Properties of the unusually short pulse sequencesoccurring prior to the first strokes of negativecloud-to-ground lightning flashesIvana Kolmašová1, Ondřej Santolík2, Thomas Farges3, William Rison4, Radek Lán1, and Luděk Uhlíř1

1Institute of Atmospheric Physics AS CR, Prague, Czech Republic, 2Faculty of Mathematics and Physics, Charles University,Prague, Czech Republic, 3CEA, DAM, DIF, Arpajon CEDEX, France, 4Electrical Engineering Department, New Mexico Instituteof Mining and Technology, Socorro, New Mexico, USA

Abstract We analyze broadband magnetic and electric field measurements of pulse sequences occurringprior to first return strokes of negative cloud-to-ground lightning flashes. Our observations take place afew tens of kilometers from the thunderstorm but we also report the first simultaneous observations ofpreliminary breakdown pulses from a distance of 400 km. Their amplitudes reach up to 50% of thecorresponding return stroke peak and typically decrease by ~40% during the sequences. A typical timeinterval between neighboring pulses was several tens of microseconds. We observe an unusually shortduration of the prestroke activity (1–7ms) reported for the first time during a summer thunderstorm, with alow height of initiation (3-4 km). A very fast propagation speed (~106m/s) is probably maintained for theentire prestroke process. A possible explanation can be based on a hypothesis of unusually strong negativecharge sources in the observed thundercloud.

1. Introduction

Processes leading to lightning initiation attract attention of scientists for decades and are still intensivelystudied by many groups of researches [e.g., Stolzenburg et al., 2013; Gurevich et al., 2013; Füllekrug et al., 2013;Stock et al., 2014]. Recently, Marshall et al. [2014] stressed the importance and complexity of electromagneticphenomena linked to lightning initiation processes by reporting that all observed negative cloud-to-ground(CG) lightning flashes show signs of preliminary breakdown.

A sequence of electromagnetic pulses lasting from a fewmilliseconds to several tens ofmilliseconds occurs priorto the first return stroke of negative CG flashes [Rakov and Uman, 2003, and references therein, pp. 119–120].This sequence is composed of three parts. It begins with a pulse train whose duration is on the order of 1ms.These initial pulses are believed to be connected with the preliminary breakdown processes. They are usuallyfollowed by a relatively low and irregular pulse activity which lasts from several tens to hundreds ofmilliseconds.The sequence ends with another short train of smaller pulses which lasts from several tens of microsecondsto several hundreds of microseconds, being attributed to the last stages of the stepped leader. Thesethree parts are known as “preliminary breakdown” (B), “intermediate stage” (I), and “stepped leader”(L) [Clarence and Malan, 1957]. The most frequent waveform pattern of electromagnetic signals radiatedby prestroke processes contains all these three parts (“BIL” type). For some flashes, the prestroke pulsesequences lack the intermediate stage (I). These “BL” type sequences have been observed during winterstorms in Albany, NY [Brook, 1992], and during Japanese winter storms [Wu et al., 2013].

Note that the term “leader” is somewhat confusingly used in two different ways in the lightning literature.Typically, a prestroke discharge activity which escapes from the cloud and propagates to the ground isconsidered to be a leader. For example, the above described period of the “L” part of the pulse sequencepattern meets this criterion. However, the whole prestroke pulse activity is also sometimes described as aleader. This is often the case of low-frequencymeasurements [e.g., Lu et al., 2009] where the waveform detailscannot be distinguished.

Clarence and Malan [1957] believed that the preliminary breakdown pulses (PBPs) are produced by avertical discharge between the main negative charge center and the lower positive charge center insidethe thundercloud. Stolzenburg et al. [2013] found that bursts of light recorded by the high-speed video(50,000 frames/s) are coincident with the largest PBPs in the electric field data. They hypothesized that

KOLMAŠOVÁ ET AL. ©2014. American Geophysical Union. All Rights Reserved. 5316

PUBLICATIONSGeophysical Research Letters

RESEARCH LETTER10.1002/2014GL060913

Key Points:• Measurements of preliminarybreakdown pulses from distancesup to 400 km

• Analysis of different types of prestrokepulse sequences and their amplitudes

• Unexpected fast processes prior tonegative strokes observed in summer

Correspondence to:I. Kolmašová,iko@ufa.cas.cz

Citation:Kolmašová, I., O. Santolík, T. Farges,W. Rison, R. Lán, and L. Uhlíř (2014),Properties of the unusually short pulsesequences occurring prior to the firststrokes of negative cloud-to-groundlightning flashes, Geophys. Res. Lett., 41,5316–5324, doi:10.1002/2014GL060913.

