A five-year effort will develop a set of efficient three-dimensional lightning models and apply these models to theoretically investigate a range of outstanding science questions related to thunderstorm electrification and development of different forms of lightning discharges. The outstanding questions which the research will seek to answer are:
(1) Can lightning models based on a fractal approach provide realistic descriptions of different types of lightning discharges? What are the specific limitations of these models in terms of description of charges, potentials and electric fields of the overall lightning-thundercloud electric system? Can these models be modified to provide a computationally efficient parameterization of the streamer zones of both positive and negative lightning leaders?
(2) What are the effective time scales of streamer-to-leader transition in air as a function of air pressure? What are the altitude range and other relevant physical conditions for which formation of leader channels is not possible in the Earth's atmosphere?
(3) What are the specific thundercloud charge configurations which allow upward extensions of lightning leaders above thundercloud tops leading to recently discovered blue jet and gigantic jet phenomena? Are pressure scaled fractal models of streamer zones of positive and negative lightning leaders capable of describing experimentally observed features of blue jets and gigantic jets?
The specific activities directed toward answering these questions will include development of three-dimensional numerical models based on a probabilistic approach to lightning modeling. These models will account for the best currently available knowledge for efficient representation of leader trees in the three dimensional computation domain and will include such features as open boundary conditions, preservation of full charge neutrality of leader trees at any point during simulations, and accurate modeling of lightning channels with finite resistivity. Model development will synergistically complement existing observationally-based research efforts at the New Mexico Institute of Mining and Technology directed toward investigation of electrical structure of thunderstorms, including studies of inverted-polarity electrical structures, lightning initiation studies, and studies of different and newly discovered types of lightning discharges.
Broader impacts of the proposed activity include student training and education, synergistic collaboration between theoretical modelers and experimentalists, and the dissemination of results in the form of models to be shared with the community and in the form of publications summarizing new results.
The goal of this project was to develop a set of efficient three-dimensional lightning models and apply these models to theoretically investigate a range of outstanding science questions related to thunderstorm electrification and development of different forms of lightning discharges. It is generally realized in the existing refereed literature that complexity and multi-scale nature of lightning discharges make it impossible to develop an effective computational model capable of resolving microphysical details of lightning phenomena in large-scale thunderstorm electrification models. The modeling of streamers, for example, which constitute essential building blocks of streamer zones of both positive and negative lightning leaders, requires micrometer and sub-nanosecond spatial and temporal resolutions, respectively, which are virtually unachievable in large-scale three-dimensional models of lightning leader trees covering distances of several kilometers in a fraction of a second. There is a need to develop efficient and robust numerical parameterizations of the large-scale charge, potential and electric field structures associated with lightning leaders, which would allow further advancement of understanding of lightning physics by direct comparisons with lightning mapping observations. Specific contributions we made under this project include: (1) development of three-dimensional stochastic model of lightning discharges accounting for realistic thundercloud electrodynamics; (2) development of unified theory of blue and gigantic jet discharges establishing a conducting link between thundercloud tops and the upper regions of the Earth's atmosphere; (3) development of first principles model of streamer-to-spark transition allowing investigation of scaling of air heating in streamer discharges at different altitudes in the Earth's atmosphere and as a function of air density. This project provided training to three graduate and five undergraduate students, and results have been published in eight refereed journal papers.
