Concentrating Vorticity Near the Ground: An Investigation of the Interaction of Precipitation Processes and Flow Dynamics in Supercells and Other Severe Thunderstorms

在1994年和1995年的春天，由美国国家科学基金会和国家海洋和大气管理局联合资助的科学家们执行了一个名为“龙卷风旋转起源验证实验”的实地项目。涡旋的主要目的是评估有关低层中气旋增强和龙卷风形成、维持和消亡的几种假设。收集的观测资料包括地基（移动和固定）和机载多普勒雷达、移动地面和气球气象仪器。根据该合同，主要研究人员将继续他们正在进行的研究，旨在提高龙卷风生命周期的知识水平。这一目标将通过仔细检查数据丰富的迪米特、弗里奥纳和惠勒（都在德克萨斯州）龙卷风风暴的案例研究，以及来自旋涡的其他风暴（有些是龙卷风，有些不是）和随后的实验来实现。由于先前的研究结果，很明显，未来的调查必须扩大到涵盖各种密切相关的过程，以了解龙卷风的形成和龙卷风的寿命。这些因素包括：-在超级单体上升气流后方的后侧翼下降气流（RFD）中形成/下降降水的数量和类型的因素；-这种降水及其类型与RFDs的形成有何关系；- RFD如何产生涡度和/或正压重新分配涡度：- RFD的流出如何被限制在上升气流下方的区域；-旋涡如何导致龙卷风的形成，以及旋涡的角动量是由旋涡维持，还是由最初的分布“向下旋转”；- RFD能否通过主要的正压过程直接形成龙卷风气旋；-龙卷风气旋是否由高旋度的流入空气向上倾斜形成；-龙卷风的形成是只需要上述一种过程，还是需要上述两种过程；- RFDs的深度、大小和形状是否与龙卷风形成有关。这些问题中的大多数可以，或者已经，以可检验的、可反驳的假设的形式提出。主要研究人员将着重于RFD涡度过程的理论和模型的发展，以及降水对中气旋尺度涡旋角动量重分布的影响。首席研究人员还将调查水成物和水相变化在迫使运动学结构中的作用。这项研究的成功完成可以减少龙卷风警报的误报率，同时保持足够的交货时间。

During the Springs of 1994 and '95, a field program entitled the Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX) was executed by a combination of NSF and National Oceanic and Atmospheric Administrations sponsored scientists. The primary objective of VORTEX was to evaluate several hypotheses pertaining to low-level mesocyclone intensification, and tornado formation, maintenance, and demise. The observations collected included ground-based (mobile and stationary) and airborne Doppler radars, mobile surface and balloon borne meteorological instruments. Under this award, the Principal Investigators will continue their ongoing research intended to advance state of knowledge of the life cycles of tornadoes. This goal will be accomplished through careful examination of data-rich case studies of the Dimmitt, Friona, and Wheeler (all in Texas) tornadic storms as well as other storms (some tornadic, some not) from the VORTEX and subsequent experiments. As a result of prior research, it has become apparent that future investigations must be expanded to cover a variety of closely related processes to understand tornadogenesis and tornado longevity. Among these are:-factors that govern the amount and type of precipitation that forms/descends in rear flank downdrafts (RFD) at the rear of supercell updrafts;-how this precipitation and its type is related to formation of RFDs;-how RFD generate vorticity and/or barotropically redistribute it:-how outflow from the RFD can be confined to the region below the updraft;-how RFDs lead to the formation of tornado cyclones, and whether the angular momentum of the tornado cyclone is maintained by the RFD or merely "spins down" from some initial distribution;-whether the RFD can lead directly to a tornado cyclone through largely barotropic processes;-whether the tornado cyclone forms from upward tilting of helicity-rich inflow air;-whether only one or both of the above processes is required for tornadogenesis;-whether the depth, size and shape of RFDs are relevant to tornadogenesis.Most of these questions can be, or have been, cast in the form of testable, refutable hypotheses. The Principal Investigators will emphasize the development of theories and models for RFD vorticity processes and the effect of precipitation in redistributing angular momentum in a mesocyclone-scale vortex. The Principal Investigators also will investigate the role of hydrometeors and water phase changes in forcing the kinematic structures.Successful completion of this research could lead to a reduction in tornado warning false alarm rate while maintaining adequate lead times.