴中，逆旋涡对的气旋性成员(由涡线拱形连接)被流向流入涡度的强烈低空倾斜所增强。这名成员随后在低层上升气流中伸展，增强为龙卷风。在这项研究中，首席调查人员试图促进对超级细胞RFD过程的理解。他们将集中讨论以下主题：下沉气流强度、浮力和环境切变廓线如何支配近地面反向旋转涡旋对的发展？假设某些剪切廓线导致一个或两个涡旋的增强，和/或一个或两个涡旋的部分或全部取消。是什么因素允许中性或正浮力(地面附近)RFDS的发展？假设可能的冷空气流入后翼的速率以及水流星的分布和类型起主导作用。是什么因素控制了斜压产生的RFD涡度集中成龙卷风涡旋的可能性？最近，已经证明美国国家气象局在龙卷风预警技能上的可测量的跃升可以归功于来自涡旋实验的新的科学知识。这些主要调查人员之前的研究成果已在实时预报产品中被引用。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进，导致财产、健康和生命损失的持续减少。此外，首席调查员将继续强调直接接触公众(网站、公开讲座、电视纪录片、电子邮件查询)以及预报人员。

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.