So what as storm chasers and photographers can we see when it comes to lightning? With the knowledge gained from data collected and analyzed from high-speed cameras and correlated electric field sensing instrumentation, we actually can see and interpret quite a bit. It has gotten to the point for me, and those that have worked with me on lightning research, where we can frequently classify the polarity of leaders and ground flashes in realtime. It is as if lightning has been slowed down for us, slow enough to identify its behavior and personality. Sounds strange but it is true, and it is my hope that others will be able to experience this after reading this blog and watching for the distinct behavior differences.
Horizontally Extensive Flashes
My favorite type of lightning is that associated with the backside or trailing stratiform portion of a mesoscale convective system. The challenge is that this area tends to have widespread rain so filming the flashes can be difficult. However, if you can get to the edge of the precipitation area of an active system, you will frequently be treated to spectacular horizontally extensive flashes that travel large distances and last multiple seconds. The reason for these types of flashes is that the charge regions in a well developed system tend to spread out over large distances in layers and the leaders propagate through these layers. +CG flashes tends to occur in this region as well and the resulting negative leader extension that frequently follows the return stroke tends to spread out over wide areas. Usually the negative leaders are incloud and seen as fast moving brightness that travels across the sky. However, positive leaders which encompass the other end of the bidirectional leader network will frequently chase behind the negative leaders in trail, below cloud base and clearly visible. These are the spectacular anvil crawlers or spider flashes that induce a collective “woah” by those that witness them. The positive leaders tend to be highly branched and therefore decay and produce prolific recoil leader activity, which displays a flickering forward progression that can be easily followed by eye due to their slower speed and immense travel distances. These are truly the displays every wizard strives for.
Another reason why these flashes travel such large distances is that they can cause repeated +CG return strokes as they travel along which reenergizes the horizontally extensive negative leader growth. We have observed two different ways in which positive leaders develop and connect with ground during these flashes. Often the extensive negative leader activity becomes cutoff from the opposite end of the leader network. When this happens, positive leaders can emerge from the cutoff back end and then propagate to ground resulting in another +CG return stroke which further extends the negative leaders.
Additionally, the cloud base crawling positive leaders that trail behind the incloud negative leaders can occasionally travel toward and connect with ground resulting in another +CG return stroke.
Negative CG return strokes also occasionally occur during a horizontally extensive flashes, and surprisingly, we have observed it is the recoil leaders on the cloud base crawling positive leaders that tend to lead to these much less common -CG return strokes. Sometimes the positive leader channel path becomes so decayed that the negative end of a recoil leader will begin propagating outside and away from the previously formed channel path as a stepped negative leader propagating in virgin air. If it travels toward and connects with ground, a negative CG return stroke occurs and further extends the positive leader network from which it originated.