Received 15 JUN 2014Accepted 14 JUL 2014Accepted article online 18 JUL 2014Published online 30 JUL 2014

each large PBP which is accompanied by a visible light pulse can be caused by a current surge that is a fewhundreds of meters long. Karunarathne et al. [2013] located PBPs using a network of 10 electric field sensors.They observed vertical motion of pulses and attributed it to a vertical motion of negative charges in the cloud.

It is not clear what is happening inside the thundercloud during the intermediate stage and why this partof the prestroke pulse sequence is sometimes missing. Clarence and Malan [1957] believed that the chargingprocess of the vertical channel continued during the intermediate stage until the field at the bottom ofthe channel was sufficient to initiate a stepped leader.Wu et al. [2013] speculated that a horizontal propagationof the leader channel is going on during the intermediate stage.

The percentage of first return strokes which are preceded by PBP trains varies from 20% to 100% in differentstudies [Nag and Rakov, 2008; Gomes et al., 1998; Baharudin et al., 2012a; Stolzenburg et al., 2013]. Marshallet al. [2014] also compared the measurements of the PBP peak amplitudes observed at different latitudes.They concluded that all negative CG flashes probably begin with PBPs. The largest pulses in the train aretypically bipolar with the initial polarity identical to the polarity of the following first return stroke. The peak-to-peak amplitude of the largest PBPs is sometimes comparable to the amplitude of the corresponding returnstroke [Gomes et al., 1998; Baharudin et al., 2012a; Marshall et al., 2014]. The time interval betweenthe initial PBP and the first return stroke (RS) is, in most cases, several tens of milliseconds. Extremelysmall values of the PBP-RS time separation, such as 1ms, were observed only during winter storms[Brook, 1992; Wu et al., 2013]. Having observed also summer thunderstorms, Brook [1992] was able tocompare his summer and winter data sets. He found that the extremely short duration of the winterprestroke pulse activity indicated that the winter leaders have a velocity greater by a factor of ~4 thanthe summer leaders. He hypothesized that the electric field at breakdown is much stronger in winterthunderstorms. He speculated that the precipitation mix in winter, which does not involve large waterdroplets, resulted in stronger electric field in thunderclouds. This hypothesis has not yet been confirmedby in situ thundercloud observations.

In the present study we analyze properties of magnetic and electric field pulse sequences occurring priorto the first return strokes of negative lightning flashes. We study 15 sequences of pulses measured by twodifferent instruments at different distances (~12–44 km, ~ 400 km) during a thunderstorm which occurredclose to Rustrel, France, on the 11 October 2012. We observe an unusually short duration of prestroke pulsesequences that is reported for the first time in a summer thunderstorm. We also compare properties of theBIL and BL types of prestroke pulse sequences, and we analyze variations of pulse amplitudes during the “B”part of BIL type sequences. In section 2, we describe our experimental setup. In section 3, we introduceour data set. In section 4, we summarize our results concerning the PBP-RS time separations and the PBP/RSpeak amplitude ratios and present an analysis of the evolution of pulse amplitudes within the BIL typesequences. In section 5, we discuss our results.

2. Instrumentation

For measurements close to the thunderstormwe use amagnetic field loop antenna coupled with a ground-basedversion of a broadband (5 kHz to 36MHz) analyzer [Kolmašová and Santolík, 2013], which is developed for the Toolfor the Analysis of RAdiations from lightNIngs and Sprites (TARANIS) spacecraft [Blanc et al., 2007]. The analyzer isplaced in a quiet electromagnetic environment of an external measurement site of the Laboratoire Souterrain àBas Bruit (LSBB) on the summit of La Grande Montagne (1028m, 43.94°N, 5.48°E) close to Rustrel, France.

