Lightning is an ubiquitous source of infrasound, and an essential climate variable. To observe lightning flashes, thunder measurement efficiently complements electromagnetic methods. Using acoustical arrays, time delays between sensors inform on the direction of sound arrival, while the difference between emission time and sound arrival provides the source distance. Combining two allows a geometrical reconstruction of lightning flashes viewed as sets of sound sources. The measured sound amplitude can also be back-propagated, compensating for absorption and density stratification. This allows us to evaluate the acoustical power of each detected source and the total power of an individual flash. This methodology has been carried out to analyse data from two campaigns in Southern continental France in 2012 and in Corsica in 2018. In Corsica, power from reconstructed sources could also be forward-propagated towards several isolated microphones and compared to measurement there, providing an additional validation of the method. A large number of events from the two campaigns has been analysed, including negative and positive cloud to ground discharges and intra-cloud ones. The analysis outlines the method efficiency, and the strong variability of lightning as sound sources, in terms of both power spatial distribution and global values.
<p>Lightning is a ubiquitous source of infrasound, and an essential climate variable. Acoustic measurements have been carried out by the CEA over the last ten years to characterize thunder within the framework of the HyMeX project. First, during the fall of 2012 in the south of France in the C&#233;vennes region during the intensive measurement campaign (SOP1) and more recently, in the fall of 2018 in Corsica (France), as part of the EXAEDRE campaign. During both the SOP1 and EXAEDRE campaigns, mini-arrays (&#8220;AA&#8221; for &#8220;Acoustic Array&#8221;) of four microphones (respectively disposed on a 50m and a 30m-wide triangle) were used. Lightning information were available thanks to three kinds of electromagnetic detection systems. Firstly, classical Lightning Location Systems (LLS) measured the low frequency range (1-350 kHz), giving the flash emission time and location, as well as its peak current. Secondly, a network of 12 antennas, Lightning Mapping Array (LMA), detecting in the very high frequency range (60-66 MHz) was used. It measured the radiation from leaders and intracloud discharges, which occur mostly inside the thundercloud, providing the 3D location of these discharges. Thirdly, the Charge Moment Change (CMC) was provided by broadband Extremely Low Frequency (< 1.1 kHz) measurements<strong>.</strong></p> <p>Time delays between AA sensors inform on the direction of sound arrival, while the difference between emission time and sound arrival provides the source distance. Combining the two allows a geometrical reconstruction of individual lightning flashes, each viewed as a set of point sound sources. Co-localization of acoustic sources with in-cloud detections provided by the LMA and with ground impacts provided by the LLS shows the efficiency and precision of the method. The measured sound amplitude can also be back-propagated, compensating for absorption and density stratification. This allows to evaluate the acoustical power of each detected source, and then the total power of an individual flash.</p> <p>In both campaigns, very heterogeneous geometrical distributions of source sound powers within a single flash are frequently observed. Most of the power is frequently located in only one portion of the lightning, most of the time in the return stroke, but also sometimes in the intracloud part. A few homogeneous cases are observed, especially in SOP1. The total acoustical power of the flashes turns out to be also extremely variable, extending over at least 4 orders of magnitude with a median value of 3 MW. It correlates quite good with the peak current or the CMC, and the nature of the correlation differs strongly with the category of lightning considered, either typical return strokes or very energetic positive flashes generating sprites. However, a high dispersion of the data is observed, so that it is not possible to correctly predict any electrical parameter using only the total acoustic power of an event, although a trend is statistically observed. This could be overcome by finding other variables to fully explain the relationship between acoustical and electrical parameters, and improving our propagation model to better account for acoustic variability.</p>
<p>From September 13th to October 12th 2018, the EXAEDRE field campaign took place in Corsica, dedicated to the characterisation of thunderstorm clouds and electrical activity. Among a wide range of observation instruments, an array of 4 microphones, arranged on a&#160; 30-m wide triangle located near the island eastern coast, recorded over the frequency band 1 to 80 Hz the acoustical signal, or thunder, associated to lightning flashes. The search for&#160; coherent signals between the four sensors within prescribed frequency bands allows to determine the thunder apparent sound velocity and azimuth (PMCC algorithm [1]). Knowing the flash emission time provided by Meteorage low frequency (1-350 kHz) electromagnetic ground lightning locating system, as well as the local speed of sound, it is possible to reconstruct the various positions of coherent sound sources within a single lightning flash. Co-localisation of acoustic sources with in-cloud detections provided by SAETTA high-frequency (60-66 MHz) electromagnetic lightning locating system, and with ground impacts provided by Meteorage, ensures the efficiency and precision of the method. This one was already used successfully in a previous field campaign (HyMeX-SOP1) in C&#233;vennes in 2012 [2,3,4]. The detection algorithm PMCC also provides the various recorded signal intensities. Assuming each sound point source radiates a spherical wave, the different propagation distances between the sources and the recording array can be compensated, so that the thunder source energies can also be localised within the flash with their relative levels. For EXAEDRE, two storms have been studied from an acoustical point of view, one with a low electrical activity on October 2nd mainly over the Mediterranean sea, and one with an intense activity on 17th September mainly overland. A significant number of flash events has been analysed, reconstructed and their energy distribution determined. For the 17th of September,&#160; acoustical events of large amplitudes are well correlated to (mostly negative) Cloud to Ground flash events. Energy localisation indicates a strong heterogeneity of its distribution within the flash, with intense sound sources concentrated inside the return strokes, mostly within the two first kilometres above the ground. Intracloud parts of the flashes appear much less energetic from an acoustical point of view. For the 2nd October, overversea events turn out quite different. [The authors acknowledge the EXAEDRE program, lead by E. Defer, for supplying the data. Present results have been obtained within the frame of the LETMA Contractual Research Laboratory between CEA, CNRS, Ecole Centrale Lyon, C-Innov, and Sorbonne Universit&#233;.]</p><p>[1] Y. Cansi, <em>Geophys. Res. Lett.</em>, <strong>22</strong>, 1021-1024, 1995</p><p>[2] L.J. Gallin, Th. Farges, R. Marchiano, F. Coulouvrat, E. Defer, W. Rison, W. Schulz, M. Nuret,<em> J. Geophys. Res. Atmos.</em>, <strong>121</strong>, 3929-3953, 2016</p><p>[3] A. Lacroix, Th. Farges, R. Marchiano, F. Coulouvrat, <em>J. Geophys. Res. Atmos.</em>, <strong>123</strong>, 12040-12065, 2018</p><p>[4] A. Lacroix, F. Coulouvrat, R. Marchiano, Th. Farges, J.-F. Ripoll, <em>Geophys. Res. Lett.</em>, <strong>46</strong>, 11479-11489, 2019</p>
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