Upward Connecting Leaders of Lightning Flashes to Flat Water and Flat Ground

The final attachment of cloud-to-ground (CG) lightning is generally assumed to include an upward connecting leader (UCL) from the grounded connection point. In recent work [Stolzenburg et al., 2018, Atmos. Res., 211, doi: 10.1016/j.atmosres.2018.04.020], video data recorded at 50,000 frames per second for negative CG flashes were used to derive estimates for the length of positive UCLs. The return strokes were 1.7 to 7.8 km distant; image resolutions were 4.25 to 19.5 m. Upper bound lengths for the presumed UCLs were determined as the height above ground or water of the stepped leader tip in the last video frame before the return stroke. Better estimates of maximum UCL lengths were determined using the downward stepped leader tip's speed and estimated return stroke time within its first video frame. For 17 strokes the upper bound length of UCLs averaged 31.6 m and ranged from 11.3 to 50.3 m. Among the closest strokes, the five which connected to water had UCL upper bound estimates averaging significantly shorter (24.1 m) than the average for the three close strokes which connected to land (36.9 m). The better estimates of maximum UCL lengths for the eight closest strokes averaged 20.2 m, with slightly shorter average of 18.3 m for the five that connected to water. These are the first known modern estimates, since 1947, of UCL lengths from water. In this presentation we will show examples of the video data and discuss the results.

No UCLs are imaged in the video data of Stolzenburg et al. [2018], indicating those features were not bright enough or did not initiate long enough in time to be resolved. Another possibility raised to explain the lack of visible UCLs in the data was that some UCLs are very short, i.e. less than a few meters long. Stolzenburg et al. [2018] speculated that for some strokes to flat water, any upward leader might not be initiated until a very short time, perhaps less than a few microseconds, before the return stroke occurs, due to lack of corona emission until the stepped leader tip is quite close to the water surface. These ideas will be expanded upon in this presentation.