For distant measurements we use a vertical electric field dipole whip antenna mounted on a mast. Receivedsignals are analyzed in the frequency range from a few hundred Hz to 5MHz with a sampling frequencyof 12.5MHz. The instrument is placed in HYF seismic station, France (47.27°N, 2.64°E) at a distance of ~400 kmfrom Rustrel. A detailed description of the instrumentation is given in Farges and Blanc [2011].

The time assignment is done by GPS receivers connected to both analyzers. The locations, the polarities, andthe peak currents of return strokes analyzed in this study are provided by the French lightning location networkMÉTÉORAGE. Our records were completed by data of a Lightning Mapping Array (LMA) [Rison et al., 1999],which was deployed in France in 2012 in the frame of Hydrological Cycle in Mediterranean Experiment SpecialObservation Period campaign [Ducrocq et al., 2013]. The LMA was located about 100 km west of the flashesdiscussed in this paper.

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3. Data Set

The data selection has been based on broadband magnetic field waveform measurements close toRustrel. All analyzed first return strokes and the corresponding prestroke pulses were measured on the11 October 2012. Altogether, we recorded 38 return strokes during a single thunderstorm. Using theMÉTÉORAGE detection network we have identified and excluded all positive cloud-to grounddischarges and all subsequent return strokes from our analysis. Our data set then consists of 17 initialCG- return strokes all of which are preceded by PBPs. Fifteen of them were reported by MÉTÉORAGE.Prestroke activity from the two initial return strokes not reported by MÉTÉORAGE was located by theLMA. In one case, the PBP sequence was followed by two return strokes occurring so close in time(~50 μs) that we have not been sure which return stroke corresponded to the observed PBPs. Therefore,we have reduced our data set to 15 events where PBPs clearly corresponded to first return stroke.The locations of the return strokes reported by the MÉTÉORAGE and the LMA are plotted in Figure 1a.Each blue dot (MÉTÉORAGE) and black dot (LMA) represents a stroke which was preceded by a trainof pulses. The red dot shows the location of our broadband receiving station. Electromagnetic signalsradiated by all 15 return strokes were also detected by the distant receiver in HYF. After removingnarrowband interferences caused by strong radio transmitters, we have found the prestroke sequencesof pulses prior to all 15 return strokes in the HYF records. To verify the correspondence of thesedistant measurements with our data set, we have calculated the distance between the receiving stationin HYF and the MÉTÉORAGE locations of individual return strokes. We have then estimated thepropagation delay using the time assignment of the return strokes and the time of the onset of thereturn stroke peaks in the analyzed electric field waveforms. We have found that radiated signalstraveled within 2% of the speed of light. This deviation is consistent with the onset time inaccuracylinked to our estimations.

We can see the BIL type pattern in eight cases and the BL type pattern in the remaining seven cases. Examplesof close (Rustrel) and distant (HYF) waveform measurements showing the BIL patterns are respectivelyplotted in Figures 1b and 1d, starting with the first preliminary breakdown pulse and ending with thereturn stroke. These plots correspond to the return stroke marked by asterisk in Figure 1a. Examples of BLpatterns for the return stroke marked by triangle are given in Figures 1c and 1e for the close and distantmeasurements, respectively. For a further analysis, the amplitudes of the PBPs and the times of their peakvalues were estimated from numerically integrated measurements of dB/dt waveforms obtained close to thethunderstorm. To identify a pulse we have chosen a threshold of 1 nT for the peak amplitude. This thresholdis several times larger than the noise level of the analyzer [Kolmašová and Santolík, 2013]. We estimatedalso the times and peak amplitudes of the largest PBPs in the distant waveform measurements.

