Archive for category Positive Leaders
On the evening of Saturday, 23 May 2020 a strong linear storm passed over the South Dakota Badlands. As the sun began to set, the stunning orange and pink light illuminated the backside of the storm and its trailing stratiform precipitation area. As is common with mesoscale convective systems, this backside region produced numerous horizontally extensive lightning flashes many of which contained positive cloud-to-ground return strokes. Also common with these types of flashes, negative leaders raced through the layered positive charge regions above cloud base, while trailing positive leaders propagated below cloud base in trail of the negative leaders presumably through negative screening layer charge or negatively charge rain. This spectacular “spider” lightning is my personal favorite and this spectacle was one I will not soon forget. My daughter and I filmed the flashes with every camera we had available and the video below shows our best captures. Recordings were made from 30 to 1,500 images per second.
Just as we have documented positive leaders developing from negative leader channels, we have also observed and analyzed negative leaders develop from positive leader channels. However, the physical process is noticeably different as it involves the development of recoil leaders in decayed positive leaders. Negative cloud-to-ground return strokes can occur during the horizontal propagation of positive leaders when the positive leaders decay and become cutoff from their original negative ends. A recoil leaders that develops along the positive leader channel paths can have its negative ends “veer off” the previously ionized channel and travel to ground via negative breakdown through virgin air causing a negative return stroke. The growing positive leader that follows the return stroke frequently decays with additional recoil leaders forming. The negative end of subsequent recoil leaders travel down the newly established channel path to ground, since it is more conductive due to its more recent ionization, causing additional negative return strokes resulting in repeated extension and growth of the horizontal positive leader end.
It is sometimes possible to recognize this type of flash solely from digital still imagery due to the geometry and shape captured during a single exposure. Below is a digital still image of the flash shown in the video above. The negative leader development that traveled to ground from the decayed positive leader channel displays recognizable negative leader patterns (erratic direction change and branching) and the brightness of the return stroke illuminates the channel back to the positive leader end which is in the left portion of the image. Notice the left curve where the negative leader return stroke channel joins the positive leader channel. If the downward negative leader was simply a branch of the initial horizontal propagating negative leader there would have been a right curve in the bright channel segment that traveled back in the direction from which the leader initially propagated (to the right).
We know from observation and analysis of horizontally extensive lightning flashes that often negative leaders travel horizontally through a layered positive charge region that spans large areas. We frequently observe that positive cloud-to-ground return strokes occur along the path the negative leaders travel but in trail of the negative leader tips. Evidence suggests that the negative leaders become cutoff from their original positive ends and then develop new positive leader ends that propagate downward to the ground and cause a +CG return stroke that then further extend the negative leaders. Although we have frequently documented the positive leaders growing toward ground after negative leaders propagate in cloud, due to the clouds, we rarely are able to see the positive leader development initially take place from the previously formed negative leader channel. This video contains three cases where we see the negative leader channel from which a new positive leader develops, propagates to ground and causes a +CG return stroke that travels toward the end of the negative leaders, thus furthering their propagation. This behavior was first documented and described in a paper by Saba et al., 2009 using high-speed camera imagery.
Saba, M. M. F., L. Z. S. Campos, E. P. Krider, and O. Pinto Jr. (2009), High-speed video observations of positive ground flashes produced by intracloud lightning, Geophys. Res. Lett., 36, L12811, doi:10.1029/2009GL038791.
This is one of the best positive cloud-to-ground flashes that I have filmed. When you watch the video remember that lightning leaders grow as bidirectionally with a positive and negative end. We see the positive leaders of this flash below cloud base and the negative end of the leader network is higher up in the clouds and therefore not visible. There are two sets of positive leaders to focus on. The farther leaders are on the left descending to ground and the right positive leaders closer to the camera spread out horizontally along cloud base. Once the far positive leaders reach ground a return stroke occurs. Once the return stroke traverses the leader network, the connected channel grows as an upward propagating negative leader higher up in the storm. The closer leaders also have a negative end that is growing unseen in the upper part of the storm but these leaders do not connect with ground and continue to spread out horizontally. Frequently, some of the positive leader branches become cutoff and develop fast moving bidirectional recoil leaders that attempt to reionize the decayed positive leader branches. The negative end of the recoil leaders travel toward the negative end of the flash by racing toward the place where the positive leaders emerged below cloud base. This continues for quite some time. You may consider this to be a hybrid flash with a ground flash component (farther) and an intracloud flash component (nearer) both raising negative charge upward toward a positive charge region. This flash was filmed at 5,600 images per second.
Citizen Science – the collection and analysis of data relating to the natural world by members of the general public, typically as part of a collaborative project with professional scientists.
