Posts Tagged lightning
On the evening of 16 July 2012, a weak cluster of storms moved north over Rapid City, South Dakota. A single visible rainshaft formed on the leading edge of the approaching development. At the time of the rainshaft formation, there was no lightning activity along the leading edge. However, lightning flashes were visible to the distant south in the more active trailing portion of the storms. At 04:20:35, (17 July 2012) UT two digital still cameras captured a ground flash near the rainshaft. This was the first visible flash along the leading edge. One camera, a Canon 5D2 Mark III, captured the image using a 16 mm lens set at f/2.8 using ISO 800 and an exposure time of 11 sec. This camera was capturing continuous 11 sec exposures for a timelapse sequence. A second camera, a Canon 7D, captured the image using a 20 mm lens set at f/8 using ISO 100 and an exposure time of 30 sec.
The captured images, which show the entire flash due to the long exposure times, showed a unique feature that I have not seen previously with any flash images that I have captured. The visible channels below cloud base show that there was a main vertical channel that connected with ground and a branch that propagated somewhat horizontally to the left and did not connect with ground. This second branch appeared to propagate toward the rainshaft and upon entering the rain, spread out vertically in both directions while branching extensively. The change in propagation direction and increase in branching appears isolated to inside the rainshaft, and is not apparent on any other channel segments.
An analysis of National Lightning Detection Network (NLDN) data revealed the NLDN recorded a corresponding 6.8 kA estimated peak current, negative cloud to ground stroke (-CG) 8 km southwest of the cameras. This location correlated in both time and direction, and all other preceding NLDN-indicated flash activity was south of the area by 20 km.
I believe that this image provides evidence that a negative leader branch propagated into a positively charged rainshaft that served as a positive potential well favorable for negative leader propagation (Coleman et al., 2003 and Coleman et al., 2008).
Coleman, L. M., T. C. Marshall, M. Stolzenburg, T. Hamlin, P. R. Krehbiel, W. Rison, and R. J. Thomas (2003), Effects of charge and electrostatic potential on lightning propagation, J. Geophys. Res., 108(D9), 4298, doi:10.1029/2002JD002718.
Coleman, L. M., M. Stolzenburg, T. C. Marshall, and M. Stanley (2008), Horizontal lightning propagation, preliminary breakdown, and electric potential in New Mexico thunderstorms, J. Geophys. Res., 113, D09208, doi:10.1029/2007JD009459.
On the night of 12 Apr 2012, the Bay Area of California experienced a storm that literally lit up the skies with upward lightning. Some iconic photographs and video were taken during this event which provided evidence that numerous tall objects developed upward leaders in response to nearby flashes. The foremost images that illustrated what happened that night were taken by Phil McGrew. He had his Canon 5D Mark III camera running continuously using 20 sec exposures. During two of these exposures, his camera captured upward leaders that developed from the Bay Bridge and additional structures on the east side of the Bay. For each of the two photographs that he posted, it is likely that all the upward leaders developed during the same flash that probably lasted less than one second. He was located in a tall building on the east side of San Francisco downtown looking east along the Bay Bridge.
Below are embedding links as provided by Phil’s Flickr page where he has posted two images. Click on the images to go to his Flickr page. The exif data on his Flickr page indicates that the first of the two images was taken at 8:38:29 pm PDT using ISO 100, f/10, 20 sec exposure and a 28 mm lens. In this image there appears to be 5 upward leaders from the Bay Bridge structure and 2 upward leaders from two separate structures on the east side of the bay (likely in the Oakland area).
Again based on the exif data, the second image that Phil captured was at 8:42:41 pm PDT (4 min and 12 sec later) and used the same camera settings. This image (which has rightfully received international acclaim) appears to show 6 upward leaders from the Bay Bridge structure and 4 additional leaders beyond the Bay Bridge likely from structures on the east side of the bay.
Phil’s photographs indicate they were separated by 4 min and 12 sec. Not know the time accuracy of Phil’s camera, we compared the indicated times and time difference between the two images with National Lightning Detection Network (NLDN) data. Based on previous research findings, we suspected that these upward leaders were triggered by positive ground flashes (+CG) within 50 km of the Bay Bridge. Two very large estimated peak current +CG strokes were recorded at 3:39:59.425 and 3:44:12.332 pm PDT. They had estimated peak currents of +129.8 kA and +270.7 kA respectively and were separated by 4 min and 12.907 sec. There was a +27.8 kA stroke at 03:39:22.773 (37 min earlier of the first big +CG) and 2 -CG strokes at 03:40:32.843 and 03:42:21.957 fell within the time spaning the two large +CGs.
