Acoustical localization, reconstruction, signal, and statistical analysis of storm electrical discharges from a 2-months long database in southern France

Abstract

Infrasound and low frequency sounds are discussed as a method to characterize lightning flashes in a complementary way to electromagnetic (EM) observations. Thunder and EM data result mainly from a 2-months long observation campaign in Southern France, dedicated to monitor atmospheric electricity as part of the hydrological cycle in Mediterranean (HyMeX program). Possibilities and limitations to follow storms by sound or infrasound (in the 1 to 40 Hz frequency range) at various distances are outlined. The influence of distance, wind, and ambient noise is examined. Several examples of individual lightning flashes acoustical reconstruction are compared to EM reconstruction by means of a Lightning Mapping Array. Both Intra-Cloud or Cloud-to-Ground (CG) discharges are investigated. Special emphasis is brought to the lower part of CGs, as many acoustic signals are localized inside the lightning CG channel. A statistical comparison between the acoustical versus EM approaches is performed, thanks to a significant number of recorded discharges in a single storm. Performances of acoustical reconstruction are detailed as function of observation range. Detailed signal analysis compared to a theoretical model shows that the tortuous channel geometry explains at least partly the low frequency content of our observations of thunder spectra.