Archive for April, 2018
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
Here are two scientific journal paper citations on the subject:
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.
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. Journal of Geophysical Research: Atmospheres, 123. https://doi.org/10.1002/2017JD027821