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Original simultaneous records of currents, close electric field, and high‐speed videos of natural negative cloud‐to‐ground lightning striking the tower of Morro do Cachimbo Station are used to reveal typical features of upward positive leaders before the attachment, including their initiation and mode of propagation. According to the results, upward positive leaders initiate some hundreds of microseconds prior to the return stroke, while a continuous uprising current of about 4 A and superimposed pulses of a few tens amperes flow along the tower. Upon leader initiation, the electric field measured 50 m away from the tower at ground level is about 60 kV/m. The corresponding average field roughly estimated 0.5 m above the tower top is higher than 0.55 MV/m. As in laboratory experiments, the common propagation mode of upward positive leaders is developing continuously, without steps, from their initiation. Unlike downward negative leaders, upward positive leaders typically do not branch off, though they can bifurcate under the effect of a downward negative leader's secondary branch approaching their lateral surface. The upward positive leader's estimated average two‐dimensional propagation speed, in the range of 0.06 × 106 to 0.16 × 106 m/s, has the same order of magnitude as that of downward negative leaders. Apparently, the speed tends to increase just before attachment.

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First and subsequent return strokes' striking distances (SDs) were determined for negative cloud‐to‐ground flashes from high‐speed videos exhibiting the development of positive and negative leaders and the pre–return stroke phase of currents measured along a short tower. In order to improve the results, a new criterion was used for the initiation and propagation of the sustained upward connecting leader, consisting of a 4 A continuous current threshold. An advanced approach developed from the combined use of this criterion and a reverse propagation procedure, which considers the calculated propagation speeds of the leaders, was applied and revealed that SDs determined solely from the first video frame showing the upward leader can be significantly underestimated. An original approach was proposed for a rough estimate of first strokes' SD using solely records of current. This approach combines the 4 A criterion and a representative composite three‐dimensional propagation speed of 0.34 × 106 m/s for the leaders in the last 300 m propagated distance. SDs determined under this approach showed to be consistent with those of the advanced procedure. This approach was applied to determine the SD of 17 first return strokes of negative flashes measured at MCS, covering a wide peak‐current range, from 18 to 153 kA. The estimated SDs exhibit very high dispersion and reveal great differences in relation to the SDs estimated for subsequent return strokes and strokes in triggered lightning.

[1] Records of natural lightning currents of negative downward strokes measured at the base of a 60 m high instrumented tower are presented to denote patterns of their early phase. Such patterns in the records of five flashes and two upward unconnected leaders are discussed. Typical profiles of this early phase, comprising the pre-return-stroke pulses of the current flowing upwards the tower in response to the downward stepped leader approaching the ground, the concavity in the initial rising part of the return-stroke waveform, and the abrupt rise around the half-peak of the first-stroke waveform, are remarked. Statistics are developed for the pre-return-stroke pulses of current to characterize their amplitude, duration, interval, and risetime for both the first stroke and upward unconnected leaders, which proved to be quite similar. The ratio of the maximum pulse peak to first stroke peak is typically in the range 0.7% to 2.2% of the analyzed currents, different from the value of around 10% found for the distant electric fields radiated by first-stroke currents.Citation: Visacro, S., M. H. Murta Vale, G. Correa, and A. Teixeira (2010), Early phase of lightning currents measured in a short tower associated with direct and nearby lightning strikes,