This aspect of scientific research and understanding is rapidly growing primarily due to increased technology advancement, smartphone integration and interest by the general public. Smart phones and associated apps can now collect valuable data quickly and easily. I use my smartphone to collect data using the Globe Observer app and have made cloud observations from both polar regions and remote ocean locations. I also make weather observations using the mPing app.
The contributions can be significant as was evident with the aurora-related phenomenon now known as STEVE. Aurora chasers have recorded (and named STEVE) for quite awhile, but now their images and documentation combined with correlated scientific observations and analysis has lead to a better understanding of this phenomenon.
As a lightning research scientist, I am frequently approached with interesting lightning observations from the general public. These usually come in the form of video and digital still images. When I am able, I try to investigate these observations using what tools and analysis methods I have available, and often involve my willing colleagues that specialize in various aspects of lightning research. I encourage these observations as they have shown us things we often cannot necessarily capture in a confined research project domain or timeframe. Many of the images and video come from storm chasers and weather spotters.
Our biggest challenge when analyzing these data is correlating them to a specific time and place so they can be compared with any other research data that might be available. There are now multiple lightning location networks operational as well as research lightning mapping arrays that continuously record lightning related data.
So what can you as a storm chaser or storm spotter do to contribute to citizen science? I have a few suggestions that will greatly increase the value and usability of your video and image recordings for scientific analysis.
- Time is incredibly important for lightning analysis especially when we are discussing events that typically last less than a second. Having the correct time set to within a couple of seconds on your cameras is very helpful if not essential for analysis. I set my camera time every time I format my sd card after downloading any images. I use the NavClock app but typically the mobile carrier time on your smartphone is accurate to within a few seconds so just using your phone time is good enough. I also set all my cameras to GMT which is the standard for data collection and analysis. It makes it easy when you don’t have to convert from the timezone where the image or video was captured.
- Location is the next important information needed for analysis. Some cameras have built in GPS that will record position to metadata (exif) as well as set the camera time accurately. I wish this was a standard feature on every camera, but unfortunately it is not. I use the app Geotag Photos Pro to capture and add position information to my images and video. The direction the camera was pointed is useful as well, but that parameter is more difficult to record as metadata. I believe Canon has an external GPS encoder that can provide heading information as well.
- Thankfully, digital cameras record extensive metadata that describes the parameters set when images are captured. These data are critical for photogrammetry analysis (e.g., sensor size, focal length, exposure time). Always keep the exif data and export it with the image when providing it for analysis. The metadata is embedded in jpg and dng file formats, but will be in a separate xmp sidecar file for native raw camera formats.
- The more data the better and the RAW camera format provides the maximum amount of data when capturing digital still images. I recommend always shooting in the RAW format.
As far as getting your observations to the scientific community, the Science Operations Officer at National Weather Service forecast offices is a great place to start. Twitter is also good for connecting with researchers.
Again, the importance of Citizen Science cannot be overstated. It is integral part of scientific exploration and understanding, and I encourage all to participate when and how they can. Besides its fun too.
Unlike most lightning that initiates in the thunderstorm cloud as a bidirectional and bipolar leader that travels both upward and downward towards oppositely charged regions, upward lightning is unique in that it initiates from a tall object and the unidirectional leader only travels upward towards opposite polarity storm charge or a preceding triggering lightning flash component. Lightning-triggered upward lightning (LTUL) is caused by a nearby triggering lightning flash which has one of its components (either leader activity or a return stroke) pass close enough to the tall object to cause a large and rapid electric field change which in turn initiates a self-propagating upward leader from the object. Self-initiated upward lightning (SIUL) does not require a preceding nearby triggering lightning flash. Instead, the electric field due to storm charge generation within the cloud reaches a point at which a self-propagating leader initiates spontaneously. However, in this case the storm charge region is usually much lower and closer to the tall object and sometimes even envelopes the object. In both cases, the shape and height of the tower enhances the electric field locally near the tip so that ionization of the air and resulting leader formation takes place much easier than that over flat ground. In essence, if the tall objects (i.e., towers, wind turbines or buildings) where not there, the upward lightning would not occur.
We have researched upward lightning in Rapid City since 2004, and our findings show that the 10 tall towers along the ridge that runs through the city all have experience upward lightning. During the summer, we only observed lightning-triggered upward lightning and during intense winter storms with heavy snow and strong winds, we only observed self-initiated upward lightning. During the summer months from 2004 through 2014 we recorded recorded 122 upward flashes from the towers all of which were LTUL.
