Archive for December, 2011
Protected: High-speed camera observations of bipolar/bidirectional lightning leader development near positive leaders
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.
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.