Lightning attachment process involving connection of the downward negative leader to the lateral surface of the upward connecting leader

Transcription

1 GEOPHYSICAL RESEARCH LETTERS, VOL. 40, , doi: /2013gl058060, 2013 Lightning attachment process involving connection of the downward negative leader to the lateral surface of the upward connecting leader Weitao Lu, 1,2 Luwen Chen, 3 Ying Ma, 1,2 V. A. Rakov, 4 Yan Gao, 1,2 Yang Zhang, 1,2 Qiyuan Yin, 3 and Yijun Zhang 1,2 Received 18 September 2013; accepted 4 October 2013; published 21 October [1] High-speed video camera records, with a temporal resolution of 20 μs and a spatial resolution of 2.4 m per pixel, of a downward negative lightning flash that terminated on a 440 m high building are examined. The attachment process in this flash exhibited an unexpected behavior in that the downward leader tip connected to the lateral surface of the ~400 m upward connecting leader (UCL) below its tip. It appears that the effect of the downward leader on the UCL is significant, while the effect of the UCL on the downward leader is negligible, except for the final 80 μs preceding the beginning of the first return stroke. The ratio of speeds of the downward leader and the UCL tends to decrease with time, ranging from 1.8 to 0.12, although the lower m or so of the UCL were too faint to allow speed measurements. Citation: Lu, W., L. Chen, Y. Ma, V. A. Rakov, Y. Gao, Y. Zhang, Q. Yin, and Y. Zhang (2013), Lightning attachment process involving connection of the downward negative leader to the lateral surface of the upward connecting leader, Geophys. Res. Lett., 40, , doi: /2013gl published a streak photograph of a lightning discharge to a 70 m tower, in which the steps of the downward leader are seen clearly and the evidence of the UCL is given. Using an eight-channel photodiode system, Wang et al.[1995]resolved the luminous features of lightning attachment to the CN tower with a temporal resolution of 0.2 μs and a spatial resolution of 29 m. In recent years, high-speed video cameras have been used to observe the attachment process of lightning to tall structures with relatively high spatial and temporal resolution [e.g., Lu et al., 2010; Warner, 2010]. [4] This paper presents a negative lightning flash with the junction point between the downward and upward connecting leaders being below the UCL tip. To the best of our knowledge, such behavior has never been reported before. For this flash, high-speed video camera records with a temporal resolution of 20 μs and a spatial resolution of 2.4 m enabled us to analyze in detail the propagation characteristics, including direction of extension, speed, and luminosity, of the downward leader and the UCL during the attachment process preceding the first return stroke. 1. Introduction [2] Understanding of the lightning attachment process is vital for improving lightning protection techniques. Unfortunately, due to the low sensitivity and low spatial or temporal resolution, most of the existing optical images of natural lightning discharges are not suitable for a thorough analysis of the propagation characteristics of the downward leader and the upward connecting leader (UCL) during the attachment process. [3] Streak camera has been used to observe the development of lightning discharges for many years [e.g., Uman, 1987; Rakov and Uman, 2003]. Berger and Vogelsanger [1966] 1 Laboratory of Lightning Physics and Protection Engineering, Chinese Academy of Meteorological Sciences, Beijing, China. 2 State Key Laboratory of Severe Weather, Chinese Academy of Meteorological Sciences, Beijing, China. 3 Lightning Protection Center of Guangdong Province, Guangzhou, Guangdong, China. 4 Department of Electrical and Computer Engineering, University of Florida, Gainesville, Florida, USA. Corresponding author: Dr. W. Lu, Laboratory of Lightning Physics and Protection Engineering, Chinese Academy of Meteorological Sciences, No. 46 Zhongguancun South Ave., Haidian District, Beijing , China. (wtlu@cams.cma.gov.cn) The Authors. Geophysical Research Letters published by Wiley on behalf of the American Geophysical Union. This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made /13/ /2013GL Experiment [5] A field experiment, mainly focusing on the observation of lightning flashes terminating on tall structures, has been conducted since 2009 in Guangzhou, Guangdong, China [Lu et al., 2010]. We established the Tall-Object Lightning Observatory in Guangzhou (TOLOG), which is located on the top of a building with a height of approximately 100 m that belongs to the Guangdong Meteorological