However, during the winter months, we only documented upward flashes during two major snow events. The first and most intense was the devastating blizzard of 4 Oct 2013. During a 21 hour period, the towers in Rapid City initiated 25 SIUL flashes. In addition, the South Dakota Public Broadcasting tower near Faith, South Dakota experienced 17 SIUL flashes. Although we focus our research during the summer months, we just happened to have an electric field meter and digital interferometer operating during the blizzard. The challenge with observing SIUL during heavy snow is that you cannot see the towers because they are obscured by the snow and low clouds. So you have to record the lightning by some other means. The electric field meter recorded the ambient electric field 5 km west of the towers, and the digital interferometer, 23 km east of the towers, mapped lightning leader activity in two dimensions (azimuth and elevation). The interferometer recorded five upward flashes before it lost power along with most of western South Dakota. Below is a video animation of the data recorded by the digital interferometer for one of the upward flashes. You can visualize that you are standing east of Rapid City looking west toward the towers. Each of the individual data points represents the azimuth and elevation to electromagnetic radiation generated by the lightning leader (and received by the sensor) as the leader propagated. The system records data in sequential 4 microsecond windows and determines the direction to the strongest signal in each time window. Since lightning tends to branch as it grows, you see the source points plot the spreading branched leaders as they grow. The leader clearly initiates from a single point and then spreads upward as it branches. Occasionally, you can see a rapid succession of source points that travel back along a branch toward the tower. These are recoil leaders which form on decayed branches in an attempt to reionize the branch.
And here is some video taken from my house during one of the upward flashes.
The only other time that we documented self-initiated upward lightning from the towers in Rapid City was during a strong snow event on Christmas Day 2016. There were three confirmed upward flashes.
So if it is snowing really hard in Rapid City and you hear thunder, chances are you can blame the towers.
If you would like to learn more about lightning, please visit my Education section
Below are journal and conference paper citations on the subject.
Asakawa, A., K. Miyake, S. Yokoyama, T. Shindo, T. Yokota, and T. Sakai (1997), Two types of lightning discharges to a high stack on the coast of the Sea of Japan in winter, IEEE Trans. Power Delivery, 12, 1222–1231.
Bech, J., N. Pineda, T. Rigo, and M. Aran (2013), Remote sensing analysis of a Mediterranean thundersnow and low-altitude heavy snowfall event, J. Atmos. Res., 123, 305-322, doi:10.1016/j.atmosres.2012.06.021.
Brook, M., M. Nakano, and P. Krehbiel (1982), The electrical structure of the Hokuriku winter thunderstorm, J. Geophys. Res., 87, 1207– 1215.
Heidler, F., M. Manhardt, and K. Stimper (2014), Self-Initiated and Other-Triggered Positive Upward Lightning Measured at the Peissenberg Tower, Germany, paper presented at the 2014 International Conference on Lightning Protection (ICLP), 13 – 17 Oct, Shanghai, China.
Lyons, W. A., T. E. Nelson, T. A. Warner, A. Ballweber, R. Lueck, T. J. Lang, S. A. Cummer, M. M. F. Saba, C. Schumann, K. L. Cummins, N. Beavis, S. A. Rudtledge, T. A. Samaras, P. Samaras and C. Young (2014), Large Scale Outbreaks of Thundersnow and Self-Initiated Upward Lightning (SIUL) During Two Blizzards, paper presented at the 23nd International Lightning Meteorology Conference, Mar 20 – 21, Tucson, Arizona.
Pineda, N., J. Figueras i Ventura, D. Romero, A. Mostajabi, M. Azadifar, A. Sunjerga, F. Rachidi, M. Rubinstein, J. Montanyà, O. van der Velde, P. Altube, N. Besic, J. Grazioli, U. Germann, and E. R. Williams (2019), Meteorological aspects of self-initiated upward lightning at the Säntis tower (Switzerland), J. Geophys. Res., doi: 10.1029/2019JD030834
Rakov, V. A., and M. A. Uman (2003), Winter lightning in Japan, in Lightning: Physics and Effects, chap. 8, pp. 308– 320, Cambridge Univ. Press, Cambridge, U. K.
Schultz, C. J., Lang, T. J., Bruning, E. C., Calhoun, K. M., Harkema, S., & Curtis, N. (2018). Characteristics of lightning within electrified snowfall events using lightning mapping arrays, J. Geophys. Res. Atmos., 123. doi: 10.1002/2017JD027821
Wang, D., N. Takagi, T. Watanabe, H. Sakurano, and M. Hashimoto (2008), Observed characteristics of upward leaders that are initiated from a windmill and its lightning protection tower, Geophys. Res. Lett., 35, L02803, doi:10.1029/2007GL032136.
Wang D. and N. Takagi, (2010), Characteristics of winter lightning that occurred on a windmill and its lightning protection tower in Japan, Proceedings of the 3rd International Symposium on Winter Lightning, Jun 13-15, Tokyo, Japan.
Wang D., and N. Takagi, Y. Takaki (2010), A comparison between self-triggered and other-triggered upward lightning discharges, Proceedings of the 30th International Conference on Lightning Protection, Sep 13-17, Cagliari, Italy.
