CN tower lightning flash characteristics based on high-speed imaging

The thesis emphasizes the analysis of fifty-eight flashes that struck the CN Tower during the last five years (2013-2017), based on video records of Phantom v5.0 digital high-speed imaging system, operating at 1 ms resolution. (A Sony HDR PJ790VB digital camera, operating at 16.67 ms resolution, provided a continuous recording of each CN Tower lightning storm.) It was observed that every recorded flash contained initial-stage current, confirming that all analyzed flashes were upward initiated. It was also observed that only 27 flashes out of the 58 contained return strokes. The number of strokes per flash varied between 1 and 9, with an average multiplicity of 2.16. The time variation of the channel luminosity of each flash was precisely analyzed for the characterization of flash components. A yearly statistical comparison regarding CN Tower lightning macroscopic (number of flashes, inter-flash times and flash durations) and microscopic (initial-stage current durations, number of the return strokes per flash, M-components and inter-stroke times) characteristics was conducted. The analysis of these extensive data (2013-2017) clearly showed that the 50% cumulative probability distribution (CPD) of flash duration in in 2017 was found to be the largest in comparison with other data (2013-2016). Whereas, the 50% CPD of inter-flash time duration in 2015 was the shortest. Furthermore, the 2014 data showed the longest 50% CPD of the initial-stage current duration as well as the inter-stroke time duration. In 2016, one flash was found to contain the highest number of return strokes. 2015 was distinguished by having the longest 50% CPD of the continuing current duration. It was also noted that two significantly major storms were captured during the nights of September 5, 2014 and September 4, 2017. During the storm of September 5, 2014, the CN Tower imaging systems recorded 13 flashes. The storm lasted for 111.4 minutes, resulting on average a flash to the tower every 9.28 minutes. Whereas, the September 4, 2017 storm continued for 49.35 minutes, producing 11 flashes for the tower every 4.9 minutes. The characterisation of CN Tower lightning is pivotal to the protection of tall structures against lightning hazards. It’s worth mentioning that flashes with longer flash duration, containing high number of return strokes and have shorter inter-stroke time durations pose severe threats to tall structures, electrical and communication systems