Bureau. Several types of instruments are installed at the TOLOG to simultaneously measure the acoustic, optical, electric field, and magnetic field signals produced by lightning discharges [Lu et al., 2012]. [6] In this paper, we analyze a downward negative lightning flash that occurred at 07:13:32 (UT) on 18 July 2011 and struck the 440 m high Guangzhou International Finance Center (GIFC). This flash is labeled F1111 in our database. F1111 is captured by two Photron FASTCAM SA5 cameras (HC-1 and HC-2) and one Photron FASTCAM SA3 camera (HC-3). The three cameras are operated at sampling rates of 10,000 frames per second (fps) (100 μs per frame), 50,000 fps (20 μs per frame), and 1000 fps (1 ms per frame), respectively. Their recording lengths are set at 0.2 s, 24 ms, and 1.2 s with pretrigger lengths of 0.1 s, 20 ms, and 0.1 s, respectively. HC-1 and HC-2 images are mainly analyzed here. The focal length of the lens mounted on HC-1 is 24 mm and 20 mm on HC-2. The distance from the TOLOG to the GIFC is approximately 2.4 km, and the corresponding spatial resolutions of the HC-1 and HC-2 images are 2.0 and 2.4 m per pixel, respectively. 5531

2 Height AGL (m) Results (a) ms 440 m AGL (b) 1.54 ms Figure 1. (a and b) High-speed video camera (HC-1) images of flash F1111 captured with a sampling rate of 10,000 fps. Figure 1a is cropped vertically to save space. Time 0 is set at the beginning of the first return stroke, which occurs during the frame (not shown here) immediately following Figure 1a with most pixels saturated. Figure 1b shows the frame 1.54 ms after the beginning of the first return stroke. [7] From analysis of the electric field change record and HC-3 images (not shown here), we determined that F1111 was a downward negative cloud-to-ground lightning flash that lasted for approximately 330 ms and contained eight return strokes (along a single path) with interstroke intervals ranging from 11 ms to 117 ms. Four return strokes (first, third, fifth, and sixth) were recorded by the Lightning Location System (LLS) of the Guangdong Power Grid Corporation, whose detection efficiency and location accuracy have been evaluated by Chen et al. [2012]. The LLS-reported peak current values are 102, 29, 63, and 26 ka, respectively. [8] Selected high-speed images of F1111 are shown in Figures 1 and 2. Time 0 is set at the beginning of the first return stroke (hereinafter, the first is omitted for brevity). The time stamps of the HC-1 and HC-2 images are synchronized by using Figures 1a and 2b with the same end time of exposure duration because the UCL lengths in those images are almost the same, namely, 333 m and 336 m, respectively. The downward leader of F1111 induces three upward leaders (one UCL and two unconnected leaders) from the top of the GIFC. The two unconnected upward leaders are very faint, so they cannot be distinguished easily from the background in high-speed camera images. The primary branch of the downward leader (PBDL) that facilitates connection to the building and all the three upward leaders, reconstructed by using the HC-2 images, are shown in Figure Two-Dimensional Extension Directions of the PBDL and the UCL [9] Like most natural downward lightning flashes, F1111 exhibits a downward leader with many branches. The branches of the downward leader extend in a large spatial region and even beyond the HC-1 field of view, as shown in the top left and top right corners in Figure 1a. The extension directions of the individual branches of the downward leader appear to be random (although a downward trend on the whole is clear), except for the PBDL during the final 80 μs prior to the beginning of the return stoke (Figures 2a 2d). From the HC-1 images, it appears that the PBDL and the UCL bend toward each other during the final 60 μs, and finally, their tips make contact, which is a generally expected lightning behavior during the attachment process. However, the HC-2 images reveal more details and give us an additional insight into the attachment process of F1111. [10] From Figures 2 and 3, it can be clearly seen that the PBDL bends sharply toward the UCL after the two-dimensional (2-D) distance between them decreases to less than approximately 60 m, then extends toward the UCL and finally makes contact with the UCL lateral surface. The connection occurs between TD (the tip of the PBDL in Figure 2d) and a point, HU, located on the UCL, but below its tip (TU) in Figure 2d. It is known that some flashes strike tall, stationary structures below their tops [e.g., Gorin et al., 1976; Hussein et al., 2007]. In our case, however, the downward leader connects