Lightning Initiation by Streamer Emission from Thundercloud Hydrometeors

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The exact mechanisms responsible for initiation of lightning within thunderstorms is a longstanding and fundamental problem in the field of atmospheric electricity. Conventional theory rests on the as-yet unproven hypothesis that electron avalanches lead to the formation and growth (through branching) of "streamers," which are comparatively low-temperature filamentary ionization channels. Existing observational evidence nonetheless indicates that electric fields within thunderstorms rarely exceed 10% of the threshold breakdown field value needed to trigger such avalanches. It has thus been suggested that streamers may originate in the vicinity of individual hydrometeors (precipitation particles) near which the electric field is locally amplified. This process has been demonstrated through controlled laboratory experiments. In pursuit of better understanding of how this process may work in the free atmosphere, the PIs will employ numerical modeling of relevant processes using simplified representations of cloud properties and evaluate their results against available observations. Some specific questions for which answers will be pursued include: 1) What are the perturbations of the thundercloud electric field caused by different types of hydrometeors? 2) How does a streamer initiate and develop in the vicinity of the hydrometeors? What is the onset condition of the streamer emission from the hydrometeors? What are the characteristics of the streamer thus formed? 3) What are the electrical interactions between streamers and hydrometeors? What are the effects of the joule heating on the streamer stem? What is the leader inception condition in the vicinity of hydrometeors?The intellectual merit of this work rests on improved understanding of the processes through which lightning is triggered within electrically charged clouds, as well as further critical evaluation of the ability of existing observations to accurately capture the upper-limit of electric field strength achievable within thunderstorms. Broader impacts will be primarily through graduate and undergraduate student education, dissemination of results via peer-reviewed journals, and the long-term potential for reduced risk to life and property that would come with a better understanding (and hence ability to predict) key processes that initiate lightning within thunderstorms.