Archive for category Bipolar/Bidirectional Leader Development
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.