Protected: High-speed camera observations of bipolar/bidirectional lightning leader development near positive leaders
Protected: Determining lightning leader positive polarity from standard-speed video and still images
On the night of 8/24/11, a leading-line/trailing stratiform mesoscale convective system developed and moved over Toronto, Canada. The heart of the trailing stratiform region passed directly over the 553 m tall CN Tower and the people of Toronto were treated to an incredible light show as the tower unleashed at least 34 upward flashes over the span of an hour. Wilke and Elizabeth See-Tho graciously provided me some video of the event and my analysis suggests that all of the upward flashes were triggered by preceding flash activity (lightning-triggered lightning) similar to what I observe in Rapid City, South Dakota. For each case there was clearly in-cloud flash activity that preceded the upward leader initiation. In addition, recoil leaders were visible in a large majority of the upward leaders suggesting they were positive polarity.
Below is a composite image where I stacked selected images from the See-Tho’s video. As you can see, the CN Tower was literally ablaze with lightning leaders over the span of the storm.
Below is the edited video provided by the See-Tho’s. This version plays in real time showing all 34 upward flashes and one spider lightning flash.
Below is the the same video sped up.
Below is video of each flash played at normal speed and in slow motion (total runtime 34 min).
Although I have not obtained nor analyzed lightning data for this storm, I suspect that a majority of the upward flashes were triggered by a preceding +CG flash within 50 km of the tower. Horizontally extensive positive charge regions that form in the trailing stratiform regions of MCSs serve as potential wells for negative leaders that can travel upwards of 100 km. This horizontally extensive negative leader development can take place during an intracloud flash and/or following a +CG return stroke. The negative field change (atmospheric electricity sign convention) experienced at a tall tower by the approach of negative leaders or nearby +CG return stroke can initiate upward propagating positive leaders. The conditions apparently were ideal for this triggering process and weather radar shows this was likely the case.
Below is a radar loop (base reflectivity, 0.5 degree tilt) of the storm development and passage over the CN Tower spanning from 00:02 – 03:41 UT, 8/25/11. The See-Tho’s stated that the first upward flash was shortly after 02:00 UT. This places the leading line convective region just east of the CN Tower with the tower in an area of decrease reflectivity between 30-40 dBz. The tower would stay under this level of reflectivity (i.e., the trailing stratiform precipitation area) until 03:41 UT. The last upward flash the See-Tho’s recorded was at approximately 03:06, but they thought there were a few more upward flashes that followed after they stopped filming.
This truly was a perfect storm to produce upward lightning flashes. I suspect that many transient luminous events (TLEs) in the form of halos and/or sprites may have also been produced by the very same triggering flashes responsible for initiating the upward leaders. The CN Tower is instrumented to measure current through the tower and there is an array of optical sensors including a high-speed camera within 3 km of the tower. Hopefully, all the instrumentation was operational and an outstanding data set was captured.
Are the visible leaders that crawl along the cloud base during spider lightning positive or negative or both? Mazur et al.,  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.
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.