Infrasound from lightning measured in northern Romania

During thunderstorms in northern Romania, numerous infrasonic signals are emitted due to the process of lightning and thunder. Association between infrasound detections into 0.5 to 7 Hz frequency band and lightning flashes detected by the Arrival Time Difference lightning network (ATDnet) managed by the Met Office within 50 km from the BURARI infrasound station is systematically investigated. Statistical results are presented based on infrasound and lightning observations during summer months (June to August) from 2020 to 2022.

Assuming direct wave propagation path, infrasound detections can be successfully correlated with ATDnet lightning detections up to distances of 50 km from the infrasound array. Long-duration trains of frequent sharp spikes in the amplitude observed into infrasound recordings during thunderstorms are associated with lightning discharges. Acoustic signatures of lightning activity show short-lived disturbances with dominant frequency of approx. 3 Hz and amplitudes ranging from 0.01 up to about 0.5 Pa. In order to associate BURARI measurements with ATDnet detections, a relationship between infrasound time-of-arrival and time of discharge signals is applied. A maximum deviation of 10^o between observed infrasound back-azimuth and back-azimuth of ATDnet detections is allowed.

For several cases (days with the largest number of lightnings), detection conditions of infrasound from lightning are detailed, and some characteristics are analyzed (e.g., amplitude, frequency, trace velocity and spectrograms in the frequency range from 0.5 to 10 Hz). Correlations with synoptic charts, regional lightning activity maps and electric field measurements could be performed.