Archive for category Lightning-Triggered Lightning
On the night of 12 Apr 2012, the Bay Area of California experienced a storm that literally lit up the skies with upward lightning. Some iconic photographs and video were taken during this event which provided evidence that numerous tall objects developed upward leaders in response to nearby flashes. The foremost images that illustrated what happened that night were taken by Phil McGrew. He had his Canon 5D Mark III camera running continuously using 20 sec exposures. During two of these exposures, his camera captured upward leaders that developed from the Bay Bridge and additional structures on the east side of the Bay. For each of the two photographs that he posted, it is likely that all the upward leaders developed during the same flash that probably lasted less than one second. He was located in a tall building on the east side of San Francisco downtown looking east along the Bay Bridge.
Below are embedding links as provided by Phil’s Flickr page where he has posted two images. Click on the images to go to his Flickr page. The exif data on his Flickr page indicates that the first of the two images was taken at 8:38:29 pm PDT using ISO 100, f/10, 20 sec exposure and a 28 mm lens. In this image there appears to be 5 upward leaders from the Bay Bridge structure and 2 upward leaders from two separate structures on the east side of the bay (likely in the Oakland area).
Again based on the exif data, the second image that Phil captured was at 8:42:41 pm PDT (4 min and 12 sec later) and used the same camera settings. This image (which has rightfully received international acclaim) appears to show 6 upward leaders from the Bay Bridge structure and 4 additional leaders beyond the Bay Bridge likely from structures on the east side of the bay.
Phil’s photographs indicate they were separated by 4 min and 12 sec. Not know the time accuracy of Phil’s camera, we compared the indicated times and time difference between the two images with National Lightning Detection Network (NLDN) data. Based on previous research findings, we suspected that these upward leaders were triggered by positive ground flashes (+CG) within 50 km of the Bay Bridge. Two very large estimated peak current +CG strokes were recorded at 3:39:59.425 and 3:44:12.332 pm PDT. They had estimated peak currents of +129.8 kA and +270.7 kA respectively and were separated by 4 min and 12.907 sec. There was a +27.8 kA stroke at 03:39:22.773 (37 min earlier of the first big +CG) and 2 -CG strokes at 03:40:32.843 and 03:42:21.957 fell within the time spaning the two large +CGs.
Below are GIS plots of the NLDN indicated return strokes and cloud events. The first figure shows the event location, event type by symbol (see legend) and estimated peak current based on relative symbol size. Notice the size of the +CG return stroke symbols relative to the other events.
The next figure shows the NLDN event locations and their times.
The last figure shows the NLDN events and a label of the estimated peak current.
We suspect that the upward leaders that developed from the Bay Bridge were positive polarity and developed following the large estimated peak current positive cloud-to-ground return strokes that occurred inside the Bay. These are examples of lightning triggered upward lightning in which the field change resulting from a preceding flash causes the development of upward leaders from nearby tall objects.
There were a number of other images from other people that showed upward leaders from tall objects during this same night and the other locations included the Golden Gate Bridge and tall buildings in Oakland. We suspect that these upward leaders also developed during the same triggering flashes that caused the upward leaders to develop from the Bay Bridge.
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.