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We combine electromagnetic measurements with meteorological and lightning detection data to explain an observation of unusually strong preliminary breakdown (PB) produced by a thunderstorm system that developed along the Mediterranean Coast of Southern France in the early hours of 19 June 2013. This multi‐cellular storm was composed of several parallel convective lines in the NW‐SE direction. Our analysis focuses on 10 sequences of energetic electromagnetic PB pulses recorded by two receivers located at different distances from this thunderstorm. The peak currents, which generated these strong PB pulses, reached −36 kA. The initial polarity of all observed energetic PB pulses confirmed the movement of the negative charge downward, as in case of PB pulses preceding negative cloud‐to‐ground discharges. The locations of PB pulses appeared in areas with none or very weak lightning activity. Most PB pulses were initiated in small, short‐living, rapidly moving convective storm cells characterized by low reflectivity values (generally <40 dBZ), weak vertical development, and low flash density. Our findings indicate that the observed thunderstorm might possess temporary strong negatively charged pockets located above a strong positive charge region at low‐level. Such charge arrangement, likely explains our observation of unusually strong PB pulses and the absence of RS pulses in electromagnetic recordings. Key Points: Energetic preliminary breakdown pulses were found in broadband electromagnetic recordings from a multi‐cell summer thunderstormAnalysis of electromagnetic and radar data placed the preliminary breakdown process in small short living cells outside or on edges of the main convective linePresence of strong negative charge pockets and a strong lower positive charge region inside the thundercloud could explain the observation [ABSTRACT FROM AUTHOR]

The common phenomenon of lightning still harbors many secrets such as what are the conditions for lightning initiation and what is driving the discharge to propagate over several tens of kilometers through the atmosphere forming conducting ionized channels called leaders. Since lightning is an electric discharge phenomenon, there are positively and negatively charged leaders. In this work we report on measurements made with the LOFAR radio telescope, an instrument primarily build for radio-astronomy observations. It is observed that a negative leader rather suddenly changes, for a few milliseconds, into a mode where it radiates 100 times more VHF power than typical negative leaders after which it spawns a large number of more typical negative leaders. This mode occurs during the initial stage, soon after initiation, of all lightning flashes we have mapped (about 25). For some flashes this mode occurs also well after initiation and we show one case where it is triggered twice, some 100 ms apart. We postulate that this is indicative of a small (order of 5 km 2 ) high charge pocket. Lightning thus appears to be initiated exclusively in the vicinity of such a small but dense charge pocket. [ABSTRACT FROM AUTHOR]

With LOFAR we have been able to image the development of lightning flashes with meter‐scale accuracy and unprecedented detail. We discuss the primary steps behind our most recent lightning imaging method. To demonstrate the capabilities of our technique we show and interpret images of the first few milliseconds of two intracloud flashes. In all our flashes, the negative leaders propagate in the charge layer below the main negative charge. Among several interesting features we show that in about 2 ms after initiation the primary initial leader triggers the formation of a multitude (>10) negative leaders in a rather confined area of the atmosphere. From these only one or two continue to propagate after about 30 ms to extend over kilometers horizontally while another may propagate back to the initiation point. We also show that normal negative leaders can transition into an initial leader like state, potentially in the presence of strong electric fields. In addition, we show some initial breakdown pulses that occurred during the primary initial leader, and even during two "secondary" initial leaders that developed out of stepped leaders. Key Points: Our new LOFAR imaging procedure can locate over 200 sources per millisecond of flash with meter‐scale accuracyThe primary initial leader breaks up into many (>10) negative leaders of which only one or two continue after 30 msSome negative leaders propagate from the positive charge layer back to get close to the initiation point [ABSTRACT FROM AUTHOR]