4. Analysis of Preliminary Breakdown Sequences

We start with the analysis of measurements close to the thunderstorm. For all 15 sequences, we haveestimated the maximum ratio of the peak-to-peak amplitudes of the PBPs relative to the peak-to-peakamplitudes of the corresponding return stroke, as well as the maximum time separation of the first pulse andthe dominant peak of the corresponding return stroke. We have found that both the first and the largestpulses are bipolar with the same initial polarity as the return stroke in all cases (see examples in Figures 1band 1c). The ratio of the peak-to-peak amplitudes of the largest PBP and the corresponding return strokevaries from 3.2% to 46.1% with the mean value of 22% (25% and 18% for the BIL and BL sequences,respectively). The time separation of the first PBP and the corresponding return stroke varies from 0.9ms to7.1ms with the mean value of 2.5ms (3.2ms and 1.8ms for the BIL and BL sequences, respectively).

In case of BL type sequences, it is difficult to distinguish where the PBPs (B part) end and where the steppedleader (L part) begins. We have therefore estimated the peak amplitudes and times of all PBPs only for eightBIL type sequences. This analysis includes the total number of 254 pulses. The duration of the individual Bparts varies from 615μs to 1768μs with a mean value of 1294μs. The number of pulses in the individual Bparts varies from 15 to 52 with a mean value of 32. Figure 2a shows a histogram of the time intervals betweenthe neighboring pulses in all eight sequences. The obtained values vary from 8μs to 253μs with amean valueof 41μs. The distribution clearly has a “heavy tail,” typical for lognormal distributions of lightning parameters[Uman, 1987, p. 339]. The distribution of the pulse amplitudes normalized by their maximum in each

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Figure 1. (a) Locations of the return strokes (blue and black dots) preceded by trains of pulses and reported, respec-tively, by MÉTÉORAGE and LMA (see text). Red dot indicates location of close magnetic field sensor. Asterisk isthe location of BIL flash for Figures 1b and 1d; triangle is the location of BL flash for Figures 1c and 1e; and plus signis the location of the flash captured by LMA in Figure 2h. (b–e) Examples of the waveforms showing sequencesof PBPs and the corresponding return strokes; Figures 1b and 1c show the close measurements of magnetic fieldwaveforms; Figures 1d and 1e show the distant measurements of electric field waveforms; Figures 1b and 1d showthe BIL type waveform pattern; Figures 1c and 1e show the BL type waveform pattern. Note: The polarity of themagnetic field pulses is determined not only by the polarity of the source currents but also by the relative positionof the antenna with respect to these currents.

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individual train for all 254 pulses is shown in Figure 2b. The minimum obtained relative amplitude is 3%with a mean value of 29%. For comparison, normalization to the amplitude of the corresponding returnstroke is used in Figure 2c. The obtained values vary from 1% to 46% with a mean value of 7%.

An example of the evolution of interpulse intervals during the B part of the prestroke sequence from Figure 1b isshown in Figure 2d. Random fluctuations of the obtained results during this train are so large that it is impossibleto determine any general trend for the evolution of the interpulse intervals. The same holds true for all otherindividual B parts of the eight analyzed BIL type pulse sequences (not shown). We have also examined the

Figure 2. (a) Histogram of the obtained values of the interpulse intervals; (b) histogram of pulse amplitudes normalized bytheir maximum in each individual sequence; (c) histogram of the ratio of pulse amplitudes and the corresponding returnstroke in each individual sequence; (d) example of evolution of the interpulse interval within the B part of the BIL typeprestroke sequence from Figure 1b; (e) example of evolution of the pulse amplitude normalized by its maximumwithin theB part of sequence from Figure 1b; (f) time series of the PBP-RS separation; (g) scatterplot of the PBP-RS time separationversus the PBP/RS ratio; (h) LMA data from 11 October 2012, (19:32:10.45 to 19:32:11.50); the triangles are the times of theCG strokes detected by MÉTÉORAGE.

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evolution of amplitudes in the eight B parts as a function of time. An example for the prestroke sequence fromFigure 1b is shown in Figure 2e, where the obtained pulse amplitudes are normalized by their maximum. Theresults suggest a general decreasing trend demonstrated by a linear model (overplotted in Figure 2e) which hasbeen calculated using coefficients of a linear least squares regression. To verify if the normalized amplitudes aretypically decreasing, we have calculated the time-amplitude rank correlation coefficients for all the analyzedsequences using the procedure described by Kolmašová and Santolík [2013]. The obtained values for sevenout of eight BIL type sequences are negative, corresponding to decreasing amplitudes. The correspondingprobabilities of randomoccurrence of negative rank correlation coefficients are less than 15% for all seven trainsand less than 7% for five trains. Only one of the sequences shows a positive correlation but a higher probabilityof its random occurrence (21%). The amplitudes are therefore typically decreasing. To roughly characterizethis trend, we have estimated the linear regression coefficients in all B parts for seven BIL type sequences withnegative correlation. The results indicate that amplitude drops on average by 38% of its maximum duringthe duration of these trails.

