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，但它的大部分基本物理学仍然有待发现。即使是其最研究的元素-回击-仍然是完全量化的。闪电物理学中的一个中心问题是，正先导和负先导传播中广泛观察到的不对称性如何映射到正极性和负极性的云对地（CG）闪电之间的整体对比差异（先导是用于描述连接到地面之前的伸长闪电通道的术语）。一个重要的例子是反冲先导的起源，据观察，反冲先导只能追溯带正电荷的先导通道。反冲先导可能是正性和负性CG之间卒中多样性不对称的根本原因。了解闪电在基本层面上的物理学是量化其在我们星球大气中的影响所必需的（例如，氮氧化物化合物的生产）并减轻其社会影响（例如，电力传输和分配中断）。通过开发新的计算机模型，该项目解决了下面第二段中概述的闪电物理学中的突出问题。这个项目的广泛影响与教育计划密切相关。我们的教育计划的支柱是基于一个简单的想法，即物理教师可以使用闪电和雷暴-任何学生都熟悉的东西-介绍复杂的物理概念，制定教学策略。该项目解决了闪电物理学中的两个关键问题。问题1：当引线通道连接到接地结构时，称为回程的强电流浪涌向上传播。目前应用最广泛的一种回击模型，假定波的传播速度及其随高度的衰减是模型的自由参数。尽管从以前的调查，一个详细的特点，回击动力学的第一原则仍然是一个悬而未决的问题。问题二：正先导和负先导具有不同的传播机制（不同的速度和通道分支率，以及连续传播与阶梯传播）。有人假设，这种极性不对称映射到如何不同的电流截止和反冲领导人的形成发生在积极和消极的CG闪光。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，以推进当前对闪电的理解：（1）回程模型，其计算电流和光学光度波的速度以及它们在向上朝向云传播时的衰减，以及（2）随机，领导渠道网络的三维模型，它考虑了概率分支，并具有不同的传播机制，为积极和消极的极端。最重要的进步是将这两个电动力学模型与等离子体通道非线性电阻的现实处理相结合。该奖项反映了NSF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.