to the UCL (plasma channel) below its tip when its extension is in progress. [11] The 2-D length of the UCL in Figure 2d is 403 m and that of the UCL part higher than HU is 67 m. It is impossible to detect propagation of the PBDL and the UCL after the end of the exposure time of Figure 2d because the return stroke causes most pixels in the following frame (not shown here) to be saturated. It can be inferred that the final 2-D length of the UCL is longer than 403 m and that of the UCL part above HU is over 67 m. As shown in Figures 2d, 2e, 2f, and 3, the UCL at heights above HU propagates predominantly upward, not toward the PBDL tip, although with a little slant toward the PBDL. We infer that the effect of the PBDL on the UCL is limited (even though the 2-D distance between them decreases to less than 60 m) and that the integrated effect of all branches of the downward leader produces a stronger attractive effect to the UCL than the PBDL alone Two-Dimensional Propagation Speeds of the PBDL and the UCL [12] The 2-D propagation speeds (hereinafter speeds ) of the PBDL and the UCL, V d and V u, respectively, calculated by using high-speed images, are plotted versus time in Figure 4a. It should be noted that not all frames were suitable for unambiguous identification of the progressing leader tip. A total of 21 V d values were obtained from the HC-1 images after the PBDL entered the HC-1 field of view and 37 V d values from the HC-2 images. The initial (before 0.7 ms) 83 m of the UCL channel cannot be discerned from the HC-1 images due to the low luminosity of the UCL. Seven HC-1 frames can be used to analyze the propagation characteristics of the UCL, and six V u values can be obtained. By using the HC-2 images, the initial (before 640 μs) 96 m of 5532

3 (a) -80 µs (b) -60 µs TU TD (c) -40 µs (d) -20 µs 1040 HU TD TU HU Height AGL (m) (e) 0.5 ms (f) Composite image 440 Figure 2. Frames of flash F1111 acquired using HC-2 operating with a sampling rate of 50,000 fps. (a e) Individual frames. (f) A 76-frame composite image. The 76 frames include 75 consecutive frames preceding the return stroke and the frame shown in Figure 2e. All the images are inverted for a better view. TD is the tip of the primary branch of downward leader (that facilitated connection to the building) in Figure 2d, the last frame preceding the return stroke. TU is the tip of the UCL in Figure 2d. HU is the highest UCL point that is discernible in the return stroke channel in Figure 2e. the UCL channel cannot be discerned. A total of 20 HC-2 frames can be used to analyze the propagation characteristics of the UCL, and 19 V u values can be obtained. [13] From Figure 4a, it can be seen that both the PBDL and the UCL have speeds mainly on the order of 10 5 ms 1. From the HC-1 images, V d ranges from to ms 1 (with an average value of ms 1 ) between the heights of 700 m and 1320 m AGL, and V u ranges from to ms 1 (average: ms 1 ) between 490 m and 720 m. From the HC-2 images, V d ranges from to ms 1 (average: ms 1 ) between the heights of 700 and 1070 m, and V u ranges from to ms 1 (average: ms 1 ) between 500 and 790 m. [14] The ratio of the speeds of the PBDL and the UCL (V d /V u ), the 2-D distance between their tips (D dt-ut ), and the 2-D distance between the PBDL tip and the closest point on the UCL channel (D dt-u ), each versus time, are shown in Figure 4b. The V d /V u calculated by using the HC-1 and HC-2 images range from 1.2 to 0.28 and from 1.8 to 0.12, respectively. Overall, V d /V u exhibits a decreasing trend because V d shows no clear change, except for the final 80 μsprior to the beginning of the return stroke, while V u tends to generally increase, as shown in Figure 4a. During the final 160 μs, as D ut-dt decreases to less than 150 m, V u sharply increases from to ms 1.Duringthefinal 60 μs, although the predominantly upward direction of the UCL extension above point HU results in the increase of the D ut-dt, D dt-u (always equal to D ut-dt before 60 μs) decreases continuously. From Figure 2a to 2d, after the PBDL bends toward the UCL, D dt-u decreases from approximately 60 m to 30 m, and V d increases from to ms 1.Duringthefinal 80 μs, although both V d and V u increase sharply, the former increases faster, so that V d /V u increases from 0.12 to Luminosities of PBDL and UCL Tips [15] In this paper, the average grey level of the five brightest pixels near the leader tip in the high-speed image is used to represent the luminosity of the leader tip. The variations of the leader tip luminosity versus time for the PBDL 5533

