We propose a scenario in which elevated ionic conductivity regions (EICRs) with dimensions of the order of 0.1-1 m are formed in the turbulent thundercloud environment. The starting point in this scenario is the occurrence of electron avalanches in the vicinity of colliding hydrometeors, leading to the formation of ion production centers. Their dimensions are of the order of 10 À3 À 10 À2 m, and their lifetime is of the order of 10 À4 À 10 À3 s. When a new ion production center is created inside the decimeter-scale residual ion concentration spot left behind by a previously established center, the local ion concentration steadily increases, which leads to the formation of decimeter-scale EICRs whose lifetime is measured in seconds. The relatively high conductivity of EICRs (up to 10 À9 S/m or so) relative to the background conductivity (10 À14 S/m or less) ensures their polarization in external electric field within a few milliseconds or so. The EICR formation mechanism requires only one condition: the rate of occurrence of ion production centers per unit time in a unit volume should exceed the percolation-theory-based critical level of 10 À1 m À3 s À1 . Hydrometeor collision rates three and even four orders of magnitude higher than this value have been reported from observations. Presence of EICRs in the cloud provides local electric field enhancements and pre-ionization levels that will lead to the formation of additional ion production centers and may be sufficient for the initiation and development of streamers and, eventually, lightning.npj Climate and Atmospheric Science (2019) 2:46 ; https://doi.

A numerical model with physical timing and grid spacing of 3 m is applied to studying the progression (including stepping and branching) of negative lightning stepped leader. The asymmetry between positive and negative streamers is taken into account via using polarity‐dependent initiation and propagation field thresholds. The stepped nature of negative leader is confirmed to be caused by this asymmetry. The step formation process of the negative leader is modeled to begin with the appearance of space stems inside and in the immediate vicinity of its streamer zone (corona streamer burst completing the preceding step). Some of those space stems evolve into space leaders, which can connect to the primary leader channel, thereby facilitating its extension. Model‐predicted morphology and dynamics of negative leaders are in good agreement with the recent recordings of lightning stepped and dart‐stepped leaders obtained using high‐speed video cameras, and their electrical parameters are in line with the current knowledge on negative lightning leaders.

In this work, we represent the lightning initiation scenario as a sequence of two transitions of discharge activity to progressively larger spatial scales: the first one is from small-scale avalanches to intermediate-scale streamers; and the second one is from streamers to the lightning seed. We postulate the existence of ion production centers in the cloud, whose occurrence is caused by electric field bursts accompanying hydrometeor collisions (or near collisions) in the turbulent thundercloud environment. When a new ion production center is created inside (fully or partially) the residual ion spot left behind by a previously established center, there is a cumulative effect in the increasing of ion concentration. As a result, the essentially non-conducting thundercloud becomes seeded by elevated ion-conductivity regions (EICRs) with spatial extent of 0.1–1 m and a lifetime of 1–10 s. The electric field on the surface of an EICR (due to its conductivity being at least 4 orders of magnitude higher than ambient) is a factor of 3 or more higher than ambient. For a maximum ambient electric field of 100 kV/m typically measured in thunderclouds, such field enhancement is sufficient for initiation of positive streamers and their propagation over distances of the order of decimeters, and this will be happening naturally, without any external agents (e.g., superenergetic cosmic ray particles) or extraordinary in-cloud conditions, such as very high potential differences or very large hydrometeors. Provided that each EICR generates at least one streamer during its lifetime, the streamers will form a 3D network, some parts of which will contain hot channel segments created via the cumulative heating and/or thermal-ionizational instability. These hot channel segments will polarize, interact with each other, and cluster, forming longer conducting structures in the cloud. When the ambient potential difference bridged by such a conducting structure exceeds 3 MV, we assume that the lightning seed, capable of self-sustained bidirectional extension, is formed.

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