| 英文摘要: | 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. |