The inaudible sound of thunderstorms

By measuring infrasound, a wide variety of atmospheric sources can be identified. In this study, infrasound associated with thunderstorm lightning discharges is investigated. For this purpose, infrasound observations are compared with discharge detections based on a electromagnetical (EM) detection system. Both systems are operated by the Royal Netherlands Meteorlogical Institute. The initial reason for carrying out the study is a study concentrating on the validation of the EM detection system by a variety of methods, for both scientific and social purposes. The main objective of the study is to investigate the detectibility of lightning discharges with infrasound. For this purpose, infrasound detection parameters are compared with discharge detections at different distances to infrasound arrays. These analyses provide results on the detectibility in terms of extent of detection, discrimination between cloudcloud and cloud-ground discharges and possible localization of discharges with infrasound. From these analyses, infrasound coherency, apparent velocity, amplitude and spectral content of the observed infrasound appears to be closely related to the distance of lightning discharges to the infrasound array. Current detection techniques limit successful association to distances up to 70-80 km. Moreover, correlations show that cloud-ground discharges can be associated with infrasound mainly; cloud-cloud discharges are not detected or can be associated to low coherent infrasound solely. In order to provide unambiguous associations of infrasound to lightning discharges, infrasound detections at multiple arrays are compared by crossbearing localization. Localization attempts do not appear to provide satisfactory results yet; the infrasound arrays being capable of detecting thunderstorm signals are too far away to relate the detection results in a meaningful way. Furthermore, the content and behaviour of associated infrasound is investigated. The form of associated infrasound resembles the blast-waveform and is coherent over the array elements. The blast-waveform is readily associated to cloud-ground discharges. The observed waveform is the key to the interpreted source mechanism. By picking amplitude values, the attenuation of the associated blastwave as function of distance is investigated. A first-order decaying trend is observed with distance, although the datapoints are relatively scattered. The results are explained in terms of attenuation and source mechanisms. Lastly, observations on subsonic pressure variations, coupled to thunderstorm fronts are described. Infrasound arrays appear to be succesful in detecting the back-azimuths of propagating fronts. In conclusion, infrasound observations correlate well with the cloud-ground discharge detections by the EM detection system. Currently, localization by crossbearing is not possible, therefore validation of the EM system is not yet possible. However, future studies may enable localization by enhancing the infrasound detection algorithm and array configurations. Advanced boundary conditions may reduce the scattering in the observed attenuation curves.