Acoustical Measurement of Natural Lightning Flashes: Reconstructions and Statistical Analysis of Energy Spectra

Abstract

Twenty-seven natural flashes of three thunderstorms which occurred during October 2012 in Southern France are acoustically reconstructed and analyzed in the [0.1 − 180]-Hz frequency bandwidth and the [0.3−20]-km distance range. A 50-m triangular array of four recalibrated microphones sampled at 500 Hz has been used for the recording. A novel method of separation of return strokes from intracloud discharges within the acoustical signal is detailed and systematically applied. It shows the possibility to separate nearby cloud-to-ground (CG) events or nearby and distant CGs of the same flash. The separation method yields a total of 36 return stroke signals and spectra, along with some intracloud signals. The combination of reconstruction, separation, and frequency analysis provides new insights on the origin of thunder infrasound, showing unambiguously that thunder infrasound originate dominantly from return strokes. Apart from the higher amplitude of CGs, no clear difference between intracloud and CG spectra is observed. No sharp frequency peaks can be put into evidence. The spectral variability with distance is highlighted, especially for the total acoustic energy and the center frequency and bandwidth. A link between acoustic energy and impulse charge moment change is also found, though only by a small number of data.The description of these two mechanisms explaining the infrasonic and audible acoustic content of thunder has been summarized by Few (1995). Regarding the audible content, the sudden core heating (25,000-30,000 K) just after the electric discharge produces a chain of strong shock waves along the lightning channel. The time dependence of pressure within the lightning channel was estimated by Orville (1968a) from the first measured time resolved optical spectra (Orville, 1968b), with peak overpressure exceeding 80 kPa. Based on a radiation thermodynamical model, Hill (1971) could simulate such values, with even higher shock amplitudes for shorter times (see his Figure 5). The different steps to switch from strong shock waves to weak shock waves and finally to acoustic waves are developed by Few (1969). Theory for self similar strong shock wave was proposed by Taylor (1950) for point sources and Lin (1954) for line sources and then numerically extended to transition and weak shock regimes by Brode (1955) for spherically symmetric shocks. The cylindrical symmetry was described by Plooster (1970). An empirical match between strong and weak shock theories has been applied to lightning by Jones et al. (1968) assuming a rectilinear channel. However, lightning channel tortuosity has been observed optically by Hill (1968), who proposed a mean value of 16 ∘ deflection of one segment to the next. Synthesizing these previous studies, Few et al. (1967) andFew (1969) assumed that the transition between strong and weak shocks takes place approximately in the same range of distances as the transition between cylindrical and spherical divergence. This distance is called relaxation radius. Indeed,

Acoustical Energy of Return Strokes: A Comparison Between a Statistical Model and Measurements

“…In the far field, fluctuations dampen in amplitude, and the influence of tortuosity fades away, while absorption becomes determinant. These characteristics are globally in good agreement with observations of Lacroix et al, (, Figure 10). For instance, at short distances, relatively flat spectra are observed, with significant fluctuations in some cases (see, for instance, Event 2 [at 380 m] in Figure 8c in Lacroix et al, ).…”

“…These characteristics are globally in good agreement with observations of Lacroix et al, (, Figure 10). For instance, at short distances, relatively flat spectra are observed, with significant fluctuations in some cases (see, for instance, Event 2 [at 380 m] in Figure 8c in Lacroix et al, ). In the far field, lower variability is observed as well as an increasing negative spectrum slope with distance.…”

“…This proves that the extended geometry favors infrasound emergence, whatever the return stroke tortuosity is. This corroborates field observations of Lacroix et al () showing an important low‐frequency content of return strokes. Also observed is the increase with distance of the negative slopes of all frequency responses, whatever the geometry is.…”

“…Moreover, some positive slopes are observed with similar proportions only in the near field: 5 of 36 experimental cases (about 13%) and 50 of 648 numerical cases (about 8%). A linear regression of the mean numerical data matches well the measurements (Lacroix et al, ). Our model simulates therefore the right order of magnitude of the spectrum slope and its evolution with distance.…”

Acoustical reconstruction and thunder energy localisation in lightning flashes measured over Corsica during EXAEDRE field campaign

“…Co-localisation of acoustic sources with in-cloud detections provided by SAETTA high-frequency (60-66 MHz) electromagnetic lightning locating system, and with ground impacts provided by Meteorage, ensures the efficiency and precision of the method. This one was already used successfully in a previous field campaign (HyMeX-SOP1) in Cévennes in 2012 [2,3,4]. The detection algorithm PMCC also provides the various recorded signal intensities.…”