Archive for November, 2011

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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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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Upward Lightning Triggering Study (UPLIGHTS)

The Upward Lightning Triggering Study (UPLIGHTS) is a three year National Science Foundation funded research campaign seeking to better understand how upward lightning from tall objects is triggered by nearby flash activity.  Using coordinated optical and electromagnetic sensors, researchers from the South Dakota School of Mines and Technology and INPE Brazil will observe upward lightning from 10 tall towers in Rapid City, South Dakota, USA during the 2012-2014 summer thunderstorm seasons.

Below are map images showing the location of the research project.

 

 

Here is a view of 6 of the 10 towers from a primary observation location.  View is looking northeast from west Rapid City.

 

The objectives of this campaign are to identify the:

1)     Types of flashes (intracloud or cloud-to-ground) and their properties (polarity, current, electrical potential, distance from tall objects and propagation speed) that affect or are critical for the initiation of upward leaders from tall towers.

2)     Types of storms (e.g., mesoscale convective systems, supercell, multicell), region of storm (e.g., anvil region, convective core, trailing stratiform precipitation area), and storm development stage (e.g., mature, dissipating) during which upward lightning occurs.

3)     Conditions for triggering upward leaders on multiple tall objects during the same flash: all upward leaders initiated by one influencing component of a triggering flash or as a result of interaction between individual upward leaders in a sequential manner.

The equipment that will be used includes:

1)     Opticial sensing: multiple high-speed cameras capable for recording rates over 100,000 images per second, standard- and high-definition video cameras, and digitial still image cameras.

2)     Electromagnetic sensing: two interferometers that can 3-dimensionally locate radiated sources from lightning leader propagation, electric field meters, fast and slow field change sensors, and National Lightning Detection Network data. Interferometers will be loaned from Vaisala, Inc.

3)     Meterological: radara data from the KUDX WSR-88D weather radar located near New Underwood, South Dakota, thermodynamic sounding data obtained from and by the Rapid City NWSFO, and meteorogical surface data observed at the Rapid City NWSFO and Rapid City Regional Airport.

This research is made possible be a grant from the National Science Foundation.  We wish to acknowledge and thank NSF and Dr. Brad Smull.

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