Abstract
Abstract. Information about lightning properties is important in order to advance the current understanding of lightning, whereby the characteristics of ground strike points (GSPs) are in particular helpful to improving the risk estimation for lightning protection. Lightning properties of a total of 1174 negative downward lightning flashes are analyzed. The high-speed video recordings are taken in different regions, including Austria, Brazil, South Africa and the USA, and are analyzed in terms of flash multiplicity, duration, interstroke intervals and ground strike point properties. According to our knowledge this is the first simultaneous analysis of GSP properties in different regions of the world applying a common methodology. Although the results vary among the data sets, the analysis reveals that a third of the flashes are single-stroke events, while the overall mean number of strokes per flash equals 3.67. From the video imagery an average of 1.56 GSPs per flash is derived, with about 60 % of the multiple-stroke flashes striking the ground in more than one place. It follows that a ground contact point is struck 2.35 times on average. Multiple-stroke flashes last on average 371 ms, whereas the geometric mean (GM) interstroke interval value preceding strokes producing a new GSP is about 18 % greater than the GM value preceding subsequent strokes following a pre-existing lightning channel. In addition, a positive correlation between the duration and multiplicity of the flash is presented. The characteristics of the subset of flashes exhibiting multiple GSPs is further examined. It follows that strokes with a stroke order of 2 create a new GSP in 60 % of the cases, while this percentage quickly drops for higher-order strokes. Further, the possibility of forming a new lightning channel to ground in terms of the number of strokes that conditioned the previous lightning channel shows that approximately 88 % developed after the occurrence of only one stroke. Investigating the time intervals in the other 12 % of the cases when two or more strokes re-used the previous lightning channel showed that the average interstroke time interval preceding a new lightning channel is found to be more than twice the time difference between strokes that follow the previous lightning channel.
Highlights
Cumulonimbus clouds are the birthplace of one of Earth’s true spectacles in nature: the lightning discharge
The flash multiplicity depends on the ability to identify all the respective strokes that occurred during the flash
The multiplicities in this study are in line with average multiplicity values published in other studies such as Rakov et al (1994), Cooray and Perez (1994), Cooray and Jayaratne (1994), Saba et al (2006), and Saraiva et al (2010) and lower than what was found by Ballarotti et al (2012) and Kitagawa et al (1962)
Summary
Cumulonimbus clouds are the birthplace of one of Earth’s true spectacles in nature: the lightning discharge. High-speed camera observations observe the light emitted directly by the lightning discharge, thereby documenting the flow of the electrically charged particles through the air, and provide, linked to electric field measurements, a means to investigate in great detail the associated optical and electromagnetic properties of natural downward lightning flashes. The characteristics deduced from this are relevant from a pure scientific perspective and essential in developing adequate lightning protection solutions as the level of lightning protection and risk to be mitigated is derived from the density of lightning terminations in a region This is based on flash density values, but there have been recommendations to increase calculated densities by a factor of 2 to account for multiple ground strike point flashes (Bouquegneau, 2014; IEC 62858 Ed. 2, 2019). It is worthwhile mentioning that the data sets described here serve as basis to investigate the ability of so-called ground strike point algorithms to correctly group strokes in flashes according to the observed ground strike points (Poelman et al, 2021, companion paper)
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