Bipolar leader development prior to a +CG return stroke

The following high-speed video recorded at 10,000 images per second shows bipolar leader development that precedes a +CG return stroke.  A non-branched positive leader emerges below cloud while negative stepped leaders are visible propagating upward from an apparent common origin/initiation area obscured within the cloud.  The negative leaders are likely a portion of the upper leader network associated with the bipolar/bidirectional leader development that initiated in the cloud.  There is in-cloud brightening to the right of the common area as well that extends away from the camera to the right, and these are likely additional leader channels associated with the flash leader network that are propagating in-cloud.  Prior to the +CG return stroke an additional non-branched positive leader emerges below cloud to the right of the first positive leader.  Upon connection of the positive leader with the ground, a bright return stroke travels up the previously formed leader network resulting in an intense brightening of the visible negative leaders.  These negative leaders appear more energetic and appear to propagate faster once ground potential (or near ground potential) is raised to their tips via the return stroke.  Their growth continues following the return stroke resulting in the continuing current seen from the leader tips to the ground connection point.  The obscured leader network extending back into the cloud to the right maintains an increased brightness as well following the return stroke and exhibits some brightness pulses which are also seen in the lower return stroke channel segment.

The NLDN recorded an optically correlated +19.1 kA estimated peak current “IC” event.

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  1. #1 by Sumedhe Karunarathna on 01/26/2012 - 6:31 am

    Tom, Do you have an idea about the dimensions of the frame? Like how high it is from the ground? What exactly NLDN detected, did it optically correlated with the return stroke or the one that happening right to the frame? Any way, a nice case. Thanks!

  2. #2 by ZTResearch on 01/26/2012 - 9:45 am

    Sumedhe. The +19.1 kA, +IC correlated with the return stroke. I have a number of cases where the NLDN classified a +CG as a +IC. We are currently looking at why, but we suspect it has to do with the geometry of the downward positive leader channel. If it has horizontal segments, it can cause a short “peak-to-zero” time in the received signal which forces the classification as a cloud flash. I will check on the 2D height of the negative leaders. I suspect they were traveling towards the camera as well as upward, so there will be some uncertainty by using the distance from the camera to the NLDN +IC location in the photogrammetry calculation. Tom

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