Two‐dimensional velocity, optical risetime, and peak current estimates for natural positive lightning return strokes

Abstract

We report velocities, optical risetimes, and transmission line model peak currents for seven natural positive return strokes. The data were taken with our return stroke velocity device mounted on the National Severe Storms Laboratory mobile laboratory. The average two‐dimensional positive return stroke velocity for channel segments of <500 m in length starting near the base of the channel is 0.8 ± 0.3 × 108 m s−1, which is slower than our corresponding average velocity for natural negative first return strokes of 1.7 ± 0.7 × 108 m s−1. This implies that positive stroke peak currents in the literature, which assume the same velocity as negative strokes, are low by a factor of 2. The average two‐dimensional positive return stroke velocity for channel segments of >500 m starting near the base of the channel is 0.9 ± 0.4 × 108 m s−1. The corresponding average velocity for our natural negative first strokes is 1.2 ± 0.6 × 108 m s−1. We find no significant velocity change with height for positive return strokes. The average 10–90% optical risetime from channel segments with an average length of 3.2 ± 1.7 m for positive return strokes within 100 m of the base of the channel is 9.4 ± 3.0 μs; the corresponding measurement for negative strokes is 3.5 ± 1.7 μs. We detected no optical leaders from the positive flashes. This limits the relative brightness of positive stroke leaders to approximately 0.08 times the optical output of the return stroke. The logarithmic average peak current from the transmission line model for positive return strokes is 59.8 kA, approximately 5 times the logarithmic average peak current we calculated for natural negative first return strokes. All differences between positive and negative natural strokes noted above are statistically different at the 99% level.