Thunderstorm electric field structural features and lightning initiation problem

Formation of elevated-conductivity regions that have a spatial scale of the order of decimeters and a lifetime of about 1 ms inside thunderclouds can lead to the enhancement of the electric field needed for lightning initiation. We show that the elevated-conductivity regions are generated due to the presence of stochastic small- scale electric field of charged hydrometeors. Fluctuations of this field can exceed, from time to time, the critical breakdown level, which can be sufficient to ensure the survival of free electrons under the conditions when the average small-scale electric field is significantly lower than the air breakdown field. The rate of above- critical electric field enhancements and hence the occurrence of elevated-conductivity regions is determined by two main factors: hydrometeor concentration and the variance of their charge magnitude distribution. Formation of elevated-conductivity regions occurs on a relativly long time interval due to the interaction of electron and ion components. Specifically, the rapid attachment of electrons is balanced by their liberation in negative ion destruction processes. Further, the drift of ions in a stochastic electric field leads to enlargement of elevated- conductivity regions. The growth of conductivity is limited to spatial-temporal clusters occupying a very small portion of the overall space-time domain, so that the average conductivity of the medium does not change significantly. The presence of elevated-conductivity regions in a dielectric medium (thundercloud) lowers its effective electric breakdown field, because of the concentration of large-scale quasi-static field across the gaps between the decimetre-scale elevated-conductivity regions, which are essentially equipotential in a quasi- static electric field