Date of Award

5-2015

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Aerospace, Physics, and Space Sciences

First Advisor

Ningyu Liu

Second Advisor

Hamid K. Rassoul

Third Advisor

Joseph R. Dwyer

Fourth Advisor

Ming Zhang

Abstract

Electric field values measured inside thunderclouds have consistently been reported to be up to an order of magnitude lower than the value required for the conventional electrical breakdown of air. This result has made it difficult to explain how lightning frequently occurs in thunderclouds. One theory that has been offered to explain the lightning initiation process is the theory of lightning initiation from hydrometeors. According to this theory, lightning can be initiated from electrical discharges originating around thundercloud water or ice particles in the measured thundercloud electric field. These particles, called hydrometeors, can cause significant enhancement of the thundercloud electric field in their vicinity, increasing the probability to initiate streamers that are precursor discharges for the hot lightning leader channel. For this dissertation research, we have focused our efforts on studying streamer initiation and propagation from thundercloud hydrometeors. For the first part of this study, we have further investigated the idea proposed by Liu et al. [2012b] to study streamers from ionization column hydrometeors in thundercloud conditions. We have performed simulations for ambient electric field values as low as 0.3Ek at thundercloud altitudes. According to our results, initiation of stable streamers from thundercloud hydrometeors in a 0.3Ek electric field is possible, only if enhanced ambient ionization levels are present ahead of the streamer. We investigate the streamer branching behavior and characteristics, and test a theory that has recently been proposed to explain this phenomenon [Savel’eva et al., 2013]. In order to verify whether an ionization column is a proper representation of a dielectric hydrometeor, for the second part of this dissertation we modify our streamer discharge model to accommodate an isolated dielectric particle representing the hydrometeor inside the computational region. The development of this model has enabled us to accurately simulate the discharges around dielectric hydrometeors with various shapes and physical states. Streamer discharge results obtained from the dielectric hydrometeor have been presented and compared with the results obtained from the first part of this work. We compare our modeling results with laboratory experiments and realistic thundercloud conditions and discuss the implications of this study to lightning initiation and other lightning related phenomena.

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