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• M-component incident wave in the +CG channel coupled with the first stroke of a distant –CG. • A branched positive leader intermittently extended via bidirectional transients separated by inactive intervals of about 5 ms. • The bidirectional transients, moving negative charge in the backward direction, triggered a downward negative leader, resulting in a three-stroke –CG. • A subsequent-stroke negative leader, moving through virgin air, entered the lower part of residual (non-luminous) first-stroke channel, in addition to creating a new ground termination about 950 m away. • Ultraviolet images of lightning are presented for a negative cloud-to-ground lightning flash with two ground terminations. Based on recent observations at the Lightning Observatory in Gainesville (LOG), Florida, we present three examples of very unusual behavior of cloud-to-ground (CG) lightning flashes: (a) One +CG and one tower-terminated -CG separated by 11 km, were briefly coupled, so that positive charge was in effect drawn from the ground at the position of –CG and injected into the ground at the position of +CG. That unusual interaction was accomplished by the initiation of M-component incident wave in the +CG channel by the leader of the first stroke of the –CG. (b) A branched positive leader intermittently extended via bidirectional transients separated by inactive intervals of about 5 ms, on average. One of the bidirectional transients, moving negative charge in the backward direction, triggered a downward negative leader, resulting in a three-stroke –CG. (c) A subsequent-stroke negative leader, moving through virgin air, entered the lower part of residual (non-luminous) first-stroke channel, in addition to creating a new ground termination about 950 m away. Studying unusual lightning behavior helps to improve our understanding of the variability of basic lightning processes and to identify new potential lightning hazards to various objects and systems. [ABSTRACT FROM AUTHOR]

Simultaneous electric field and high-speed video camera records of two flashes, labeled 1593 and 1594, were obtained at the Lightning Observatory in Gainesville (LOG), Florida. Flash 1593 was an upward negative flash whose upward positive leader initiated from a 257-m high tower, located 8.8 km from LOG. It contained six leader/return-stroke sequences, all of which developed along the upward leader path and terminated on the 257-m tower. Flash 1594 was a three-stroke downward bipolar flash whose first stroke was positive and terminated on a 60-m tower. The two subsequent strokes were negative and terminated on the same 257-m tower as the six strokes of flash 1593. All strokes terminated on the 257-m tower exhibited very similar narrow bipolar electric field waveforms with damped oscillatory tails. Characteristics of those electric field waveforms are examined and compared to similar observations found in the literature. [ABSTRACT FROM AUTHOR]

• An upward flash was triggered from a 257-m tower in florida by a +CG 45 km away. • Six return strokes of the upward flash were all initiated by bidirectional leaders. • All six return-stroke field waveforms at 8.8 km exhibited early zero-crossings. • Two scenarios giving rise to the narrow field waveforms are discussed. We examined in detail the morphology and evolution of an upward negative flash initiated from a 257-m tower in Florida. High-speed video camera images and wideband electric field records, as well as ENTLN data, were used. The upward negative flash was induced (triggered) by a single-stroke 50-kA +CG that occurred about 45 km from the tower. The 257-m tower flash contained 6 leader/return stroke sequences and 1 attempted leader that almost terminated on the tower. All the leaders exhibited bidirectional extension. Electric field waveforms produced by the return strokes (measured at a distance of 8.8 km) were bipolar and abnormally narrow (exhibited earlier zero crossings). In order to examine the origin of the observed earlier zero crossings, we used two return-stroke models of transmission line type (MTLL and MTLE) to see what model input parameters are responsible for this feature. Within the limits of those models, the observed earlier zero crossings could be explained only by a narrow input current waveform or/and its fast amplitude decay with height. [ABSTRACT FROM AUTHOR]