Posts Tagged lightning-triggered lightning
Early in the morning of 26 June 2018 in southwest Kansas, something wonderful happened. A lightning flash occurred that caused additional lightning to rise up from 14 wind turbines filling the sky with blinding channels of light. Hank Schyma (an accomplished storm chaser, photographer/videographer and all around interesting guy also known as Pecos Hank) was there to witness this amazing spectacle and captured it on video. A huge mesoscale convective system had developed earlier in the evening, and he had positioned himself on the trailing side in hopes of capturing massive horizontally extensive lightning flashes that tend to develop in the trailing stratiform region. He was not disappointed. He witnessed numerous spectacular flashes and a number of these involved upward leaders developing from a wind turbine complex nearby. He reached out to me and other scientists to share his observations, and we were floored by what he captured.
I have been studying upward lightning flashes since 2004 primarily in Rapid City, South Dakota where there are 10 towers positioned along a ridgeline that runs through the middle of town.
In 2013, we participated in a project to observe upward lightning from a wind turbine farm in north central Kansas. We managed to capture a few events with one involving 4 wind turbines.
Our research, analysis and findings show that most upward flashes in the summer convective season are triggered by preceding nearby positive ground flashes and/or cloud flashes in which horizontally extensive negative leader activity passes nearby tall objects. The rapid electric field change from the negative leader activity or positive cloud-to-ground return stroke combined with the shape of the tall object, which enhances the electric field locally, results in the initiation or triggering of upward positive leaders from the objects.
I had always wondered just how many wind turbines could initiate upward leaders when triggered by a nearby flash. Hank’s capture showed that up to 14 wind turbines could initiate upward leaders in a single flash. As far as I know, this is the most that has been observed to date. This flash was truly a Perfect Upward Flash and followed the textbook on how preceding flashes can trigger upward leaders.
Hank’s video shows incloud brightening that propagates toward the camera and over the wind turbines. This is negative leader activity that frequently travels through layers of horizontal positive charge that build up in the trailing stratiform region of mesoscale convective complexes. Lightning develops as a bidirectional leader which ionizes the neutral air due to the strong electric field caused by charge regions within a thunderstorm. The bidirectional leader has a negative end that has a surplus of electrons and the a positive end with a deficit of electrons.
Often when the negative leaders travel a large distance, they tend to become cutoff from the other end of the leader. Due to the still present strong electric field, the cutoff segment, which is still conductive, can polarize and develop a new positive end resulting in new positive leader propagation and corresponding renewed negative leader growth. Frequently, the new positive leader end will travel to ground and connect causing a positive cloud-to-ground return stroke, and that is exactly what happened as recorded by Hank’s camera. Positive leaders propagate to ground on the right side of the video and connect to ground causing a return stroke. This return stroke, which involves an incredibly fast propagating region of rapid electron acceleration, heating and intense light emission, travels up the channel at about 1/3rd the speed of light and through to the negative end of the leader network that was overlying the wind turbines. The resulting electric field change causes positive leaders to initiate and grow from the highest of the wind turbine blades. These upward positive leaders travel upward driven by the newly modified electric field created by the return stroke.
To have so many upward positive leaders develop shows that the area covered by the triggering leader network and magnitude of the electric field change from the return stroke was very large influencing all the wind turbines nearly at once. It truly was a Perfect Upward Flash and something to behold.
I would like to thank Hank for sharing this video with me so I could share its explanation with all of you. He recently created an excellent video on How Lightning Works which you can see on his YouTube channel. It is definitely worth seeing and explains our latest scientific understanding of lightning using his amazing video and imagery.
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.