Lightning Initiation by Streamer Emission from Thundercloud Hydrometeors

雷暴中闪电产生的确切机制是大气电学领域一个长期存在的基本问题。传统的理论建立在一个尚未证实的假设上，即电子雪崩导致“流光”的形成和生长（通过分支），这是一种相对低温的丝状电离通道。然而，现有的观测证据表明，雷暴中的电场很少超过触发此类雪崩所需的击穿场阈值的10%。因此有人提出，飘带可能起源于个别水成物（降水粒子）附近，在其附近电场被局部放大。这一过程已通过控制实验室实验证明。为了更好地了解这一过程如何在自由大气中发挥作用，pi将使用云特性的简化表示对相关过程进行数值模拟，并根据现有观测结果对其结果进行评估。研究的具体问题包括：1)不同类型的水成物对雷云电场的扰动是什么？2)流光是如何在水成物附近形成和发展的？水成物的流光发射的开始条件是什么？这样形成的流线的特征是什么？3)飘带和水成物之间的电相互作用是什么？焦耳加热对拖缆杆的影响是什么？水成物附近的先导启动条件是什么？这项工作的智力价值在于改进了对在带电云中触发闪电的过程的理解，以及对现有观测准确捕获雷暴中可实现的电场强度上限的能力的进一步批判性评估。更广泛的影响将主要通过研究生和本科生教育，通过同行评议的期刊传播结果，以及更好地理解（从而能够预测）雷暴中引发闪电的关键过程，从而降低生命和财产风险的长期潜力。

The exact mechanisms responsible for initiation of lightning within thunderstorms is a longstanding and fundamental problem in the field of atmospheric electricity. Conventional theory rests on the as-yet unproven hypothesis that electron avalanches lead to the formation and growth (through branching) of "streamers," which are comparatively low-temperature filamentary ionization channels. Existing observational evidence nonetheless indicates that electric fields within thunderstorms rarely exceed 10% of the threshold breakdown field value needed to trigger such avalanches. It has thus been suggested that streamers may originate in the vicinity of individual hydrometeors (precipitation particles) near which the electric field is locally amplified. This process has been demonstrated through controlled laboratory experiments. In pursuit of better understanding of how this process may work in the free atmosphere, the PIs will employ numerical modeling of relevant processes using simplified representations of cloud properties and evaluate their results against available observations. Some specific questions for which answers will be pursued include: 1) What are the perturbations of the thundercloud electric field caused by different types of hydrometeors? 2) How does a streamer initiate and develop in the vicinity of the hydrometeors? What is the onset condition of the streamer emission from the hydrometeors? What are the characteristics of the streamer thus formed? 3) What are the electrical interactions between streamers and hydrometeors? What are the effects of the joule heating on the streamer stem? What is the leader inception condition in the vicinity of hydrometeors?The intellectual merit of this work rests on improved understanding of the processes through which lightning is triggered within electrically charged clouds, as well as further critical evaluation of the ability of existing observations to accurately capture the upper-limit of electric field strength achievable within thunderstorms. Broader impacts will be primarily through graduate and undergraduate student education, dissemination of results via peer-reviewed journals, and the long-term potential for reduced risk to life and property that would come with a better understanding (and hence ability to predict) key processes that initiate lightning within thunderstorms.