1Royal Meteorological Institute, Brussels, Belgium
2Austrian Lightning Detection and Information System (ALDIS), Vienna, Austria
3Météorage, Pau, France
4Scientific Lightning Solutions LLC (SLS), Titusville, Florida, USA
5National Institute for Space Research, INPE, São José dos Campos, Brazil
6The Johannesburg Lightning Research Laboratory, School of Electrical and Information Engineering, University of Witwatersrand Johannesburg, Johannesburg, South Africa
7Institute of High Voltage Engineering and System Performance, Graz University of Technology, Graz, Austria
8ZT Research, Rapid City, South Dakota, USA
1Royal Meteorological Institute, Brussels, Belgium
2Austrian Lightning Detection and Information System (ALDIS), Vienna, Austria
3Météorage, Pau, France
4Scientific Lightning Solutions LLC (SLS), Titusville, Florida, USA
5National Institute for Space Research, INPE, São José dos Campos, Brazil
6The Johannesburg Lightning Research Laboratory, School of Electrical and Information Engineering, University of Witwatersrand Johannesburg, Johannesburg, South Africa
7Institute of High Voltage Engineering and System Performance, Graz University of Technology, Graz, Austria
Received: 11 Jan 2021 – Accepted for review: 09 Feb 2021 – Discussion started: 11 Feb 2021
Abstract. Lightning properties of a total of 1174 negative downward lightning flashes are analyzed. The high-speed video recordings are taken in different regions around the world, including Austria, Brazil, South Africa and USA, and are analyzed in terms of flash multiplicity, duration, interstroke intervals and ground strike point (GSP) properties. 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 ground in more than one place. It follows that the channel creating a GSP is re-used by a factor of 2.3. 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 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 stroke order of two create a new GSP in 60 % of the cases, while this percentage quickly drops for higher order strokes. Further, the possibility to form a new channel to ground in terms of the number of strokes that conditioned the previous 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 channel showed that the average interstroke time interval preceding a new channel is found to be more than twice the time difference between strokes that follow the previous channel.
This paper presents a comprehensive study of negative downward lightning flashes based on high-speed video camera recordings of negative cloud-to-ground lightning in several regions around the globe. This study presents solid statistics that help improve the current lightning protection standard (change from flash density to ground-strike point density). The subject is suitable for this journal. Several comments follow. I recommend this paper be accepted after minor revisions.
Specific Comments:
Did the authors include upward lightning in South Africa dataset? If yes, I think those upward lightning contradicts your title (negative downward flashes). If not, please state so in the paper.
“Note that in Austria two flashes are observed whereby a new GSP is created by the tenth stroke in the flash, while the channel belonging to the previous GSP was used four and seven times, respectively.” It would be interesting to know the interstroke interval preceding the 10th
Flash characteristic studies solely relying on high-speed cameras have limitations. I hope the authors could discuss those limitations and how those limitations could possibly influence the statistics presented. Two limitations that I can think of: (1) strokes creating a new termination could be missed by the camera (e.g., the stroke can occur at the back of cameras or simply out of view). (2) It is likely camera record length is not long enough to cover the entire flash. I see that length for SA dataset is only 1s with manual trigger setup (not sure what’s the pre-trigger and post-trigger during manual trigger setup), maybe this partially explains why most SA flashes are single-stroke flash. Simultaneous electric/magnetic field measurements/LLS data might help mitigate some of those limitations. They could be used to see if there are additional strokes in the vicinity but outside the field of view of camera or outside the duration of the camera records.
“It follows that the channel creating a GSP is re-used by a factor of 2.3” I think the word “re-used” is ambiguous. Sounds like the termination created by a previous stroke will be re-struck by 2.3 subsequent return strokes on average. Your statement “A ground contact point is struck 2.35 times on average” in Line 166 is more accurate.
Minor editorial suggestions:
Line 65: “Hence, the role of high-speed camera observations.” This Is not a complete sentence.
Lightning properties of negative downward lightning flashes are analyzed. High-speed video recordings are taken in different regions around the world and analyzed in terms of flash multiplicity, duration, interstroke intervals and ground strike point properties.
Lightning properties of negative downward lightning flashes are analyzed. High-speed video...