Using Romanian infrasound observations to analyze thunderstorms generated by extratropical cyclones over the Black Sea

A new 4-element infrasound array (AGIR) of 0.2 km aperture was deployed by National Institute for Earth Physics (NIEP) in August 2024 in Eastern Romania, on the Black Sea coast. Between August and October, during the strong thunderstorms that crossed this region, long-duration trains of frequent sharp spikes in the amplitude associated with lightning discharges were observed into AGIR infrasound recordings. Some of these storm episodes could be correlated to cyclones moving over the Black Sea and greatly affecting Romania's regional climate in 2024.

We examined data from NIEP's current infrasound network – BURARI, IPLOR and AGIR stations –, in order to study the possibilities of infrasound-based monitoring of extratropical cyclones over the Black Sea. Association between infrasound detections into 0.5 to 7 Hz frequency band and lightning flashes detected by MTG Lightning Imager within 50 km from the AGIR infrasound station was investigated, assuming direct wave propagation path. Acoustic signatures of lightning activity show short-lived disturbances with dominant frequency of approx. 3 Hz and amplitudes up to about 3.5 Pa.

In addition to the strong lightning discharges during the storms, the cyclones were also accompanied by strong winds that produced waves in the Black Sea. We can consider these waves to be the cause of the significant fluctuations observed into the microbarom detections in the Black Sea region at BURARI and IPLOR stations, into the 0.1 Hz to 1 Hz frequency band. Microbarom power spectral noise amplitudes peak was observed around 0.3 Hz. Microbaroms detections are strongly influenced both by seasonally dependent stratospheric winds and local turbulence-induced pressure fluctuations. Local wind was averaged for the BURARI and IPLOR locations using NRL-G2S wind fields.

Infrasound signatures linked with certain extratropical cyclonic episodes were well identified. To improve storm track estimates, these infrasound-based detections were subsequently combined with conventional meteorological data including surface observations, electric field measurements, and satellite data. This study shows the potential of Romania's current infrasound infrastructure to support extratropical cyclone surveillance and improve forecasting capability in the region, even when more calibration of detection thresholds and source characterisation is required.