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年春季，由美国国家科学基金会和国家海洋和大气管理局赞助的科学家联合执行了一项题为验证龙卷风自转起源的实地方案。涡旋的主要目标是评估与低层中气旋加强以及龙卷风的形成、维持和消亡有关的几个假说。收集的观测包括地面(移动和固定)和机载多普勒雷达、移动地面和气球载气象仪器。根据这一奖项，首席调查人员将继续他们正在进行的研究，旨在促进对龙卷风生命周期的了解。这一目标将通过仔细研究数据丰富的案例研究(都在德克萨斯州)的迪米特、弗里奥娜和惠勒以及来自涡旋的其他风暴(一些是龙卷风的，一些不是)以及随后的实验来实现。作为先前研究的结果，很明显，未来的调查必须扩大到涵盖各种密切相关的过程，以了解龙卷风的发生和龙卷风的寿命。其中包括：-控制在超级单体上升气流后侧下沉气流(RFD)中形成/下降的降水的数量和类型的因素；-这种降水及其类型与RFDS形成的关系；-RFD如何产生涡度和/或正压重新分配它：-RFD的外流如何限制在上升气流下方的区域；-RFDS如何导致龙卷风气旋的形成，以及龙卷风气旋的角动量是由RFD保持还是仅仅从一些初始分布中“向下旋转”；-区域热带气旋能否通过大体上的正压过程直接导致龙卷风气旋；-龙卷风气旋是否由螺旋度高的流入空气向上倾斜形成；-龙卷风的形成是否只需要上述两个过程中的一个；-龙卷风的发生是否需要上述两个过程之一；-龙卷风的深度、大小和形状是否与龙卷风的发生有关。这些问题大多可以或已经以可测试、可驳斥的假设的形式提出。首席研究人员将着重发展RFD涡度过程的理论和模式，以及降水对中气旋尺度涡旋角动量重新分配的影响。首席调查员还将调查水流星和水相变化在推动运动结构方面的作用。成功完成这项研究将在保持足够的提前时间的同时，降低龙卷风警报的误警率。

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.