Common physical processes are identified in various types of natural and artificially triggered lightning flashes, both in summer and winter storms By applying an electrostatic model of bidirectional, uncharged and monopolar, charged leaders, the main physical principles are defined for interpretation of common lightning processes These principles focus on lightning initiation, charges on the leader, the leader's electrical potential, the electrical break-down at the leader tip, leader branching and current cutoff, the occurrence of recoil streamers, and the conclusion of lightning propagation The bidirectional, uncharged leader model is compared, on both physical grounds and by analyzing the electric field changes, with the conventional model of a unidirectional, uniformly charged leader which originates from a space charge source These two models are also tested against experimental data obtained with a VHF mapping system and in situ measurements of electric fields in storms on the heights of lightning origins in cloud-to-ground flashes

Common physical processes in natural and artificially triggered lightning

Characteristics of lightning discharges that transport either positive charge or both positive and negative charges to the ground are reviewed. These are termed positive and bipolar lightning discharges, respectively. Different types of positive and bipolar lightning are discussed. Although positive lightning discharges account for 10% or less of global cloud-to-ground lightning activity, there are five situations that appear to be conducive to the more frequent occurrence of positive lightning. These situations include 1) the dissipating stage of an individual thunderstorm, 2) winter thunderstorms, 3) trailing stratiform regions of mesoscale convective systems, 4) some severe storms, and 5) thunderclouds formed over forest fires or contaminated by smoke. The highest directly measured lightning currents (near 300 kA) and the largest charge transfers (hundreds of coulombs or more) are thought to be associated with positive lightning. Two types of impulsive positive current waveforms have been obse...

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A review of positive and bipolar lightning discharges

Many outstanding problems in lightning physics are linked with a difference in macroscopic behaviour between positive and negative polarity. Such differences are referred to broadly as 'polarity asymmetry'. As specific examples, the positive and negative ends of lightning propagate at different speeds, with different degrees of steadiness, and with different radiated electromagnetic energy. Positive and negative flashes to ground transfer their charge in markedly different ways—negative flashes with multiple discrete strokes (often) and positive flashes with single strokes followed by continuing current. Positive ground flashes cause sprites and negative flashes do not (generally). Positive intracloud flashes send gamma radiation upward to space and negative intracloud flashes do not (generally). Speculative arguments are presented that all of these macroscopic asymmetries are rooted in the microscopic asymmetry in mobility for free electrons and positive ions.

https://iopscience.iop.org/article/10.1088/0963-0252/15/2/S12/pdf

Problems in lightning physics—the role of polarity asymmetry

Three triggered lightning flashes, initiated during the Thunderstorm Research International Program (1981) at Langmuir Laboratory, New Mexico, are examined on the basis of three-dimensional return stroke propagation speeds and peak currents. Nonlinear relationships result between return stroke propagation speed and stroke peak current for 56 strokes, and between return stroke propagation speed and dart leader propagation speed for 32 strokes. Calculated linear correlation coefficients include dart leader propagation speed and ensuing return stroke peak current (32 strokes; r = 0.84); and stroke peak current and interstroke interval (69 strokes; r = 0.57). Earlier natural lightning data do not concur with the weak positive correlation between dart leader propagation speed and interstroke interval. Therefore, application of triggered lightning results to natural lightning phenomena must be made with certain caveats. Mean values are included for the three-dimensional return stroke propagation speed and for the three-dimensional dart leader propagation speed.

Correlated observations of three triggered lightning flashes

A 300-MHz interferometric imaging system has been used for the characterization of the VHF-UHF electromagnetic radiation from lightning flashes. Results are presented, with emphasis on typical phenomenology, in particular through correlations with the electrical activity description from an E field sensor network. At the microsecond time scale the VHF-UHF radiation appears impulsive, either as low rate pulsed emissions or as bursts of pulses. The low-rate emissions (below 20 pulses/ms) are usually present during important intracloud charge transfers and correspond to spread out sources over distances of the order of 1 km. The bursts last from several hundred microseconds to a few milliseconds, and they are associated with highly organized propagations of sources in the 107-m/s velocity range extending over distances of several kilometers. They appear in all phases of the lightning flash and are not associated with large charge transfers. Their spatial evolution and chronology with respect to the electrical phenomenology, in particular during precursor phenomena, indicate a decorrelation between electromagnetic phenomenology and the typical electrical phenomenology.

Results of spatial and temporal characterization of the VHF‐UHF radiation of lightning

During the 1978 campaign of the triggered lightning program at Saint-Privat d•'Allier (France), simultaneous data from a movie camera, a coaxial shunt amperometer, a network of electric field mills, and a weather radar, were collected during the initial phase of a particular triggered event. These data are shown to exhibit a high degree of consistency, leading to the conclusion that charges totaling several coulombs were present in cloudfree air in the vicinity of a stormy area.

/pdf/triggered-lightning-strokes-originating-in-clear-air-2fikbr4ja5.pdf

P. Waldteuffl

Papers

Triggered lightning strokes originating in clear air