An advanced 3‐D numerical model of lightning development is presented. The key features of the model include the probabilistic branching, bidirectional propagation, nonzero internal electric field, simultaneous growth of multiple branches, physical timing, channel decay, and, for the first time, probabilistic propagation field threshold. The new model can be used for computing electrical parameters of individual branches, including conductivity, current, and longitudinal electric field, each as a function of time, in different parts of the discharge tree. For illustrative purposes, the model was applied to studying the occurrence of lightning flashes of different type depending on the cloud charge structure, with emphasis on the lower positive charge region (LPCR). We demonstrated with the new model that the presence of relatively large (excessive) LPCR can prevent the occurrence of negative CG flashes by “blocking” the progression of descending negative leader from reaching ground. The blocking effect of excessive LPCR was found to occur when the vertical component of electric field near the cloud bottom was negative (downward directed). Further, we showed that significant reduction or absence of LPCR can eliminate the possibility of negative CG flashes and lead to normal‐polarity IC flashes instead. The model predicts the polarity‐asymmetry, which suggests that the amount of collected charge depends not only on the number of branches but also on the dynamics of their conductivity (lifetime) and the local cloud charge density.

This paper aims at analyzing the broadband part of electromagnetic emission from thunderclouds in a frequency range of tens of kilohertz to hundreds of megahertz. A model of the intracloud lightning discharge formation is presented. The lightning formation is described as a stochastic growth of the branching discharge channels, which is determined by the electrostatic field. The dynamics of the electric field and of the charge distribution over the lightning structure is calculated deterministically. The effect of the initial charge density in the cloud and the parameters of the conducting channels on spatio-temporal characteristics of the currents and structure of the lightning discharge is studied. The discharge radio emission is calculated by summing up the radiation fields of each channel at the observation point. The standard model for a separate discharge current is adopted, and the electromagnetic radiation in the far zone is estimated. It is found that the obtained frequency spectra exhibit a universal power-law behavior. The results of the modeling agree with known experimental data.

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It is traditionally accepted to define the dielectric strength of air as an electric field corresponding to the balance between the rates of impact ionization and electrons’ attachment to neutrals. Its reduced value is known to be about 110 Td regardless of the altitude above the mean sea level. In this study, the altitude profile of the critical electric field of atmospheric air in the 0–40 km altitude range is specified. Unlike the conventional approach, a wide range of additional plasma-chemical processes occurring in atmospheric air, such as electron detachment from negative ions and ion-ion conversion is taken into account. Atmospheric air is considered to be a mixture of N2:O2 = 4:1 containing a small amount of chemically active small gas components, such as water vapor, atomic oxygen, ozone, and several types of nitrogen oxides. It is shown that the dielectric strength of air falls noticeably compared to its conventional value. The results of the study can be important to solve the problems of initiation and propagation of lightning discharges, blue starters, and blue jets.

A new mechanism of charge transport inside a thundercloud is suggested and numerically investigated. The considered mechanism can be called “relay” because it is provided by a dynamical network of a relatively small amount of continuously decaying and arising conducting plasma formations. It manifests itself in two consecutive modes corresponding to pre-streamer and streamer/leader stages of thundercloud development. The first one is provided by dynamics of conducting ionic spots recently described by Iudin et al.1 that prepare conditions for initiation of positive streamers. The second mode relies on dynamical network of streamer/leader discharges and finally results in the formation of a compact well-conducting structure that bridges an area of strong electric field inside a thundercloud and can be associated with a lightning “seed”. The effectiveness of relay charge transport strongly depends on the relative proportion of conductive elements (plasma formations) and drastically increases in the field-dependent case.