Unraveling In-Cloud Lightning Development Through Ground-Based And Space-Borne Observations

Abstract

Lightning, a phenomenon occurring within thunderclouds, has spurred numerousstudies seeking to understand its origin and behavior. The initiation of lightning remains elusive, particularly due to difficulties in conducting direct observations within thunderstorms. Subsequent evolutions of intracloud (IC) ashes characterize bi-level IC phenomena and have been a focal point of recent research. Advancements in observation techniques have significantly enhanced our understanding of these lightning processes within thunderclouds. Broadband interferometry, a prevalent method for radio imaging of lightning, has consistently been re ned across various studies to adeptly capture the nuanced progression of lightning breakdown. Furthermore, the recent launch of the space-based instrument, the Atmosphere-Space Interactions Monitor (ASIM) aboard on the International Space Station, provides a more detailed insight into the optical radiance of IC lightning activities compared to ground-based observations. ASIM particularly highlights the 337.0 nm and 777.4 nm bands, which are associated with non-thermal discharges and swift conductive channel formation, respectively. LF magnetic eld measurements capture a substantial amount of energy from lightning's sudden current discharges. Due to their long-range reach, they are also key in detecting and tracking extensive areas of lightning activity. By leveraging comprehensive observational data, this dissertation focuses on the intricacies of IC lightning. First, the high bandwidth and fast time resolution VHF interferometry enables us to examine the preliminary stages of fast breakdown (FB) in lightning initiation. Through observing thousands of FBs, a consistent pattern showed they start as either a positive polarity streamer or a mix of both polarities. We also identified a new FB variant, mixed FB, which further suggests positive and negative streamers are both likely propagating simultaneously from the initiation point. Our simulations on VHF emissions from these streamers revealed how slight changes in emission intensities affect streamer paths, enhancing our understanding of dielectric discharge and setting the stage for future research on streamer behaviors. Shifting from streamer dynamics, we studied the evolution of IC leaders in thunderclouds. By combining the LF magnetic eld measurements, ASIM optical observations, and VHF interferometry data, we charted IC lightning's growth from an upward path to a branching horizontal spread. We identified three key IC stages: Ascending, Transition, and Horizontal, revealing IC's transition from streamer activities to leader-like behaviors. This dissertation also delved into uncommon pulse trains associated with the late rapid discharge phase of IC, focusing on chaotic pulse trains (CPTs) and regular pulse trains (RPTs). Over four years of observations, we captured a full negative IC ash event in both the ASIM optical system and Duke observation network. This comprehensive data highlighted the distinct nature of CPTs from RPTs and provided a fresh understanding of their origins. In conclusion, this dissertation, enriched by a wide collection of observational techniques, offers a holistic view of IC lightning in thunderclouds.