Observations of High Peak Current 'lull' Lightning Flashes

Abstract:

A new and peculiar type of negative cloud-to-ground (-CG) lightning flash has been found from LMA observations of storms. Called a 'lull' flash, the discharge begins with a short duration (~5-10 ms), stepped leader and high peak current return stroke, followed by a brief period of inactivity. After a few tens to a few hundreds of milliseconds, the discharge reignites in the same location as a normal -CG. VHF observations show a complete lack of activity during the lull. The flashes were first identified from LMA observations during a 2008 lightning study in West Virginia (Krehbiel, 2012 ILDC), but are seen in other normally electrified storms. They are characterized by large negative peak currents (-50 to -150 kA) in NLDN data.

 Here we report detailed LMA, interferometer, and fast antenna observations of lull flashes obtained at Langmuir Laboratory in 2013. A series of six lull CGs occurred over an 11 min time interval in a small storm on August 5. Analysis of the observations shows the breakdown immediately produced a stepped leader that went straight to ground. The durations from initial breakdown to the return stroke (RS) was 2-15 ms, corresponding to leader speeds of 4×10⁵ to 3×10⁶ m/s. Peak currents of the RSs were -32 to -188 kA. The lulls began 10-29 ms after the RSs, and lasted 40-100 ms before the discharge reignited. By contrast, the durations, speeds and peak current values of the reignited first strokes were those of normal -CGs, namely, 9 to 105 ms, -11.5kA to 55.8kA, and 0.6×10⁵ to 6.7×10⁵ m/s. Further analysis indicates the lull flashes occurred when the storm had substantial negative charge but little lower positive charge, allowing the initial stroke to go straight to ground at a high speed. The energetic stroke somehow quenches the discharge a short time afterward while much negative charge remains, which leads to reignition and the second part of the discharge. Exactly how and why this happens remains an open question.