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Two downward negative flashes (F1612 & F1613) with multiple return strokes struck to the 356-m Shenzhen Tower were analyzed. The high-speed camera captured upward positive connecting leaders (UCL) for the first strokes of both flashes and two subsequent strokes of F1612. The observations also show that none of the downward negative leader (DNL) completely propagated to the tower tip before the occurrence of the return stroke (RS), so that we inferred that the UCL initiated from the tower tip might exist for each RS of the two flashes. The possible height of the UCL and DNL connecting point above the tower tip for each RS was estimated, which showed a positive correlation to the speed of corresponding DNL. The striking distances estimated for 8 subsequent RSs in F1612 were in a range from 14.6 m to 85.4 m and that for 3 subsequent RSs in F1613 were from 24.2 m to 66.2 m. There was a weak positive linear relationship between the DNL speed and striking distance for subsequent RSs in the two flashes. The DNL speed showed a positive correlation to the peak current of corresponding RS. • Two negative flashes with multiple return strokes to a tall tower were studied based on high-speed camera measurements. • Upward connecting leaders from the tower tip were observed for the first time for two subsequent return strokes of a flash. • Heights of upward and downward leader connecting points showed a positive correlation to the downward leader speeds. [ABSTRACT FROM AUTHOR]

Using video data recorded at 50,000 frames per second for nearby negative lightning flashes, estimates are derived for the length of positive upward connecting leaders (UCLs) that presumably formed prior to new ground attachments. Return strokes were 1.7 to 7.8 km distant, yielding image resolutions of 4.25 to 19.5 m. No UCLs are imaged in these data, indicating those features were too transient or too dim compared to other lightning processes that are imaged at these resolutions. Upper bound lengths for 17 presumed UCLs are determined from the height above flat ground or water of the successful stepped leader tip in the image immediately prior to (within 20 μs before) the return stroke. Better estimates of maximum UCL lengths are determined using the downward stepped leader tip's speed of advance and the estimated return stroke time within its first frame. For 17 strokes, the upper bound length of the possible UCL averages 31.6 m and ranges from 11.3 to 50.3 m. Among the close strokes (those with spatial resolution <8 m per pixel), the five which connected to water (salt water lagoon) have 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 close strokes average 20.2 m, with slightly shorter average of 18.3 m for the five that connected to water. All the better estimates of UCL maximum lengths are <38 m in this dataset [ABSTRACT FROM AUTHOR]

The influence of the corona space charge layer created at ground level on lightning attachment to high (20–100 m) slender objects is numerically studied. The shielding effect of corona space charge produced near the object tip on the electric field at ground level is considered. The behavior of a non-stationary streamer-free corona developed from ground irregularities in a time-varying thundercloud electric field is discussed. The initiation of an upward leader from the tip of a high grounded object is simulated in the electric field created by a storm-cloud, approaching downward leader and space charge produced by corona at ground level and near the object tip. The corona space charge produced on the ground surface is shown to affect only slightly the inception of an upward connecting leader from the object when the local electric field near the object tip rises rapidly due to an approaching downward leader. The effect of neutral aerosol particles on lightning attachment to high grounded slender objects is also small. [ABSTRACT FROM AUTHOR]

Six downward negative flashes terminated on tall structures in Guangzhou are analyzed. The three-dimensional (3-D) lightning channels are reconstructed from dual-station optical observations. For each reconstructed 3-D upward connecting leader (UCL) channel, its 3-D length and speed are calculated. The 3-D length values of the six positive UCLs range from 180 to 818 m. There are 38 3-D speed values which are calculated combining the 3-D UCL channel and the high-speed images for the six UCLs. The 3-D speed values range from 0.8 to 14.3 × 10 5 m s − 1 and four of them (11%, 4/38) are on the order of 10 6 m s − 1 . For comparison, the corresponding two-dimensional (2-D) parameters are calculated using the single-station high-speed images. The values of the 2-D length and 2-D speed range from 147 to 610 m and 0.3 to 10.6 × 10 5 m s − 1 , respectively. From the statistical analysis, we determine that the average value of the 3-D speed is 1.3 times that of the 2-D speed. When the time is approaching the return stroke (RS), the propagation speed of the UCL is increasing. All of the four 3-D speed values on the order of 10 6 m s − 1 occur less than 0.2 ms prior to the RS. When the 3-D length is shorter than 300 m, 77% (20/26) of the corresponding 3-D speed values are smaller than 5 × 10 5 m s − 1 . When the 3-D length is longer than 300 m or the UCL tip height is higher than 650 m, all of the corresponding 3-D speed values are faster than 5 × 10 5 m s − 1 . [ABSTRACT FROM AUTHOR]

Abstract: The principal results of triggered-lightning experiments conducted at the International Center for Lightning Research and Testing (ICLRT) at Camp Blanding, Florida, from 1993 through 2002 are reviewed. These results include (a) characterization of the close lightning electromagnetic environment, (b) first lightning return-stroke speed profiles within 400 m of ground, (c) new insights into the mechanism of the dart-stepped (and by inference stepped) leader, (d) identification of the M-component mode of charge transfer to ground, (e) first optical image of upward connecting leader in triggered-lightning strokes, (f) electric fields in the immediate vicinity of the lightning channel, (g) inferences on the interaction of lightning with ground and with grounding electrodes, (h) discovery of X-rays produced by triggered-lightning strokes, (i) new insights into the mechanism of cutoff and reestablishment of current in rocket-triggered lightning. Selected results are discussed. [Copyright &y& Elsevier]