4 the downward leader branches appears to be negligible, except for the final 80 μs, which is evidenced by the lack of changes in the extension direction, speed, and luminosity of the downward leader until the distance between the PBDL and the UCL decreases to less than approximately 60 m (see Figures 2, 3, 4a, and 4c). The speeds of the PBDL and the UCL, V d and V u, are both mainly on the order of 10 5 m s 1. V d exhibits no clear trend before 80 μs and sharply Figure 3. The primary branch of the downward leader that facilitated connection to the building and the three upward leaders reconstructed from the HC-2 images. The channel between TD and HU can be only reconstructed from unsaturated post-return-stroke images (e.g., Figure 2e). and the UCL obtained from the HC-2 images are shown in Figure 4c. Similar to the variation of speed, the variation of luminosity of the PBDL tip exhibits no clear trend until the final 80 μs prior to the beginning of the return stroke, while the luminosity of the UCL tip exhibits an overall increasing trend, although the trend before 300 μs is not clear. 4. Summary and Discussion [16] High-speed video camera records showing the details of the connection between the downward leader and the UCL in a natural downward negative lightning flash that terminated on the 440 m high Guangzhou International Finance Center, China, are analyzed. In this flash, the presence of multiple branches of the downward leader appears to be causing the occurrence of multiple upward leaders from the top of the building and the propagation of the UCL without clear bending toward the PBDL (the primary branch of the downward leader that facilitated connection to the building). The latter behavior apparently resulted in the connection of the PBDL to the UCL over 67 m below the UCL tip. There is no information on how the PBDL and the UCL propagated after Figure 2d. V d was increasing sharply prior to the return stroke and reached ms 1. It is possible that the PBDL propagated all the 42 m between TD and HU (see Figure 3) and finally made contact with the UCL at HU, i.e., the junction point may be HU. It is also possible that a branch of the UCL initiated from HU and made contact with the PBDL, i.e., the junction point may be located somewhere between TD and HU. It should be mentioned that the flash presented here is not the only event inwhichweobservedtheconnectionoccurringbelowtheucl tip, although there are cases that do show connections between the tips of the downward leader and the UCL. Further analysis of the phenomenon presented in this paper is in progress. [17] The predominantly upward direction of the UCL extension (see Figures 2 and 3) and the overall increase of both speed and luminosity of the UCL (Figures 4a and 4c) suggest that the integrated effect of (electric field produced by) all branches of the downward leader on the UCL is significant, especially during the final 160 μs preceding the beginning of the return stroke. In contrast, the effect of the UCL on Figure 4. (a) Variations of 2-D speeds of the PBDL and the UCL, V d and V u. (b) Variations of V d /V u, the 2-D distance between the tips of the PBDL and the UCL (D dt-ut ), and the 2-D distance between the PBDL tip and the closest point on the UCL channel (D dt-u ). (c) Variations of the leader tip luminosity for the PBDL and the UCL. 5534

5 increases from to ms 1 during the final 80 μs, while V u tends to increase in general and sharply increases from to ms 1 during the final 160 μs. The ratio of V d and V u exhibits an overall trend to decrease, with the range of variation being from 1.8 to 0.12, although the lower m or so of the UCL were too faint to allow speed measurements. [18] The optical properties of the downward negative stepped leader have been studied by many researchers [e.g., Krider, 1974; Orville and Idone, 1982; Chen et al., 1999; Lu et al., 2008; Hill et al., 2011]. Observations of downward and upward connecting leaders during the attachment process preceding the first return stroke in natural lightning flashes are limited [e.g., Lu et al., 2010; Warner, 2010]. The ranges and average values of the speeds of the downward leader and the UCL presented in this paper are consistent with those reported previously. Detailed information on variations of the ratio of speeds of downward and upward connecting leaders and the luminosities of leaders during the attachment process is not found in the previous literature. The observed characteristics of leaders are important for understanding the lightning attachment process, which is one of the least understood lightning processes. The results presented in this paper