Posts Tagged positive leaders

Characteristics of Upward Positive Leaders: Initial Development

Upward positive leaders (UPLs) that develop from the towers in Rapid City, SD usually exhibit low luminosity during their initial development.  For UPLs that do not initially branch, they tend to exhibit pulsing/stepping within the first 500 m of their growth.  The luminosity pulses originate at or near the tip of the leader with a luminosity front that travels down the leader toward the tower tip.  As the leader grows in length and brightens, the pulse frequency decreases and becomes more irregular.  The leader also then exhibit a more continuous development unlikely negative leaders which continue to clearly step during their propagation.  These observations are similar to those reported by Idone, 1992 and most recently by Biagi et al., 2011.  Below is high-speed video of an UPL’s initial development filmed at 54,000 images per second.

Below is another high-speed video of an UPL filmed at 100,000 ips.  There are two bright luminosity pulses that travel from the tip of the leader down to the tower tip during its development.

UPLs that branch shortly after initiation tend to branch widely and remain weakly luminous.  They do not exhibit pulsing/stepping like the bright non-branched leaders, however, they are very difficult to see at the higher recording speeds due to their weak luminosity.

Idone, V. P., 1992: The luminous development of Florida triggered lightning. Res. Lett. Atmos. Electr., 122, 23–28.

Biagi, C. J., M. A. Uman, J. D. Hill, and D. M. Jordan, 2011: Observations of the initial, upward-propagating, positive leader steps in a rocket-and-wire triggered lightning discharge. Geophys. Res. Lett., 38, L24809, doi:10.1029/2011GL049944.

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Recoil Leaders in Lichtenberg Figure Formation??

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.

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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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Determining lightning leader positive polarity from standard-speed video and still images

Insight gained from the analysis of high-speed camera observations and correlated electric field measurements has allowed for lightning leader polarity classification in some standard-speed video and still image recordings.  To date, recoil leaders appear to be solely associated with positive leader development and therefore provide a unique signature that can be identified in standard-speed video recordings (60 ips).  A majority of recoil leaders that form on positive leader branches tend to fade/decay without connecting to a main luminous channel, and their bipolar/bidirectional development can only be seen at recording rates greater than 5,000 ips.  Even though their duration is typically less than 500 µs, their intense brightness will record well on standard-speed video camera sensors.  During a single standard-speed video image exposure of 17 ms, numerous recoil leaders may form.  If any of the recoil leaders that form during the long exposure do not connect with a main luminous channel their integrated luminosity traces will appear detached from a main channel.  In essence, they appear as floating leader segments.  Furthermore, the positive end of the recoil leaders, upon arrival at the positive leader tip, tend to illuminate a short forked segment.  This forked segment also records clearly on standard-speed exposures and occasionally digital still images.

The video segment below shows the development of an upward positive leader recorded at 7,207 ips with a high-speed camera as well as with a standard-speed video camera (60 ips).  The high-speed recording resulted in 135 µs exposures (139 µs image intervals) and 17 ms exposures for the standard-speed recording.  A total of 122 high-speed images were recorded during each standard-speed video exposure.  The standard-speed video image is, therefore, an integration of the activity recorded by the high-speed camera during the 17 ms exposure.  Annotations on the standard-speed video show the features that identify the leader as positive due to the recoil leader production.

The following is an integrated high-speed video segment that corresponds in time to a single standard-speed video image from the previously shown upward flash.  The detached recoil leaders are clearly visible in both images.

Here are more standard-speed video images showing recoil leader development during upward positive leader propagation.

The decreased sensitivity of digital still camera sensors compared to video sensors and the longer exposure times used at night (i.e., 20 s) results in recoil leaders recording as faint leader segments.  Below is a video showing positive leader development captured at 1,000 ips.  Three different positive leaders of differing intensity show the spectrum of behavior modes exhibited by positive leaders.  The weak positive leader  (top) was weakly luminous, highly branched and produced numerous recoil leaders.  The middle positive leader was brighter and only branched a few times near the end of the recording and produced fewer recoil leaders.  The bright positive leader branch at the bottom did not branch and did not produce any recoil leaders.

The image of this event below shows how the spectrum of positive leader development appears when captured by a digital still camera.  The image was captured using ISO 100, f/6.3 and a 20 s long exposure.  Although the recoil leaders where intensely bright in the high-speed video, their short duration and the decreased ISO sensitivity of the digital still camera results in them appearing faint in the upper portion of the image.  The non-branched lower leader channel remained brightly luminous during its entire development and this recorded as a brightly luminous leader on the still image.

Below are additional examples of positive leader development associated with +CG flashes as captured by digital still camera.  The recoil leader producing positive leader branches are the primary indicator of leader positive polarity.

Negative leaders do not exhibit similar recoil leader behavior as shown in a related post.

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