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

尽管闪电是一种常见的自然现象，但其大部分基本物理原理仍有待发现。即使是研究得最多的部分——回击——也还没有完全量化。闪电物理学的一个核心问题是，在正、负引线传播中广泛观察到的不对称性是如何映射到云对地（CG）的正、负极性（引线是用来描述连接到地面之前的延伸闪电通道的术语）之间的总体对比差异的。一个重要的例子是后坐力先导的起源，它被观察到只回溯带正电的先导通道。反过来，后坐力先导可能是阳性和阴性cg之间卒中多重性不对称的根本原因。为了量化闪电对地球大气的影响（例如，产生氮氧化物化合物）和减轻闪电对社会的影响（例如，输电和配电中断），需要在基本水平上了解闪电的物理特性。通过开发新的计算机模型，本项目解决了下文第二段概述的闪电物理中的突出问题。这个项目的广泛影响与教育计划密切相关。我们教育计划的核心是基于这样一个简单的理念，即物理教师可以利用闪电和雷暴——任何学生都熟悉的东西——来介绍复杂的物理概念，从而制定教学策略。这个项目解决了闪电物理中的两个关键问题。问题1：当先导通道连接到地面结构时，一股强电流浪涌（称为回冲）向上移动。应用最广泛的回冲程模型假定波的传播速度及其随高度的衰减是模型的自由参数。尽管以前的研究做了一些尝试，但从第一原理详细描述回冲程动力学仍然是一个悬而未决的问题。问题2：积极和消极的领导者有不同的传播机制（不同的速度和渠道分支率，连续和阶梯传播）。据推测，这种极性不对称反映了在正、负CG闪烁中电流切断和后坐力先导形成的不同。然而，没有可用的计算模拟工具可以完全回答这个复杂的问题。该项目的主要目标是通过引入两个新的基于物理的模型来描述闪电的主要阶段，并解决上述两个问题，从而提高对闪电的当前理解：(1)计算电流和光学光度波向上向云传播时的速度及其衰减的返回行程模型；(2)考虑概率分支且正负极值具有不同传播机制的领导通道网络的随机三维模型。本文最重要的进展是这两种电动力学模型的耦合，并对等离子体通道的非线性电阻进行了实际处理。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

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.