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.

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.

No abstract

[1] Records of M-component currents of first return strokes in natural negative cloud-to-ground lightning measured in a short instrumented tower are shown along with correlated video records, electric field changes, and relative luminosity. The synchronized fast-camera records clearly match the increase of luminosity of the faint lightning channel. The parameters of these M components are all consistent with those of triggered lightning. According to records of electric field change of lightning events near Morro do Cachimbo Station and of currents measured at the instrumented tower, the frequency of first return strokes exhibiting M components is within the range of tens of percent and not of a few percent, as usually accepted. This expectation of a few percent is based on a very low number of first return strokes expected to be followed by continuing currents. It was found that most of the analyzed first-return-stroke M components of natural lightning occur at the final phase of the return-stroke process and not during the flow of continuing currents. Apparently, this also holds true for most of the first M components of subsequent return strokes.

The development of downward and upward leaders that formed two negative cloud‐to‐ground return strokes in natural lightning, spaced only about 200 µs apart and terminating on ground only a few hundred meters away, was monitored at Morro do Cachimbo Station, Brazil. The simultaneous records of current, close electric field, relative luminosity, and corresponding high‐speed video frames (sampling rate of 20,000 frames per second) reveal that the initiation of the first return stroke interfered in the development of the second negative leader, leading it to an apparent continuous development before the attachment, without stepping, and at a regular two‐dimensional speed. Based on the experimental data, the formation processes of the two return strokes are discussed, and plausible interpretations for their development are provided.