Lightning return stroke velocities in the thunderstorm research international program (TRIP)

Abstract

We have used high‐speed streaking photographic techniques to time‐resolve the luminous components of cloud‐to‐ground lightning flashes. Two‐dimensional return stroke velocities have been measured for 63 strokes representing, we believe, the largest set of return stroke velocity measurements obtained to date. All recordings were made during our participation in the Thunderstorm Reseach International Program conducted at the Kennedy Space Center, Florida, during the summers of 1977 and 1978 and at the Langmuir Laboratory near Socorro, New Mexico, during the summer of 1979. The mean return stroke velocity, near ground (channel length ≤1.3 km), was found to be 11×107 m/s, with a maximum relative error estimate in most cases of 35% or less. The distribution of velocities peaks strongly at approximately 9×107 m/s. Thirty‐two of the 63 values (51%) fall within the interval of 8–12 × 107 m/s. The range of observed velocities spans the interval of 2.9–24×107 m/s. Based on the presence of branches in the time‐resolved recordings, 17 strokes are considered to be first return strokes, with a mean velocity, near ground, of 9.6×107 m/s. The mean velocity for subsequent strokes is 12×107 m/s. A further breakdown of the results for Florida and New Mexico, respectively, reveals mean first return stroke velocities of 6.6×107 m/s and 15 × 107 m/s as well as mean subsequent stroke velocities of 11×107 m/s and 13 × 107 m/s. Velocity variations for 17 of the best events are presented, with the return stroke velocity observed to decrease with height in every case except one. The velocity reduction can be substantial; velocities in upper channel lengths were often reduced by 25% or more relative to velocities near ground, even for subsequent strokes. The variation of velocity between strokes in multistroke flashes was found to be significant in some cases and minor in others. The results of this study are compared with the earlier major works of Schonland and of McEachron; substantial disparities are found and examined. We show, in fact, that Schonland's South African return stroke velocity measurements were obtained, in part, from a lightning storm atypical of the region. It should be noted, however, that our minimum return stroke velocity value of 2.9×107 m/s is in excellent agreement with the results of Schonland and of McEachron, despite the fact that our work, Schonland's, and McEachron's were obtained in widely differing geographical areas by using different instrumentation and spanning a period of 50 years.