Leandro Z. S. Campos

Positive leader characteristics from high-speed video observations

values range from 0.3 to 6.0 10 5 ms 1 with a mean of 2.7 10 5 ms 1 . Contrary to what is usually assumed, downward +CG leader speeds are similar to downward CG leader speeds. Our observations also show that the speeds tend to increase by a factor of 1.1 to 6.5 as they approach the ground. The presence of short duration, recoil leaders (RLs) during the development of positive leaders reveal a highly branched structure that is not usually recorded when using conventional photographic and video cameras. The existence of the RLs may help to explain observations of UHF-VHF radiation during the development of +CG flashes. Citation: Saba, M. M. F., K. L. Cummins, T. A. Warner, E. P. Krider, L. Z. S. Campos, M. G. Ballarotti, O. Pinto Jr., and S. A. Fleenor (2008), Positive leader characteristics from high-speed video observations, Geophys. Res. Lett., 35, L07802, doi:10.1029/2007GL033000.

High-speed video and electromagnetic analysis of two natural bipolar cloud-to-ground lightning flashes

High-speed video records of two bipolar cloud-to-ground flashes were analyzed in detail. They both began with a single positive return stroke that was followed by more than one subsequent weak negative stroke. Due to the elevated cloud base height of its parent thunderstorm, the preparatory processes of each subsequent negative stroke were documented optically below cloud base. In the first event (Case 1) it was observed that all four subsequent negative strokes were initiated by recoil leaders that retraced one horizontal channel segment previously ionized by the positive leader. Those recoil leaders connected to the original vertical channel segment and propagated toward ground, producing four subsequent strokes that had the same ground contact point as the original positive discharge. The second event (Case 2), in contrast, presented 15 subsequent strokes that were initiated by recoil leaders that did not reach the original channel of the positive stroke. They diverged vertically toward ground, making contact approximately 11 km away from the original positive strike point. These results constitute the first optical evidence that both single- and multiple-channel bipolar flashes occur as a consequence of recoil leader activity in the branches of the initial positive return stroke. For both events their total channel length increased continuously at a rate of the order of 104 m s−1, comparable to speeds reported for typical positive leaders.

On β2 stepped leaders in negative cloud‐to‐ground lightning

In their seminal lightning studies using streak cameras, Schonland et al. (1938) identified four negative stepped leader events that they term “β2,” a “rather rare variant of the type β leader”, and in it, “the second and slower stage of the leader is associated with the appearance of one or more fast dart streamers, which travel rapidly down from the cloud along the previously formed track and cease when they have caught up with the slower leader tip.” Seven negative downward leaders that agreed with the description given by Schonland et al. for type β2 were recorded in Tucson, Arizona, USA, and in Sao Jose dos Campos, Sao Paulo, Brazil. All cases were recorded by a high-speed camera operating at 4000 frames per second, and electric field changes were measured for three of them. Their “dart streamers” had speeds between 106 and 107 m s−1, compatible with previous observations of recoil leaders (RLs). Also, during the development of the three cases with correlated electric field changes, it was possible to identify sequences of microsecond-scale pulses preceding the propagation of a dart streamer in the channel. It is proposed that the luminous process that occurs during the development of a type β2 stepped leader is the visible manifestation of one or more RLs that begin inside the cloud and connect to the in-cloud, positive portion of the bipolar, bidirectional leader, and then travel downward to the lower end of the negative stepped leader path.

Characterization of lightning observed by multiple high-speed cameras

This study aims to analyze the visible parameters of the cloud-to-ground discharges such as the average multiplicity, continuing current duration and interstroke interval. Several authors already analyzed these parameters for groups of thunderstorms in some regions. Although some authors did not find differences between those characteristics between different regions, there has not been a comprehensive amount of lightning recordings from the same thunderstorm, in order to evaluate such parameters in a storm-to-storm basis. The lightning data for this work was obtained by four high-speed cameras (Phantom v9.1) set to record 2500 frames per second. They were located in Sao Jose dos Campos, Brazil, as part of the project RAMMER (Automated Multi-Camera Network for Monitoring and Study of Lightning Flashes). Five thunderstorm days were selected for this study and some of the data were recorded manually, which provided a higher number of confirmed cloud-to-ground lightning records. A total of 357 flashes were recorded. As far as the authors know, this is the first time that four high-speed cameras are looking to the same thunderstorm from different locations. Because of the number of cameras and their positions, the coverage area is larger, thereby increasing the number of flashes recorded from the same thunderstorm. These samples allow a more representative analysis of the lightning parameters mentioned above.

High-speed video observations of positive lightning flashes

Although positive lightning flashes to ground are not as frequent as negative flashes, their large amplitudes and destructive characteristics make understanding their parameters an important issue. This study summarizes the characteristics of 103 positive cloud-to-ground (+CG) flashes that have been recorded using high-speed video cameras (up to 8000 frames per second) in three countries together with time-correlated data provided by lightning location systems (LLS). A large fraction of the +CG flashes (81%) produced just a single-stroke, and the average multiplicity was 1.2 strokes per flash. All the subsequent strokes in multiple-stroke +CG flashes created a new ground termination except one. 75% of the +CG flashes contained at least one long continuing current (LCC) ≥ 40 ms, and this percentage is significantly larger than in the negative flashes that produce LCCs (approximately 30%). The median estimated peak current, (Ip) for 116 positive strokes that created new ground terminations was 39.4 kA. Positive strokes with a large Ip were usually followed by a LCC, and both of these parameters are threats in lightning protection. The characteristics presented here include the multiplicities of strokes and ground contacts, the percentage of single-stroke flashes, the durations of the continuing current, and the distributions of Ip.

Electric field waveforms of M components in negative and positive ground flashes A comparative analysis

There has been some discussion concerning the nature of luminosity pulses observed during the continuing currents of positive cloud-to-ground flashes. It still undefined whether or not these pulses can be considered M components when compared to similar events observed in negative flashes. In the present paper we expand this discussion by analyzing electric field waveforms associated with channel luminosity enhancements of both negative and positive ground flashes. Typically, negative M components are preceded by high-frequency pulses similar to those that can are associated to recoil leaders that are visible below cloud base. On the other hand, luminous processes in positive flashes (similar to M components) did not present a particularly common electric field signature, and no pulses could be positively correlated to their inception inside the cloud. We believe that this polarity asymmetry favors previous theories on the production of negative M components according to the bidirectional leader model. The distinctive features of the positive M components suggest that they are related to different processes.