Lightning can be classified into two types according to whether it reaches the ground or not. About three-quarters of lightning discharges do not involve ground, most of which belong to the intracloud (IC) lightning category (Rakov, 2016). Lightning discharges that reach the ground are referred to as cloud-toground (CG) lightning. Whether a lightning discharge strikes the ground or remains inside the cloud can be determined from their low-frequency (LF) electromagnetic field waveforms. Pulses generated by lightning processes inside the cloud differ from those generated when lightning strikes the ground (the process referred to as return stroke). Typical return-stroke field waveforms measured at a distance beyond several tens of kilometers or so are characterized by a short rise time of several microseconds and a relatively slow decay time of several tens of microseconds (Haddad et al., 2012; Lin et al., 1979). Compared to return strokes, field pulses of most intracloud lightning processes are generally narrower. Also, due to the greater diversity of lightning processes inside the cloud, pulse waveshapes tend to have larger variation from one event to another. The differences in field waveforms produced by in-cloud lightning processes and return strokes are used in the multiparameter waveform classification methods implemented in many regional and continental scale ground-based lightning detection networks (e.g., U.S. National Detection Network (NLDN), Earth Networks Total Lightning Network (ENTLN), and European Cooperation for Lightning Detection (EUCLID) network) to classify the detected lightning field pulses into two categories: cloud pulses (referred to as ICs) and return strokes (referred to as CGs). In recent validation studies for the abovementioned networks, estimated classification accuracy for return strokes ranges from 70% to 92% (

Previous studies have shown that subsequent leaders in positive cloud‐to‐ground lightning (+CG) flashes rarely traverse pre‐existing channels to ground. In this paper, we present evidence that this actually can be common, at least for some thunderstorms. Observations of +CG flashes in a supercell storm in Argentina by Córdoba Argentina Marx Meter Array (CAMMA) are presented, in which 54 (64%) of 84 multiple‐stroke +CG flashes had subsequent leaders following a pre‐existing channel to ground. These subsequent positive leaders are found to behave similarly to their negative counterparts, including propagation speeds along pre‐existing channels with a median of 8 × 106 m/s, which is comparable to that of negative dart leaders. Two representative multiple‐stroke +CG flashes are presented and discussed in detail. The observations reported herein call for an update to the traditional explanation of the disparity between positive and negative lightning.

Using visible-range and infrared (3-5 μm) high-speed video cameras, we observed luminosity transients that reilluminated decayed branches of two close (2 to 4 km) negative stepped leaders in Florida. Leader branches were energized via stepping at their tips and, as a result, were most heated near their lower ends, with the hotter sections being connected via cooler sections to the trunk. In the modeling of lightning leaders, usually a single tip is considered. In contrast, in the present study, many (up to 30 per major branch) tips were active at the same time, forming a network-like structure with a descending multitip "ionization front" whose transverse dimensions were of the order of hundreds of meters. The front exhibited alternating stepping, with each step necessarily generating a positive charge wave traveling from the leader tip up along the channel, like a mini return stroke. We inferred that the step-related waves can cause luminosity transients in the remnants of decayed negative branches at higher altitudes. Such reactivated branches, in turn, may facilitate further leader stepping at lower altitudes, as first reported by Stolzenburg et al. (2015, https://doi.org/10.1002/2014JD022933). The reactivation process is likely to involve multiple steps, as evidenced by a large number of active tips (some tens per 50-μs frame) and corresponding electric field pulses occurring at time intervals of 2 μs or less. Additionally, our observations suggest that a transient in one decayed branch can trigger (or assist with triggering of) a transient in another branch.

Using the high‐speed optical and electric field records, in conjunction with Earth Networks Total Lightning Network and radar data, we examined in detail the morphology and evolution of an upward negative flash containing six downward leader/upward return stroke sequences terminated on a 257‐m tower in Florida. The upward flash was induced (triggered) by a single‐stroke 50‐kA +CG that occurred about 45 km from the tower. The in‐cloud part of +CG was optically detected to extend toward the tower and appeared to stop at a height of about 3 km above the tower top. The six leader‐return‐stroke sequences were each initiated by a bidirectional leader utilizing the remnants of branches created during the initial stage. Electric field signatures of bidirectional leaders were similar to K‐changes. The upper end of the return‐stroke channel in all six cases exhibited branching and appeared to extend to higher altitudes or/and move closer to Lightning Observatory in Gainesville with increasing stroke order.

Positive lightning discharges to ground (+CGs) are relatively rare and considerably less studied than negative ones (-CGs). We present observations of unusual transient phenomena occurring in +CGs and discuss their mechanisms. One of them is a brief electric coupling to a concurrent -CG initiated from a 257-m tall tower located 11 km from the +CG channel. A transient process (stroke) in the -CG flash appears to cause a transient luminosity enhancement (M-component) in the +CG channel. In the course of these essentially simultaneous transients, positive charge is in effect taken from the ground at the position of the tower and injected into the ground at the position of the +CG channel. Recoil leaders reactivating decayed +CG branches near the cloud base are each observed to cause a transient luminosity decrease (dip), as opposed to the expected luminosity increase, in the +CG main channel.