Sustainable upward leaders from tall structures: Their development and effects

Upward lightning is known to be the most common type of lightning that interacts with tall structures on the surface of the earth. The initial stage of every upward lightning flash is an upward leader. A dart leader-return stroke cycle, i.e., a phase of a cloud-to-ground flash, may follow it. The objective of our study was to identify and interpret the various processes during development of upward positive leaders from tall structures. The objects of our observational setup were two TV towers (324 m and 295 m tall) situated in downtown Kansas City, Missouri, separated by a distance of 1.1 km. Measurements of electrical field changes (dE and dE/dt) were made using a network of five stations in close proximity to the towers. We also used a high-speed video system (1000 frames/sec) that viewed the tops of both towers. We provide a physical interpretation for both the varying luminosity of the upward leader channel during its initial ascent and the pulsing luminosity during much later stages of the established leader channel. Our analysis confirms that pulsed-channel luminosity such as this is a result of recoil leaders (the product of previous stages of the branched upward leader) intercepting the current-carrying leader channel that is still connected to ground. Applying the bidirectional leader concept and electrostatic approximation, we introduce a physical model that interprets the pulse-type luminosity of the existing upward leader channel, referred to in the literature as an “M-component”.