Combining VHF and optical observations to reconstruct an upward flash

In this study we present, to the best of our knowledge, the first three-dimensional (3D) reconstruction of an upward lightning flash using combined very high frequency (VHF) interferometric and high-speed camera (HSC) observations. Based on this reconstruction, we estimate the 3D velocities of different pulse fronts along the channel branches. Comparable 3D reconstructions have previously been reported for downward lightning flashes [1].

The Mt. Säntis Lightning Research Facility [2], located in the Appenzell region of Eastern Switzerland, features an electric field and current measurement system, as well as a Phantom HSC situated at a distance of 5 km from the namesake mountaintop tower. Additionally, during the summer 2021 experimental campaign, a VHF interferometer (IFM) belonging to New Mexico Tech was installed at the base of Mt. Säntis, 2 km away from the tower. The HSC operated at 24,000 fps and the IFM at 200 MS/s, corresponding to respective time resolutions of 42 μs and 5 ns. The spatial resolutions of the HSC and IFM were 512 x 512 pixels and 0.1°, respectively, both corresponding to ~3 m at the location of the tower tip. These two instruments were used in combination to reconstruct in three dimensions the bottom ~600 m of an upward negative flash that initiated from the Säntis Tower on July 30, 2021, at 15:38:10 UTC. This particular flash featured numerous “mixed-mode” pulses superimposed on the initial continuous current (ICC), in addition to the standard dart leader–return stroke sequences, identified as such from their current and E-field waveforms. The ICC pulses propagated downward along 4+ different visible branches; altitude change rates averaged -5.6 ± 2.0 x 10⁶ m/s and were observed to decrease slightly as the pulse fronts approached the strike point. 3D speeds of ~2 x 10⁷  m/s were observed, punctuated by spikes (spaced on the order of 10 μs apart) at times exceeding 1e8 m/s, indicative of step-like behaviour. Such an analysis of ICC pulse velocities is heretofore absent in the literature and lends itself to an improved understanding of leader dynamics and charge transfer mechanisms in upward lightning.

 

References:

[1] Li, Y., Qiu, S., Shi, L., Huang, Z., Wang, T., Duan, Y., 2017. Three‐Dimensional Reconstruction of Cloud‐to‐Ground Lightning Using High‐Speed Video and VHF Broadband Interferometer. JGR Atmospheres 122. https://doi.org/10.1002/2017JD027214

[2] Rachidi, F., Rubinstein, M., 2022. Säntis lightning research facility: a summary of the first ten years and future outlook. Elektrotech. Inftech. 139, 379–394. https://doi.org/10.1007/s00502-022-01031-2