The mechanisms of lightning leader propagation and ground attachmen

Lightning data were collected at the International Center for Lightning Research and Testing at Camp Blanding, Florida from 2009 to 2011. Data were obtained for 12 natural negative cloud-to-ground lightning discharges and 46 rocket-and-wire triggered lightning discharges. The mechanisms and characteristics of upward and downward leader propagation and downward leader attachment to ground were examined using data from high-speed framing cameras, electric field derivative (dE/dt) sensors, plastic and lanthanum bromide (LaBr3) energetic radiation (x-ray) scintillation detectors, channel-base currents, a Lightning Mapping Array (LMA), and a C-band dual-polarimetric radar. The dE/dt and energetic radiation measurements form a 10-station time-of-arrival (TOA) network used to determine the locations and emission times of sources within about 750 m of ground. High-speed video images of a stepped leader are analyzed to determine characteristics of optical phenomena, such as space stems/leaders, associated with the sequence of electrical breakdowns that occur during the formation of a leader step. Observations are compared to those obtained for dart-stepped leader steps in triggered lightning. dE/dt waveforms of "chaotic" dart leaders preceding triggered and natural lightning strokes are analyzed using TOA techniques and their characteristics are compared to dart and dart-stepped leader processes. "Chaotic" dart leaders are found to emit copious x-rays in a nearly continuous manner prior to the return stroke. The initial stage (IS) processes of nine triggered lightning discharges are mapped in three-dimensions with the LMA. The geometrical and electrical properties of IS branching are determined and compared to channel-base currents. The LMA and radar are used to examine the effects of hydrometeor structure on the propagating IS channels. Vertically-propagating IS channels are observed to turn horizontal at 3-6 km altitude, near the 0° C level, often propagating for many kilometers along the tops of high-reflectivity rain-shafts. The propagation characteristics and attachment processes of triggered and natural lightning dart-stepped leaders are analyzed. Properties of the dE/dt pulses following the final downward leader step are discussed with respect to the measured channel-base current. Upward leader lengths, speeds, and durations are calculated. A total of 30 x-ray sources are TOA-located and compared spatially and temporally to corresponding dE/dt source locations.