On the evening of 16 July 2012, a weak cluster of storms moved north over Rapid City, South Dakota. A single visible rainshaft formed on the leading edge of the approaching development. At the time of the rainshaft formation, there was no lightning activity along the leading edge. However, lightning flashes were visible to the distant south in the more active trailing portion of the storms. At 04:20:35, (17 July 2012) UT two digital still cameras captured a ground flash near the rainshaft. This was the first visible flash along the leading edge. One camera, a Canon 5D2 Mark III, captured the image using a 16 mm lens set at f/2.8 using ISO 800 and an exposure time of 11 sec. This camera was capturing continuous 11 sec exposures for a timelapse sequence. A second camera, a Canon 7D, captured the image using a 20 mm lens set at f/8 using ISO 100 and an exposure time of 30 sec.
The captured images, which show the entire flash due to the long exposure times, showed a unique feature that I have not seen previously with any flash images that I have captured. The visible channels below cloud base show that there was a main vertical channel that connected with ground and a branch that propagated somewhat horizontally to the left and did not connect with ground. This second branch appeared to propagate toward the rainshaft and upon entering the rain, spread out vertically in both directions while branching extensively. The change in propagation direction and increase in branching appears isolated to inside the rainshaft, and is not apparent on any other channel segments.
An analysis of National Lightning Detection Network (NLDN) data revealed the NLDN recorded a corresponding 6.8 kA estimated peak current, negative cloud to ground stroke (-CG) 8 km southwest of the cameras. This location correlated in both time and direction, and all other preceding NLDN-indicated flash activity was south of the area by 20 km.
I believe that this image provides evidence that a negative leader branch propagated into a positively charged rainshaft that served as a positive potential well favorable for negative leader propagation (Coleman et al., 2003 and Coleman et al., 2008).
Coleman, L. M., T. C. Marshall, M. Stolzenburg, T. Hamlin, P. R. Krehbiel, W. Rison, and R. J. Thomas (2003), Effects of charge and electrostatic potential on lightning propagation, J. Geophys. Res., 108(D9), 4298, doi:10.1029/2002JD002718.
Coleman, L. M., M. Stolzenburg, T. C. Marshall, and M. Stanley (2008), Horizontal lightning propagation, preliminary breakdown, and electric potential in New Mexico thunderstorms, J. Geophys. Res., 113, D09208, doi:10.1029/2007JD009459.