title: Observations of narrow bipolar events reveal how lightning is initiated in thunderstorms.
abstract: A long-standing but fundamental question in lightning studies concerns how lightning is initiated inside storms, given the absence of physical conductors. The issue has revolved around the question of whether the discharges are initiated solely by conventional dielectric breakdown or involve relativistic runaway electron processes. Here we report observations of a relatively unknown type of discharge, called fast positive breakdown, that is the cause of high-power discharges known as narrow bipolar events. The breakdown is found to have a wide range of strengths and is the initiating event of numerous lightning discharges. It appears to be purely dielectric in nature and to consist of a system of positive streamers in a locally intense electric field region. It initiates negative breakdown at the starting location of the streamers, which leads to the ensuing flash. The observations show that many or possibly all lightning flashes are initiated by fast positive breakdown.
authors.affiliation: Langmuir Laboratory for Atmospheric Research, Geophysical Research Center, New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801, USA.
authors.forename: William
authors.initials: W
authors.lastname: Rison
authors.affiliation: Langmuir Laboratory for Atmospheric Research, Geophysical Research Center, New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801, USA.
authors.forename: Paul R
authors.initials: PR
authors.lastname: Krehbiel
authors.affiliation: Langmuir Laboratory for Atmospheric Research, Geophysical Research Center, New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801, USA.
authors.forename: Michael G
authors.initials: MG
authors.lastname: Stock
authors.affiliation: Langmuir Laboratory for Atmospheric Research, Geophysical Research Center, New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801, USA.
authors.forename: Harald E
authors.initials: HE
authors.lastname: Edens
authors.affiliation: Space and Remote Sensing Group, Los Alamos National Laboratory, Los Alamos, New Mexico 87544, USA.
authors.forename: Xuan-Min
authors.initials: XM
authors.lastname: Shao
authors.affiliation: Langmuir Laboratory for Atmospheric Research, Geophysical Research Center, New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801, USA.
authors.forename: Ronald J
authors.initials: RJ
authors.lastname: Thomas
authors.affiliation: Langmuir Laboratory for Atmospheric Research, Geophysical Research Center, New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801, USA.
authors.forename: Mark A
authors.initials: MA
authors.lastname: Stanley
authors.affiliation: Laboratory of Lightning Physics and Protection Engineering, Chinese Academy of Meteorological Sciences, Beijing 100081, China.
authors.forename: Yang
authors.initials: Y
authors.lastname: Zhang