Wang D., and N. Takagi (2012), Three Unusual Upward Positive Lightning Triggered by Other Nearby Lightning Discharge Activity, paper presented at the 22nd International Lightning Detection Conference, 2 – 3 April, Broomfield, Colorado, USA
Warner, T. A., T. J. Lang, and W. A. Lyons (2014), Synoptic scale outbreak of self-initiated upward lightning (SIUL) from tall structures during the central U.S. blizzard of 1–2 February 2011, J. Geophys. Res. Atmos., 119, doi:10.1002/2014JD021691.
On May 13th, my daughter and I went out to chase storms that were forming over the Black Hills. A nice cluster of storms moved over Sturgis, South Dakota (home of the Sturgis Motorcycle Rally), and we filmed some close flashes as the storms passed over us. We then followed the cluster toward Bear Butte which is an isolated uplifted hill on the east side of the Black Hills, northeast of Sturgis.
Our primary target decayed and so we focused on new storms that had formed over the Black Hills and were moving directly toward us. They put down some nice CGs, and as they reached us, I repositioned to have Bear Butte in my field of view. A few minutes later we were treated to two spectacular CG lightning flashes directly in front of us and close. They were very bright and very loud. I suspected they were +CGs given their long duration continuing current and exceptional brightness. The Black Hills area and Northern High Plains for that matter exhibits an atypically high percentage of +CG flashes, and trying to understand and explain this anomoly was part of a study I was involved in during the UPLIGHTS research campaign.
For the first flash, I had my infrared triggered cameras set to f/8 and ISO100 in aperture priority mode. Although this setting is ideal for the average CG flash between 5-15 km, the LCD image review showed significant saturation. I reset the aperture to f/11 and the second flash was still somewhat saturated.
Below is the image for the first flash. You will notice there is two CG channels, one in front of Bear Butte and one beyond.
National Lightning Detection Data provided by Vaisala, Inc. indicated the closer CG was in fact positive (electrons traveled upward along the channel) with an impressive 159.6 kA estimated peak current. It struck 2.5 km away. NLDN data indicated the second channel was also a positive CG 12.6 km away and had an estimated peak current of 58.4 kA.
The second flash which is shown below only had one CG termination point.
NLDN data indicated it was a +CG, 2.2 km away with a peak current of 143.1 kA.
Positive CG flashes tend to exhibit higher peak current compared to negative CGs on average and usually do not have multiple return strokes. If my memory serves, I believe the latest published scientific literature has the average peak current for -CGs around 30 kA and 50 kA for +CGs. So these flashes were exceptionally strong. Unlike what we were taught in school, they DO NOT always originate from the top of a thunderstorm or anvil area and DO NOT always strike away from the main storm and rain area. It all depends on where the charge regions form, and in the Northern High Plains, we see a lot of storms with inverted charge regions, which leads to more +CGs. In the near future, I will be adding an education section on my blog which explains this in more detail.
Below is video of the two flashes captured on a Panasonic HPX-170 at 1280x720p60 which uses a global shutter (no annoying rolling shutter artifacts). In the slow playback you will see an artifact on the frame preceding the return stroke. This is saturating brightness bleed over from the subsequent return stroke that occurs in the following frame. After the CCD records a frame, the voltage values from each photosite (which corresponds to each pixel in the image) are shifted to an adjacent storage photosite that is covered. The voltage is then read out from the covered storage photosites while the next exposure is taking place in the non-covered photosites. If the non-covered photosites experience a saturating brightness, some of the voltage can bleed over into the adjacent storage photosites during their readout adding a voltage increase to their recorded values. Since the covered photosites are readout row by row with the data shifting up the CCD array to higher covered photosites after each row is read, the artifact will usually show up lower in the image as the “image data” from the previous frame has moved up when the saturating brightness occurs. These artifacts are often misidentified as attempted leaders that occur close to the camera, when in fact they are only “ghost images” of the bright return stroke channel that occurs in the subsequent frame but shows up on the previous frame (forward in time…que Twilight Zone music.)
You will also notice the integrated recoil leader activity associated with descending positive leaders in the distant second CG during the first flash. This integrated recoil leader activity is a clear identifying characteristic of positive leaders, and I explain this in the previous post.
Below are some additional images from flashes we captured before the storm moved over Bear Butte.
Below is a video on the creation of Lichtenberg figures. Interesting is the subsequent bright short discharges that continue to take place after the initial discharge. These seem similar in appearance to recoil leaders, which form on positive leaders branches that become cutoff from a main channel. Compare the two videos below.
YouTube video of Lichtenberg creation.
Upward lightning (upward positive leaders) from a tower filmed at 9,000 images per second.