Evaluation and development of lightning return-stroke models at elevated objects

点击翻译按钮获取中文摘要

Simultaneous measurement of the lightning current and its associated electric and magnetic fields, and thedetermination of lightning channel characteristics are necessary for the quantitative assessment of lightningreturn-stroke models. Such assessments will enhance our understanding of the lightning return-stroke process,help in the formulation of new models that can be more successfully used in lightning detection algorithms, andpermit the development of more sophisticated measures for lightning protection. Simultaneous measurementsof the lightning current and its radiated electromagnetic pulse are possible using instrumented tall structures, orrocket- or laser-triggered lightning. The CN Tower, with its extreme height, presents one of the best options inthe world to carry out this investigation. This fact led to the initiation of an important phase of the CN Towerlightning studies. By the start of the summer of 1991, five measurement stations were operational tosimultaneously capture all relevant lightning parameters including the lightning current derivative and itsassociated electromagnetic field. Many lightning return-stroke current models have been evaluated on the basisof unrelated statistical information about lightning currents and lightning-generated electromagnetic fields;notable exceptions include the use of instrumented towers or rocket-triggered lightning. During the proposedproject, a detailed and rigorous analysis of this transient electromagnetic field problem will be carried out withthe aim of developing a more experimentally verifiable return-stroke model. Although CN Tower lightningflashes are mostly of the upward-initiated type, downward-initiated flashes, similar to ordinary ground flashes,have been observed at the tower. Therefore, the developed model can be evaluated using the two types oflighting. In present models, it is assumed that lightning channels are straight and vertical, contrary to mostobservations. In this project, a three-dimensional image of the lighting channel will be constructed from videorecords and used, in addition to the measured (height-dependent) return-stroke velocity, for the development ofan experimentally verified lightning return-stroke model.