Archive for category Spider Lightning

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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Positive leaders associated with Spider Lightning

Are the visible leaders that crawl along the cloud base during spider lightning positive or negative or both?  Mazur et al., [1998] suggested they were negative for a decaying thunderstorm in Florida.  Optical observations using high-speed cameras suggest that most of those observed on the Great Plains may be positive polarity.  However, negative leaders appear to propagate horizontally in-cloud (above cloud base) prior to the formation of the visible positive leaders that propagate below cloud base.

The video below shows four flashes from the backside of a linear MCS that passed over Rapid City, SD on 6/13/11.  These four flashes exhibited the characteristics of spider lightning and correlated electric field data is being analyzed.  I have annotated what I think is the negative and positive leader development during these flashes.  Positive leaders that produce recoil leaders can be identified in standard-speed video as having detached (floating) leader segments and brightly forked tips.  The features are actually integrated recoil leaders that occur during each of the 17 ms video images.  A future post will discuss how positive leaders can be identified on standard-speed video and digital still images.

In 2008, I captured an impressive spider lightning flash that passed over Rapid City, SD on 6/25.  A portion of this flash was recorded with a high-speed camera at 7,207 images per second (ips).  Here is my analysis of the standard-speed video recording

As shown in the preceding video, a portion of the visible channels in the lower left side of the flash were recorded by a high-speed camera.  Here is the video from that recording.

Below is a time-integrated (stacked) image from the high-speed video segment.

There are many recordings of spider lightning (also called anvil crawlers by storm chasers) on YouTube.  Here are some of the better recordings.  Note the similar pattern of preceding in-cloud brightness followed by apparent positive leader development.

I have personally reviewed the 1,000 ips high-speed video recorded by Dr. Mazur and although the quality suffered from compression, there does not appear to be any recoil leader activity or forked tips on the visible leaders that is characteristic of positive leaders.  The pattern of branching by the leaders also resembles that of negative leaders.  However, I cannot confidently say they are definitely negative leaders due to the quality of the recording and the fact that most of the recording had a large portion of the image saturated by the brightness of the flash.  Dr. Marcelo Saba showed me a high-speed recording he captured, and there were clearly negative leaders visible just below cloud base.  A positive leader developed after the negative leader passed and a +CG return stroke resulted.  If spider flashes have extensive horizontal negative leader development that spatially precedes the visible positive leaders, then supporting electric field sensor data should indicate a negative field change (atmospheric electricity sign convention) due to the approach of the negative leaders.  This should change to positive if positive leaders later approach and dominate the signal.  However, in many of the standard-speed recordings, the positive leaders do not seem to travel as far as the preceding incloud activity.

Correlated observations using a LMA or interferometer and high-speed camera along with electric field sensors would likely show the relative location and timing of the negative leader development in-cloud (abundant spatially coherent LMA sources due to noisier propagation, visible in-cloud brightening and negative field change) and the positive leader development below cloud (less LMA sources with those produced by recoil leaders being spatially incoherent, visible recoil leaders and positive field change).

Furthermore, the horizontal leader development (both in-cloud and visible leaders) must be put in context of the entire flash they are associated with.  Did they develop as part of an intracloud flash or ground flash?  Where (and when relative to the spider) did any ground strokes associated with the flash occur?  I have witnessed positive leaders propagating below cloud base that produce both +CG and -CG strokes.  In the +CG case, a branch of the positive leader goes to ground.  In the -CG case, the negative end of a recoil leader formed during positive leader development goes to ground.  I will show and discuss examples of these to processes in a future post.

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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