CAREER: Self-consistent and Data-constrained Simulations of the Leader and Return Stroke Processes in Lightning Discharges

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Despite lightning being such a common natural phenomenon, a large portion of its fundamental physics remains to be uncovered. Even its most studied element — the return stroke — remains to be fully quantified. A central question in lightning physics has to do with how the widely-observed asymmetry in positive- and negative-leader propagation maps into the overall contrasting differences between cloud-to-ground (CG) flashes of both positive and negative polarities (leader is the term used to describe the elongation lightning channel before its connection to the ground). One important example is the origin of recoil leaders, which are observed to only retrace the channels of positively-charged leader channels. Recoil leaders in its turn may be the root cause for the asymmetry in stroke multiplicity between positive and negative CGs. Understanding the physics of lightning at a fundamental level is required for quantifying its effects in our planet's atmosphere (e.g., production of nitrogen oxide compounds) and to mitigate its societal impacts (e.g., power transmission and distribution disruptions). Through the development of novel computer models, this project addresses outstanding questions in lightning physics outlined in the second paragraph below. The broader impacts of this project are heavily tied with the educational plan. The backbone of our educational plan is to develop teaching strategies based on the simple idea that a physics instructor can use lightning and thunderstorms — something that any student is familiar with — to introduce complex physics concepts. This project tackles two key problems in lightning physics. Problem 1: When leader channels connect to a ground structure, a strong current surge known as the return stroke travels upward. The most widely-employed type of return stroke model assumes that the wave propagation velocity and its attenuation with height are free parameters of the model. Despite a few attempts from previous investigation, a detailed characterization of the return stroke dynamics from first-principles remains an open problem. Problem 2: Positive and negative leaders have different propagation mechanisms (different velocities and channel branching rates, and continuous vs. stepped propagation). It has been hypothesized that this polarity asymmetry maps into how differently current cutoff and recoil leader formation takes place in positive and negative CG flashes. However, there is no computational simulation tool available that can fully answer this complicated question. The main goal of this project is to advance the current understand of lightning by introducing two novel physics-based models to describe the main stages of a lightning flash, and address the two problems outlined above: (1) a return stroke model that calculates the velocity of current and optical luminosity waves and their attenuation as they propagate upward towards the cloud, and (2) a stochastic, 3-dimensional model of the leader channel network, which accounts for probabilistic branching and has different propagation mechanisms for positive and negative extremities. The most important advance aimed here is the coupling of these two electrodynamics models with a realistic treatment of the plasma channel's nonlinear resistance.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.