also provide information needed in developing leader progression models [e.g., Dellera and Garbagnati, 1990; Rizk, 1994; Mazur et al., 2000]. In particular, there is no consensus on the time variation of V d /V u. For example, Dellera and Garbagnati [1990] assumed that the ratio of speeds of the negative downward leader and the positive UCL changes from 4 to 1 during the attachment process, while Rizk [1994] assumed that V d /V u =1,andMazur et al. [2000] assumed that V d /V u =2.Our data show that the V d /V u exhibits an overall trend to decrease. We also found that V u can significantly exceed V d, and hence, V d /V u can be significantly less than 1. To the best of our knowledge, this latter possibility has never been considered in the models. [19] Acknowledgments. This work was supported in part by the National Natural Science Foundations of China under grant , , and ; by the Basic Research Fund of Chinese Academy of Meteorological Sciences under grant 2010Z004; and by the U.S. National Science Foundation. [20] The Editor thanks two anonymous reviewers for their assistance in evaluating this paper. References Berger, K., and E. Vogelsanger (1966), Photographische Blitzuntersuchungen der Jahre auf dem Monte San Salvatore, Bull. Schweiz. Elektrotech. Ver., 57, Chen, M., N. Takagi, T. Watanabe, D. Wang, Z.-I. Kawasaki, and X. Liu (1999), Spatial and temporal properties of optical radiation produced by stepped leaders, J. Geophys. Res., 104(D22), 27,573 27,584, doi: /1999jd Chen, L., Y. Zhang, W. Lu, D. Zheng, Y. Zhang, S. Chen, and Z. Huang (2012), Performance evaluation for a lightning location system based on observations of artificially triggered lightning and natural lightning flashes, J. Atmos. Oceanic Technol., 29(12), , doi: / JTECH-D Dellera, L., and E. Garbagnati (1990), Lightning stroke simulation by means of the leader progression model. I. Description of the model and evaluation of exposure of free-standing structures, IEEE Trans. Power Delivery, 5(4), , doi: / Gorin, B. N., V. I. Levitov, and A. V. Shkilev (1976), Distinguishing features of lightning strokes to high constructions, in Proceedings of the Fourth International Conference on Gas Discharges, IEE Conf. Publ. no. 143, pp , Institution of Electrical Engineers, Swansea, UK. Hill, J. D., M. A. Uman, and D. M. Jordan (2011), High-speed video observations of a lightning stepped leader, J. Geophys. Res., 116, D16117, doi: /2011jd Hussein, A. M., M. Milewski, W. Janischewskyj, F. Noor, and F. Jabbar (2007), Characteristics of lightning flashes striking the CN Tower below its tip, J. Electrost., 65(5 6), , doi: /j. elstat Krider, E. P. (1974), The relative light intensity produced by a lightning stepped leader, J. Geophys. Res., 79(30), , doi: / JC079i030p Lu, W., D. Wang, N. Takagi, V. Rakov, M. Uman, and M. Miki (2008), Characteristics of the optical pulses associated with a downward branched stepped leader, J. Geophys. Res., 113, D21206, doi: / 2008JD Lu, W., Y. Zhang, L. Chen, E. Zhou, D. Zheng, Y. Zhang, and D. Wang (2010), Attachment processes of two natural downward lightning flashes striking on high structures, paper presented at 30th International Conference on Lightning Protection, Power and Energy Soc., Cagliari, Italy. Lu, W., L. Chen, Y. Zhang, Y. Ma, Y. Gao, Q. Yin, S. Chen, Z. Huang, and Y. Zhang (2012), Characteristics of unconnected upward leaders initiated from tall structures observed in Guangzhou, J. Geophys. Res., 117, D19211, doi: /2012jd Mazur, V., L. H. Ruhnke, A. Bondiou-Clergerie, and P. Lalande (2000), Computer simulation of a downward negative stepped leader and its interaction with a ground structure, J. Geophys. Res., 105(D17), 22,361 22,369, doi: /2000jd Orville, R. E., and V. P. Idone (1982), Lightning leader characteristics in the Thunderstorm Research International Program (TRIP), J. Geophys. Res., 87(C13), 11,177 11,192, doi: /jc087ic13p Rakov, V. A., and M. A. Uman (2003), Lightning: Physics and Effects, Cambridge Univ. Press, New York. Rizk, F. A. M. (1994), Modeling of lightning incidence to tall structures. I. Theory, IEEE Trans. Power Delivery, 9(1), , doi: / Uman, M. A. (1987), The Lightning Discharge, Dover, New York. Wang, D., Z. I. Kawasaki, K. Yamamoto, K. Matsuura, J.-S. Chang, and W. Janischewskyj (1995), Luminous propagation of lightning attachment to CN tower, J. Geophys. Res., 100(D6), 11,661 11,667, doi: / 95JD Warner, T. A. (2010), Upward leader development from tall towers in response to downward stepped leaders, paper presented at 30th International Conference on Lightning Protection, Power and Energy Soc., Cagliari, Italy. 5535