[1] A brief review of hypothesized mechanisms of lightning initiation is presented, with the suggestion that these mechanisms provide an incomplete picture of lightning initiation. This is followed by two ideas: (1) a combination of previously hypothesized lightning initiation mechanisms as a means for local intensification of the thundercloud electric field, and (2) a process for the formation of a hot lightning leader channel that is analogous to the space leader phase of the laboratory negative stepped leader. Thundercloud electric field observations have consistently yielded peak values that are an order of magnitude weaker than the dielectric strength of air. Various mechanisms have been proposed to explain how lightning can initiate in such weak electric fields, including hydrometeor-initiated positive streamers and cosmic ray-initiated runaway breakdown. The historically favored positive streamer mechanisms are problematic due to requiring electric fields two to three times larger than peak observed fields. The recently favored runaway breakdown mechanisms appear capable of developing in conditions comparable to peak observed fields although it is not clear how these diffuse discharges can lead to creation of a lightning leader. This paper proposes a solution whereby runaway breakdown and hydrometeor-initiated positive streamer systems serve to locally intensify the electric field. Following this local field intensification, it is hypothesized that formation of the initial lightning leader channel is analogous to the formation of a space leader in a laboratory negative stepped leader.

[1] We present new high-speed video observations of a natural negative stepped leader and a subsequent negative dart-stepped leader. Observations were made at a distance of 770 m using a high-speed video camera at 10,000 frames per second, a microwave-frequency radio receiver, a broadband electric field antenna, and an avalanche photodiode array. Lightning leader breakdown was observed in detail for both the negative stepped leader and the subsequent dart-stepped leader. During the negative stepped leader breakdown, detailed images were captured of the discharge structures near the leader tips. These structures bear a remarkable resemblance to the corona streamer zone and space leader discharges that have been observed in laboratory-generated negative stepped leaders. During the dart-stepped leader breakdown, no corona streamer zone was observed outside of the decaying return stroke channel, but small luminous structures that are suggestive of space leaders were observed just ahead of the main dart leader tip. Two distinct low-luminosity zones were observed just ahead of the dart leader tip, suggestive of two distinct breakdown regimes. A multipath junction was observed in the main channel to ground following the first return stroke but was not observed following the second return stroke. Finally, microwave-frequency radio emissions for both leader types and their return strokes were recorded, and their time domain behavior is compared and discussed.Citation: Petersen, D. A., and W. H. Beasley (2013), High-speed video observations of a natural negative stepped leader and subsequent dart-stepped leader,

SUMMARYPositive streamer discharges have been observed from ice crystals grown from the vapour under controlled temperature and air pressure corresponding to the standard atmosphere and also constant air pressure of 850 hPa. Crystals of size comparable to larger precipitation crystals were grown in a vapour diffusion chamber at various temperatures and pressures, and were subject to electric fields using an in situ electrode assembly. Results show that individual positive streamer discharges can occur at temperatures from near freezing down to at least −38 • C when subject to electric fields on the order of 500 kV m −1 , in contrast to previous reports of continuous positive streamer discharges occurring only at temperatures warmer than −18 • C for pure ice crystals. These results suggest that positive streamer discharges can occur on frozen precipitation particles at high altitudes in thunderclouds, given a sufficiently strong electric field. Such discharges may be a necessary element of the lightning-initiation process.

This article reports on a new laboratory investigation of the initiation of corona discharges by small vapour‐grown ice crystals. This study was motivated by questions regarding the initiation of lightning and the possible role played by corona discharges on ice hydrometeors. Ice crystals were grown in a static diffusion chamber on a horizontally suspended glass fibre, with environmental conditions selected to mirror those of the thunderstorm environment where initiation of lightning is believed to occur. An in situ parallel plate electrode assembly was then rotated into position, so that the crystals were at the centre. The crystals were then subjected to slowly increasing electric fields until they underwent corona discharge. The discharges were observed and recorded under dark ambient conditions using an image‐intensified video microscope, along with a synchronous record of the applied voltage. The threshold electric fields for corona discharge were determined as a function of air pressure, temperature and ice crystal length in the direction of the applied electric field. The new results are relevant to a better understanding of hydrometeor‐mediated electrical discharge activity in thunderstorms.

In this Letter, we report the first experimental study of binary heterogeneous gas-liquid nucleation. Onset activities were obtained for nucleation of various n-propanol-water vapor mixtures on 6.9 nm NaCl particles soluble in water and practically insoluble in n-propanol. The Fletcher theory of vapor nucleation on insoluble particles provided a reasonable approximation for n-propanol rich mixtures only, whereas the Köhler theory of activation of soluble particles worked only for water rich mixtures. A new theory was formulated providing a satisfactory description of the transition from activation of soluble particles to vapor nucleation on insoluble particles.