Archive for category Negative Leaders

Unique Image Showing Lightning Leader Development in a Possible Potential Well

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.

Negative cloud-to-ground flash in which a negative leader branch propagated into a rainshaft and spread out vertically

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.

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Bipolar leader development prior to a +CG return stroke

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.

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Positive leader development on decayed/cutoff negative leaders

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 [2005] 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., [1998] 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.

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Protected: High-speed camera observations of bipolar/bidirectional lightning leader development near positive leaders

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Differences in positive and negative leader behavior as observed by high-speed cameras

Analysis of high-speed camera observations of lightning correlated with NLDN data and electromagnetic sensor data has shown clear differences in the appearance and propagation behavior between positive and negative polarity leaders.

Positive leaders tend to either be single bright luminous leaders that exhibit little or no branching or they are weakly luminous, branch prolifically and produce recoil leaders.  All positive leaders observed so far appear to propagate in a continuous fashion when compared to negative leaders.  Brightly luminous, non-branched positive leader channels can appear to meander with no clear sense of urgency in reaching ground or maintaining a particular direction.  However, they individually do not change direction as erratically as negative leaders.

The following is an intracloud flash in which positive leader propagated below cloud base.  Filmed at 1,000 ips, three branches are highlighted, each of which displays differing luminosity and recoil leader production.  The upper branch is weakly luminous, branches widely and produces numerous recoil leaders.  The middle branch has moderate luminosity and branches only a few times at the end of it progression and produces a lesser amount of recoil leaders.  The lower branch is bright, does not branch and exhibits no recoil leader activity.

Here is a time-integrated (stacked) image of the video segment

Weakly luminous positive leaders that produce recoil leaders are very difficult to capture with high-speed cameras.  During daylight, they are only slightly brighter than the background illumination of the sky or cloud and at night brightness from other flash activity or components can wash out the weak leaders.  Typically, exposures of at least 50 µs are needed to record them.  Below are three high-speed video segments showing weak, highly branched positive leader propagation.  The first two examples are from upward propagating positive leaders from towers and recorded at 9,000 ips (110 µs exposure).

The third example below is also an upward positive leader from a tower.  However, this video was recorded at 1,000 ips (1 ms exposure).  The longer exposure allowed for increased visualization of the weak positive leader branches that produced recoil leaders.  Three branches are annotated with the branch point and tip of the positive leader.  The branches are luminous when they first form, but then the segments nearer the branch point fade before recoil leaders begin to develop.  The yellow and red annotated branch segments produce recoil leaders that do not connect back to the branch point.  Whereas, the white annotated branch segment produces recoil leaders that connect with the branch point and cause a luminosity increase in the lower segment from the branch point to the tower tip.  The longer exposure time fails to highely resolve the recoil leader initiation points nor their bipolar/bidirectional development.  This is the challenge in observing recoil leader initiation and development relative to the weakly luminous leader branch.

Downward propagating positive leaders associated with positive cloud-to-ground flashes (+CG) also tend to be either bright and non-branched or weakly luminous, branched and produce recoil leaders.  Below is an example of a bright non-branched positive leader that propagated at a shallow angle toward the camera and connected to ground approximate 1 km in front of the camera.  The first video was recorded at 10,000 ips and the second at 100,000 ips.  In the 100,000 ips video, there appears to be luminosity variations of fairly regular intervals (on the order of 10-30 µs).  If the leader is stepping, it displays a significantly different appearance than negative leaders which will be shown later.  The NLDN indicated a +23.3 kA estimated peak current with this return stroke.

The following is an example of a highly branched, weak luminosity, recoil leader producing downward propagating leader that produces a +CG return stroke.  Frequently, a downward propagating highly branched, weakly luminous positive leader will become brighter and an accelerate as it nears the ground.  Recoil leader production in the lower segment ceases as the leader brightens.  The NLDN indicated a +33.9 kA estimated peak current return stroke.

Here is a time-integrated (stacked) image of the video segment.

Below are additional time-integrated images from downward positive ground flashes (+CG) captured with high-speed cameras.

And here is a time-integrated high-speed video image of upward positive leader development with recoil leaders from 4 towers.

 

Negative leaders, on the other hand, tend to display a pronounced stepping progression with branch leader tips much more independently erratic in their movement. Sometimes negative leaders will have branches that even appear to curl back in their propagation direction.  Furthermore, weakly luminous positive leaders are dim along their entire length and branch profusely, whereas faint negative leaders tend to have a bright tip and do not branch profusely like positive leaders.   Frequently there is a main negative channel that has its entire length luminous while its branches have less (sometimes faint) luminosity with bright tips.

Negative leader branches that decay (fade completely in luminosity) redevelop in a different fashion than positive leaders.  Instead of developing recoil leaders that initiate between the branch point and tip of the cutoff leader, negative leader redevelopment (reionization) typically initiates from the branch point of the decayed branch.  This redevelopment can initiate without any apparent triggering luminosity along the main channel from which the decayed negative branch formed or there may be a fast bright luminosity pulse that travels down the main channel which, upon arrival at the branch point, appears to initiate the redevelopment.  When redevelopment initiates at the branch point with no preceding luminosity increase along the main channel, luminosity appears to increase back along the main channel from the branch point back toward the direction from which the main channel initially propagated.  The redevelopment that initiates at the decayed negative leader branch point will propagate in a fast and continuous fashion (10×6 or 10×7 m/s) until reaching the outer extent of the initial propagation that took place before decay.  The fast, continuous leader then transitions to stepping propagation into virgin air and the corresponding speed decreases to 10×5 m/s typical of negative stepped leaders.

Below is a high-speed video segment of a negative cloud-to-ground (-CG) flash captured at 7,207 ips.

Below is a time-integrated (stack) image of the high-speed video segment ending with the beginning of the -CG return stroke.

Below is a high-speed video segment of extensive negative leader development captured at 7,207 ips prior to a -CG return stroke.  Notice the redevelopment in the decayed leader branches.

Here is a time-integrated (stacked) image of the video segment.

Below are additional high-speed video image stacks showing negative leader development.  Compare with those of positive leader development.

Below is a high-speed video segment of negative stepped leader propagation captured at 100,000 ips at a distance of approximately 1 km.  Clear stepping is visible and some of the branch segments have weak luminosity trailing a bright stepping tip.

Below is a high-speed video segment of negative leader redevelopment in two branches that decayed (7,207 ips).  The redevelopment initiates at the branch points and the reionization of the branch segment transitions from a fast continuous leader to a stepped leader upon reaching the outer extents of the initial leader development before it decayed.

Below is a high-speed video segment of negative leader redevelopment in a single decayed branch captured at 54,000 ips.  Again the redevelopment travels in a continuous fashion until reaching the end of the initial leader extent and then begins stepping.  There is a wiper in the middle of the image which obscures part of the leader segment.

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