Acoustic/Infrasonic analysis and modeling of thunder from a long-term recording in Southern France

Abstract

Thunder produces complex signals with a rich infrasonic and audible frequency spectrum. These signals depend both on the source and the propagation to the observer. However, there is no mutual agreement on the link between the observed spectral content and the generation mechanisms. The objectives of this study is to provide additional experimental and theoretical investigations, especially on the return stroke, based on a database of several thousands of acoustic and electromagnetic signals recorded in Southern France during autumn 2012 (HyMeX campaign). It contains a sufficient number of events close to the source (<1 km) to minimize propagation effects and to focus on the source effects. Source localization and lightning acoustical reconstruction indicate that infrasonic and low frequency audible part (1-40 Hz) of the spectrum show no clear differences between the return stroke and the intracloud discharges. These observations are compatible with a source mechanism due to the thermal expansion associated to the sudden heating of the air in the lightning channel. An original model inspired by Few's string pearl theory has been developed. It shows that the tortuous channel geometry explains at least partly the low frequency content of observed thunder spectrum.