Characterization and Localization of Chemical Explosions and Lightning Flashes by a Flotilla of Stratospheric Balloons

Abstract On 3 August 2021, a flotilla of four Heliotrope solar hot air balloons was launched from central New Mexico (USA). The balloons, equipped with microbarometers, flew from sunrise to sunset at altitudes between 17 and 22 km. During the flight, eight chemical explosions ranging from 45 to 136 kg TNT equivalent and a thunderstorm occurred in the vicinity of the balloons. Acoustic waves from explosions measured in the stratosphere are characterized, including overpressure, spectral content, and positive phase duration. The detection, or lack of detection, of these acoustic waves makes it possible to delimit the shadow zone in the lower stratosphere. We verify that the empirical laws of point source acoustic wave propagation are still valid in the lower stratosphere, taking into account the variation in atmospheric density. Eight flashes from a thunderstorm cell close to three of the balloons produced clear acoustic waves. Analysis confirms that their propagation is similar to that of a spherical source, whose acoustic energy decreases as the inverse square of the distance. One acoustic signal from an intracloud flash very close to a balloon showed characteristics of the “electrostatic” mechanism, which is rarely observed at the ground level. Finally, a Bayesian method for localizing all these sources (explosions and flashes), taking into account the local meteorology, is evaluated. It highlights the importance of the network geometry and the propagation path on the localization error.