Lightning Initiation and Propagation with an Advanced Computer Model and Code

该奖项将允许通过开发先进的计算机模型和代码、执行数值模拟和进行理论分析来研究闪电的引发和传播。闪电如何在雷云内部产生并在空气中传播是大气电学领域一个长期存在的基本问题。闪电的引发和传播有三个关键步骤：相对较冷的等离子体通道流光的引发和传播，从流光向明亮、高温、高导电性引导者的转变，以及引导者的传播。最近的研究已经确定，在迄今测量到的最大雷云电场的约束下，可以从雷云水流星雨中启动流光。然而，闪电引发和传播的另外两个步骤仍然知之甚少。目前尚不清楚来自雷云流星的流光是否能够在其末端产生第一个领导者，如果可以，流光到领导者的转变是如何发生的，这种转变如何取决于雷云的条件，如雷云的电荷和场。此外，实验和观察表明，最常见的一种被称为负引头的闪电引头是以一种步进式的方式发展的。也许是大气电学中最“神秘”的现象之一，被称为“太空茎”的发光斑点结构被发现出现在负引头尖端的前面，它们发展成为双向的“空间引导者”，以支持负引导者的步进传播。目前还没有理论来解释空间茎和空间引线的形成。该项目的目标是开发一种先进的计算机模型和代码来研究闪电的引发和传播。将开发一个三维计算机代码，其中包括所有必要的等离子体动力学和气体动力学组件，以自洽地模拟流光传播、引导器启动以及空间茎和空间引导器的形成。利用所开发的模型和程序，将进行以下研究活动：(1)研究长(10厘米)流光在接近或大于流光传播阈值电场中的传播特性；(2)研究从相反极性的膨胀加速流光的尾迹开始的流光及其对空间茎和空间先导形成的影响；(3)研究源自水流星的流光通道中的空气加热，在其顶部形成第一个先导通道的可能性，智力价值：这些活动的智力价值取决于对闪电研究中最具挑战性的问题之一的直接攻击，以及对闪电过程最不了解的调查。近年来，随着闪电实验、观测和理论的发展，闪电研究取得了长足的进步。这项工作将填补闪电电击穿过程方面的知识空白，以进一步促进目前对闪电的理解，并将导致开发一种先进的计算机模型和代码，将在未来几年用于闪电研究。广泛影响：这些活动的更广泛影响包括：(1)学科科学知识的进步，这可能有助于改进当前的实践和/或开发新技术，以更好地预报恶劣天气，特别是在适应气候变化的压力越来越大的情况下；(2)改进科学、技术、工程和数学(STEM)教育，(3)发展STEM劳动力，(4)妇女和任职人数偏低的少数群体参与科学研究；(5)开发模拟软件和方法，以利于实验室等离子体放电和高空精灵和喷气式飞机等其他领域的研究工作；(6)以经评审的出版物、会议报告、公开讲座和新闻稿等形式广泛传播成果。

This award will allow for the investigation of the initiation and propagation of lightning through developing an advanced computer model and code, performing numerical simulations, and conducting theoretical analysis. How lightning originates inside thunderclouds and propagates through air is a longstanding fundamental problem in the field of atmospheric electricity. It has been suggested that there are three critical steps in lightning initiation and propagation: initiation and propagation of streamers that are relatively cold plasma channels, transition from streamers to bright, hot, highly conductive leaders, and propagation of leaders. Recent studies have firmly established that streamers can be initiated from thundercloud hydrometeors subject to the maximum thundercloud electric field measured so far. However, the other two steps of lightning initiation and propagation remain poorly understood. It is unknown whether the streamers originating from thundercloud hydrometeors will be able to generate the first leader at their stem, and if they can, how the streamer-to-leader transition occurs and how this transition depends on thundercloud conditions such as thundercloud charge and field. In addition, experiments and observations have indicated that the most common type of lightning leaders called negative leaders develops in a stepping manner. Perhaps being one of the most "mysterious" phenomena in atmospheric electricity, luminous blob structures known as "space stems" are found to appear in front of the negative leader tip and they develop into bidirectional "space leaders" to support the stepping propagation of negative leaders. There is currently no theory that offers an explanation for the formation of space stems and space leaders.The objective of the project is to develop an advanced computer model and code to study lightning initiation and propagation. A three-dimensional computer code, that includes all required plasma kinetic and gas dynamic components to self-consistently model streamer propagation, leader initiation, and formation of space stems and space leaders, will be developed. With the developed model and code, the following research activities will be conducted: (1) investigating the characteristics of long (10 cm) streamers propagating in electric fields near or greater than the streamer propagation threshold fields, (2) studying the initiation of streamers from the trail of an expanding, accelerating streamer of opposite polarity and its implications for the formation of space stems and space leaders, and (3) investigating air heating in the channels of streamers originating from hydrometeors, the possibility of the formation of the first leader channel at their stem, and the formation of space stems and space leaders in front of a negative leader.Intellectual Merit :Intellectual merit of these activities is defined by the direct attack on one of the most challenging problems in lightning research and by the investigation of the least-understood processes of lightning. With the recent development of lightning experiments, observations and theories, great progress has been made in lightning research. The work will fill a gap in the knowledge of electrical breakdown processes of lightning to further advance the current understanding of lightning, and it will also result in development of an advanced computer model and code that will be used for lightning research for years to come.Broader Impacts :Broader impacts of these activities include: (1) advancement of scientific knowledge in a subject that may contribute to improving the current practice and/or developing new techniques to better forecast severe weather, especially under the increasing pressure of adapting to climate change, (2) improvement of science, technology, engineering, and mathematics (STEM) education, (3) development of STEM workforce, (4) participation of women and underrepresented minorities in scientific research, (5) development of simulation software and approaches that will benefit research work in other areas such as laboratory plasma discharges and high-altitude sprites and jets, and (6) broad dissemination of the results in the forms of refereed publications, conference presentations, public lectures and press releases.