An electric force facilitator in descending vortex tornadogenesis

Abstract

[1] We present a novel explanation of the physical processes behind one type of cloud and ground-level tornadogenesis within a supercell. We point out that the charge separation naturally found in these large thunderstorms can potentially serve to contract the preexisting angular momentum through the additional process of the electric force. On the basis of this, we present a plausible geometry that explains why many tornado vortices begin at storm midlevel and build downward into ground-level tornadoes. A simple model based on this geometry is used to demonstrate the strength of the electric force compared to the required centripetal acceleration to maintain cloud midlevel tornado vortices measurable as tornado vortex signatures (TVSs). Furthermore, a model based on this geometry is used to get a time estimate for tornado vortex formation. From this we are able to identify a plausible value for the threshold charge density that would lead to tornadogenesis and tornado maintenance on the timescale of a few minutes. We show that the proposed geometry can explain the observations that ground-level tornadoes thrive in regions with high shear and large convective available potential energy (CAPE) and are able to make some predictions of specific measurable quantities. Motivation[2] Many of the physical conditions required to initiate tornadoes are well understood on a broad qualitative level but many quantitative details remain unknown. On the qualitative level, it is apparent that the deep rotation of a supercell thunderstorm along with a buoyant updraft and a rain-driven downdraft create a probable environment for tornadogenesis. Yet, we still do not quantitatively understand the full range of energy inputs which allow particular storms to spawn tornadoes while other storms with similar macroscopic properties do not. Furthermore, we cannot currently predict the length (minutes to hours) or intensity of the tornadoes that do form [Davies-Jones et al., 2001]. This suggests the presence of one or more physical thresholds in the system that, once exceeded, can spawn a tornado. Systems in which the relevant threshold is not achieved, therefore fail to form a tornado. As an example, evidence indicates that a certain amount of boundary layer shear in conjunction with a certain level of convective available potential energy (CAPE) is required for a tornado to occur [Rasmussen and Blanchard, 1998].[3] One clear attribute is that most tornadoes spawn from supercells and associated convective activity. In particular, Trapp et al. [2005] showed that 79% of all measured tornadoes for the years 1998 -2000 came from supercells. We also know that around two thirds of this 79% of supercellspawned tornadoes begin their rotation aloft (2 -7 km) and then descend to form ground-level tornadoes in what is measured as a descending TVS [Trapp et al., 1999]. This means that roughly half of all the sampled tornadoes began their intense rotation high in a supercell cloud and then built downward to make ground-level tornadoes. This suggest...

Electrical Role for Severe Storm Tornadogenesis (and Modification)

“…No obvious role of cloud electrification would be involved in the generation of these type tornadoes. More recently, a primary role for electrical force was brought in as a facilitator of IC tornadogenesis by Patton, Bothun and Sessions [54] with focus on the fact that many tornadic vortices begin at storm mid-level and build downward into ground level tornadoes, more or less in the same manner as originally described by Vonnegut [2,3]. Important connection was established with conditions of high wind shear and large CAPE, in the latter case as already mentioned for the description given in the detailed analysis reported by Carey and Buffalo [20].…”