Below are GIS plots of the NLDN indicated return strokes and cloud events. The first figure shows the event location, event type by symbol (see legend) and estimated peak current based on relative symbol size. Notice the size of the +CG return stroke symbols relative to the other events.
The next figure shows the NLDN event locations and their times.
The last figure shows the NLDN events and a label of the estimated peak current.
We suspect that the upward leaders that developed from the Bay Bridge were positive polarity and developed following the large estimated peak current positive cloud-to-ground return strokes that occurred inside the Bay. These are examples of lightning triggered upward lightning in which the field change resulting from a preceding flash causes the development of upward leaders from nearby tall objects.
There were a number of other images from other people that showed upward leaders from tall objects during this same night and the other locations included the Golden Gate Bridge and tall buildings in Oakland. We suspect that these upward leaders also developed during the same triggering flashes that caused the upward leaders to develop from the Bay Bridge.
Over the past six years my research colleagues and I have filmed lightning using high-speed digital cameras. In total we have captured 776 naturally occurring lightning flashes with recording speeds as high as 100,000 images per second. 158 of these flashes were cloud flashes in which some of the lightning leaders propagated outside of the clouds. 372 of theses flashes were negative cloud-to-ground flashes (-CG) and 206 were positive cloud-to-ground flashes (+CG). 41 of the flashes were upward flashes originating from tall towers in Rapid City.
During this last summer, we pursued a storm into the Badlands of South Dakota. The Badlands are a beautiful area of erosion in the plains creating incredibly photogenic landscapes, and it is personally one of my favorite places to photograph lightning. On this particular day, I was filming from the Pinnacles Overlook looking east across a road. I filmed a number of flashes, but during one instance I not only captured a +CG flash, I also captured a rare wild tourist roaming the South Dakota plains. Because I film from a highly modified truck with cameras and gadgets sticking out of it, he was a bit curious by the appearance of my vehicle. However, he was clearly more interested in getting to the next viewpoint and quickly scurried off never to be seen again. Here is the video…
Below is a video on the creation of Lichtenberg figures. Interesting is the subsequent bright short discharges that continue to take place after the initial discharge. These seem similar in appearance to recoil leaders, which form on positive leaders branches that become cutoff from a main channel. Compare the two videos below.
YouTube video of Lichtenberg creation.
Upward lightning (upward positive leaders) from a tower filmed at 9,000 images per second.
The following high-speed video recorded at 10,000 images per second shows bipolar leader development that precedes a +CG return stroke. A non-branched positive leader emerges below cloud while negative stepped leaders are visible propagating upward from an apparent common origin/initiation area obscured within the cloud. The negative leaders are likely a portion of the upper leader network associated with the bipolar/bidirectional leader development that initiated in the cloud. There is in-cloud brightening to the right of the common area as well that extends away from the camera to the right, and these are likely additional leader channels associated with the flash leader network that are propagating in-cloud. Prior to the +CG return stroke an additional non-branched positive leader emerges below cloud to the right of the first positive leader. Upon connection of the positive leader with the ground, a bright return stroke travels up the previously formed leader network resulting in an intense brightening of the visible negative leaders. These negative leaders appear more energetic and appear to propagate faster once ground potential (or near ground potential) is raised to their tips via the return stroke. Their growth continues following the return stroke resulting in the continuing current seen from the leader tips to the ground connection point. The obscured leader network extending back into the cloud to the right maintains an increased brightness as well following the return stroke and exhibits some brightness pulses which are also seen in the lower return stroke channel segment.
The NLDN recorded an optically correlated +19.1 kA estimated peak current “IC” event.
In a previous post I discussed the observation that positive leaders are often visible below cloud base in association with so called “Spider Lightning.” I suspect that these positive leaders form when horizontally extensive negative leader development that propagates just above cloud base decays or becomes cutoff from their initial bipolar development or from the ground termination point in the case of the extensive horizontal negative leader development that frequently follows a +CG return stroke. These positive leaders, which are below cloud base, tend to lag behind or trail the negative leader propagation which is just above cloud base. On some occasions a positive leader associated with this secondary positive leader development will connect with ground resulting in a +CG return stroke. Since the positive leader initiated on part of the original negative leader network that formed, the subsequent return stroke will traverse this previously formed network and cause further extension of the negative leaders (through new negative breakdown in virgin air) once it reaches the outer extents of the negative leader development that formed prior to the return stroke. This can result in a continuation of horizontally extensive negative leader development that travels 10s of kilometers. If another cycle of negative leader cutoff followed by positive leader formation and subsequent +CG return stroke occurs, this horizontal extension of negative leaders can continue for very large distances exceeding 100 km. The resulting field change (or charge moment change) associated with these horizontally extensive flashes can initiate transient luminous events (TLEs) and/or upward positive leaders from tall towers.
