Lightning Initiation Due to Hydrometeor Collisions in Thunderclouds

Open Access
Author:
Cai, Qiheng
Graduate Program:
Electrical Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
March 16, 2018
Committee Members:
  • Victor P. Pasko, Dissertation Advisor
  • Jim Breakall, Committee Chair
  • Julio Urbina, Committee Member
  • David B. Spencer, Committee Member
  • Jaroslav Jansky, Outside Member
Keywords:
  • lightning initiation
  • similarity law
  • avalanche-to-streamer transition
Abstract:
In order to initiate streamers and leaders under thunderstorm conditions, the electric field should reach values higher than the critical breakdown field. However, the maximum electric field in thunderstorms measured by balloons is much smaller than the breakdown field. In order to investigate the electric field amplification required for streamer initiation in thunderclouds, in this dissertation work, we introduce two numerical models of two hydrometeors in uniform electric fields. The first model is two separated spherical hydrometeors. We have found the relationship between minimum electric field required for streamer initiation and the particle radius. The larger the radius, the lower electric field required. Furthermore, we consider the model of two particles with different radii for streamer initiation including the process of photoionization feedback, which could initially produce photons and then trigger a secondary electron avalanche to reduce the threshold electric field required for streamer initiation. The relationship between radii of particles for successful streamer initiation in the gap is investigated. We also demonstrate that for realistic particle dimensions, the avalanche to streamer transition is possible in a local electric field just slightly above the minimum field required for propagation of positive streamers in air. Considering that the connection of two spheres would lead to increase of the electric field at the outer periphery, and outside space represents a larger volume available for the photoionization feedback in comparison with the relatively small gap between two spheres, the second model of two spherical particles connected with a discharge channel is investigated. We also investigate the relationship of the radii of these two particles for streamer initiation on the outside periphery. Furthermore, we estimated the frequency of streamer initiation using three realistic hydrometeor size model distributions available in the literature and found that the scenario of streamer initiation on the outside periphery is only possible for relatively high electric fields, which are close to half of the breakdown electric field, at typical thundercloud altitudes of 3 and 6 km.