On the evening of February 13th, 2023, Kristy Sharkey captured a lightning flash with her digital camera that contained evidence of upward lightning leaders from multiple objects in the field of view. What was interesting about this image was that the upward leaders did not seem to initiate from dominantly tall objects. Rather, the objects that were visible with upward leaders included evergreen trees. Upon detailed analysis of the image, 11 leaders were identified with origins in the foreground and one branching leader was visible in the upper right side of the image, but it could not be determined if it terminated on a grounded object as it extended out of frame to the right. 7 of the 11 leaders had upward branching, and 4 of the 7 upward branching leaders initiated from evergreen trees. On these 4 upward leaders, visible orange channel segments were identified that extended down the tree a short but noticeable distance. There was no evidence based on channel brightness or downward branching that one of the 11 identified grounded leaders was the result of a downward propagating leader and return stroke sequence.
Analyzed image showing traces of identified upward leaders.
Two upward leaders from douglas fir trees with visible orange segments where the lightning channel traversed the upper tree portion of the tree. These are later identified as trees C and D below.
Kristy was located in the east part of Renton Washington which is southeast of Seattle. Her camera, a Sony A7R Mark III with 24 mm focal length, lens was facing west (268º). The single exposure was 5 seconds with an ISO setting of 100 and aperture of f/6.3.
An attempt was made to match the flash recorded by the camera with National Lightning Detection Network data. Chris Vagasky of Vaisala, Inc. conducted a search and found 4 NLDN-indicated events in that region. Three of these events, which span 302 milliseconds, appear to correlate with the time the image was taken as the camera time was later determined to be ahead of actual time by 50+ minutes.
The NLDN is shown below.
Date 2023-02-14 UTC
|Time UTC||Lat||Long||Estimate Peak Current||Type|
A plot of the last three NLDN events and Kristy’s location is shown below.
The sequence of NLDN events begins with a strong (65.3 kA) positive cloud-to-ground return stroke at 03:12:47.846 UTC 13.6 km west of the camera. The next recorded event was a 6.1 kA estimated peak current positive intracloud impulsive event at 03:12:47.910 UTC which was located 2.2 km behind the camera. Finally, there was another positive intracloud event (10.4 kA estimated peak current) 27.8 km east of the camera at 03:12:48.148.
The light blue arrow traces on the figure show the progression of the NLDN-indicated flash given the sequence of events. Given that the first event was a positive cloud-to-ground return stroke, horizontal negative leader growth likely occurred once the return stroke reached the upper extent of the previously formed bidirectional leader network. The negative leader growth propagated eastbound over the camera location continuing further east. The fact that two recorded intracloud events were both positive suggests that these were upward moving impulsive negative leaders or recoil leaders that formed on subsequent downward developing positive leaders that tend to trail in-cloud negative leaders during horizontally extensive flashes. Since no horizontally propagating leaders were visible in the captured image, it is assumed that these were above the cloud base as they passed through the field of view and overhead. There is a noticeable area of diffuse brightening in the upper middle of the image which supports increased illumination from in-cloud leader activity.
Looking at the time and distance between the NLDN events yielded an estimated speed of 2.5 x 105 m/s and 1.8 x 105 m/s which is characteristic of negative leader propagation speeds.
In looking at the weather radar data from KLDX radar site, the reflectivity showed light to moderate reflectivity elongated along the same apparent path as the lightning flash. The hydrometer classification shows graupel, dry snow and ice crystals which would act as the charge carriers in an electrified storm.
Weather Radar Reflectivity
Hydrometer Classification showing Dry Snow and Graupel
A closer look at some of the Douglas Fir trees involved in the flash shows that their heights were likely 50 m or less, however, they were somewhat isolated from other objects nearby and the tallest in each area.
Google Earth image showing locations of the camera and four trees that initiated upward positive leaders.
Zoomed in view from camera location showing trees identified as A and B.
A close up view of Trees A and B
View from camera location showing trees C and D.
Based on the analysis of the optical evidence of upward branched leaders and correlated NLDN data suggesting a horizontally extensive positive cloud-to-ground lightning flash, it seems likely that the numerous leaders recorded in the image were positive leaders that initiated from objects in response to the electric field change created by negative leaders passing overhead the objects. The electric field change coupled with the local electric field enhancement caused by the height and shape of the objects created favorable conditions for the initiation and growth of upward propagating positive leaders.
What I find remarkable in this case is the number of upward leaders and that the objects that initiated them are not significantly higher than surrounding objects. In my observations and analyses to date, only dominantly tall objects tend to initiate upward positive leaders in response to negative leaders passing overhead (Warner et al., 2012). So, what is different about this case? Kristy indicated that the cloud ceiling was fairly low at the time and that graupel was reaching the ground. This would suggest that cloud charge was fairly low in altitude. For a given amount of cloud charge, the lower the altitude, the greater the electric field strength due to the decreased distance between it and the ground. Since leaders propagate in and through cloud charge regions, the electric field change experienced during the flash and passage of negative leaders could have been higher than typically observed resulting in more objects reaching the threshold for upward leader initiation.
Another interesting aspect of this flash worth mentioning was that both of the NLDN recorded flashes had positive cloud-to-ground return stroke (+CG) locations that were directly north of Seattle-Tacoma airport. A search of flight tracking data using the FlightRadar24 app revealed that in both cases, an aircraft was within a few kilometers of the NLDN +CG locations on approach to the airport at the time of the flash. For the 03:08:09 event an Alaska Airlines 737 aircraft was 6 km north of the airport at 1950 ft mean sea level (MSL) altitude and flying at a groundspeed of 176 knots. At 03:12:47 an ABX Air 767 was 5 km north of the airport at 1625 ft MSL flying at 148 knots. After reaching out to Alaska Airlines, they stated that no flight reported any lightning strikes during this timeframe, and I was unable to get a response from ABX Air. While there was no confirmation of lightning strikes to aircraft, it is well documented that aircraft can initiate lightning flashes when flying through regions of high electric field. The shape of the aircraft and its movement through the electric field can create favorable conditions for the initiation of bidirectional leaders from opposing ends of the aircraft (Mazur et al., 1989). In fact, it has been shown that most lightning strikes to aircraft are initiated by the aircraft. I have personally witnessed and documented this myself when I flew the T-28 Storm Penetrating Aircraft.
In conclusion, the image taken by Kristy Sharkey is very worthy of analysis and although some of the lightning behavior can be confidently implied, it does frame the question of what conditions are required to produce a large number of upward leaders from non-dominantly tall objects.
Mazur, V. (1989), Triggered lightning strikes to aircraft and natural intracloud discharges, J. Geophys. Res., 94(D3), 3311-3325.
Warner, T. A., K. L. Cummins, and R. E. Orville (2012), Upward lightning observations from towers in Rapid City, South Dakota and comparison with National Lightning Detection Network data, 2004–2010, J. Geophys. Res., 117, D19109, doi:10.1029/2012JD018346.