On 8/30/11 UT, I was able to record this apparent process with a high-speed camera operating at 10,000 ips. The flash originated to the northeast of my location and a +54.7 kA estimated peak current, +CG return stroke occurred 28 km away at 04:29:11.708 UT based on the NLDN. A standard-speed video camera recorded this correlated return stroke and a sharp brightness increase just outside of the high-speed camera’s field of view also correlated with the return stroke. Horizontal negative leader development following the return stroke propagated just above cloud base towards my location (about 1 km south of Tower 6, see UPLIGHTS post). A few of the leaders were visible just below clouds base and these had the appearance and propagation characteristics of negative leaders. Additionally, electric field sensing equipment located about 5 km to my west recorded a negative field change (atmospheric electricity sign convention) that correlated with the approach of negative leaders. As this development passed over the towers (and overhead the camera) short duration attempted upward leaders were visible from multiple towers. Eventually, weak upward leaders from three towers initiated in close succession (within 7 ms). Two of these leaders exhibited weak recoil leader activity suggesting they were positive polarity. A wide field of view standard-speed camera located 5 km further west than the high-speed camera captured more of the visible negative leaders that emerged just below cloud base as they passed over the towers and the high-speed camera. (See the standard-speed video below).
After the upward leaders decayed and the brightness associated with the horizontal negative leader development decreased, positive leaders were seen to develop downward from multiple locations along the path the negative leaders passed previously. All of the weakly luminous positive leaders had branches that exhibited recoil leader activity. One of the positive leaders connected with the ground at 11.938 UT (in the high-speed camera’s field of view) and the NLDN recorded a corresponding +12.8 kA estimated peak current cloud flash, “+IC” even though there was a clear connection with the ground. The return stroke resulted in a reillumination of the western portion of the original negative leader network path that formed prior to the return stroke, and in fact a negative leader was clearly visible following the return stroke in the same area traversed previously by the horizontal negative leader development. One leader appeared to be new negative leader breakdown in virgin air likely forming a new channel near the previously formed channels. In addition, negative leaders were again visible just below cloud base, but further west than before as seen in the standard-speed video.
As observed frequently with +CG flashes, recoil leaders continued to be active on branches of the downward positive leaders even after the return stroke suggesting these branches were cutoff from the main downward propagating positive leader at the time of the return stroke. These branches did not, therefore, participate in the return stroke (i.e., the return stroke did not travel into these branches during its upward travel from the ground connection point).
The second return stroke did not initiate any upward leaders from the other towers nor did it reinitiate upward leaders from those towers that previously developed upward leaders.
Below is the high-speed camera recording from this flash.
Ron Thomas at New Mexico Tech, gave me an LMA animation showing extensive horizontal negative leader development with 4 sequential +CG return strokes that trailed behind the VHF sources (leading edge of the negative leader development). I suspect that this flash was similar to the one presented here in that positive leaders formed on cutoff ends of the negative leader development and connected with the ground forming +CGs in trail of the preceding negative leaders.
Furthermore, Carey  discussed an LDAR II’s depiction of a horizontally extensive flash in which the “long-lived, spatially extensive, and horizontally stratified lightning channels are clearly reminiscent of the spider lightning activity observed by Mazur et al.,  in stratiform precipitation as part of the intracloud component of a positive CG lightning flash.” He described that the LDAR II recorded VHF sources for one segment as becoming noisy and spatially incoherent in the area of the previously identified channel segment (i.e., there were previously coherent VHF sources that first traveled along the segment). This was followed by a +CG return stroke after which the sources become more spatially coherent and spatially extensive. I believe his description illustrates the initial horizontal negative leader development (first coherent sources that form the channel segment), the subsequent recoil leader activity associated with the positive leaders that form on the cutoff negative leaders (noisy and spatially incoherent sources generated by the spatially separated and non-coherent initiation and propagation of the negative polarity end of recoil leaders that form on cutoff positive leader branches), the +CG as one of the positive leaders connects with ground, and the expansion of new negative leader development following the +CG return stroke.
Carey, L. D., M. J. Murphy, T. L. McCormick, and N. W. S. Demetriades (2005), Lightning location relative to storm structure in a leading-line, trailing-stratiform mesoscale convective system, J. Geophys. Res., 110, D03105, doi: 10.1029/2003JD004371.
Mazur, V., X. Shao, and P. R. Krehbiel (1998), ‘‘Spider’’ lightning in intracloud and positive cloud-to-ground, J. Geophys. Res., 103(D16), 19,811 –19,822.