In order to analyze the differences in the properties of the BIL and BL patterns of prestroke pulse sequences,we have plotted the development of the PBP-RS time separation for all the sequences which occurred duringthe observed thunderstorm (Figure 2f). Figure 2g shows a scatterplot of the PBP-RS time separation versusthe PBP/RS ratio using different plotting symbols for the two types of sequence patterns.

We have also estimated the PBP/RS ratio and the PBP-RS separation for all distant waveform measurements.We have obtained exactly the same values for the PBP-RS separation times as in the analysis of closemeasurements. The PBP/RS ratio varies from 5.4% to 49.1% with a mean value of 22% for all trains. This is alsovery similar to the results of close measurements. In contrast with the similarity of these average values, theratios obtained from each individual sequence separately are different for close and distant measurements.The observed differences vary from4% to 71%. This can be explained by a relatively lower frequency band of theHYF receiver, which could not fully reproduce the high-frequency part of the radiated signal.

The LMA detected prestroke activity before 14 of the 15 return strokes. The return stroke carries groundpotential to the top of the leader channel, which results in very strong electric fields at the top of the channel,and leads to the additional electrical breakdown a few microseconds after the return stroke. The LMAdetected sources associated with this subsequent breakdown in 11 of the 15 return strokes. Figure 2h showsthe LMA sources for one of the 15 return strokes (marked by a plus symbol in Figure 1a), with an expandedview of the initial prestroke activity and the subsequent breakdown at the top of the channel. The LMAdata shows that the prestroke activity in all cases started at 3 to 4 km and had durations of 2 to 4 ms.

5. Discussion

We have observed the pulse activity prior to 85% of the first return strokes recorded by our analyzer close tothe thunderstorm on 11 October 2012 (17 cases out of 20). The analyzer was triggered by PBPs in these cases.The analyzer was triggered by return strokes in the remaining 15% of cases where the PBPs have not beenrecorded. We cannot distinguish if the PBPs were not strong enough to trigger the analyzer in these cases orif they were missing. When we compare the close and distant waveforms of the individual prestrokesequences, we can recognize similar pulse patterns as a function of time in all measured waveforms. Thepulses are distinguishable in the ~400 km distant records, even if the PBP/RS ratio drops down to 3%. Such adistant observation of PBPs is reported, to our knowledge, for the first time.

The PBP/RS ratios and the PBP-RS time separations were previously measured in different geographiclatitudes. The results are summarized in Table 1. Our PBP/RS ratios are more similar to the results obtained inthe tropical regions. Figure 2c shows that the amplitudes of all pulses in all analyzed sequences did notexceed one half of the amplitude of the corresponding return stroke. This behavior was observed in lowlatitudes [Gomes et al., 1998; Baharudin et al., 2012b]. Nag et al. [2008] showed the histogram of normalizedpulse amplitudes for one cloud and one cloud-to-ground discharge observed in Florida during a summerstorm (their Figures 5 and 12). The normalized amplitudes of overwhelming majority of pulses in their studydid not exceed 25%. In our case, the distribution of amplitudes of all PBPs normalized by their maxima inindividual sequences (Figure 2b) drops more slowly in comparison with the results obtained by Nag et al.[2008]. In addition, it exhibits a small peak at 90–100%. A small group of larger PBPs in each train observed

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during our analyzed summer thunderstorm can indicate strong negative charge sources, which are usuallyattributed to winter thunderclouds [Brook, 1992].

We have obtained a low value of the average PBP-RS time separation, which is also typical for winterthunderstorms (see Table 1). We can exclude that such a low value was obtained as a result of wrongassignment of the prestroke pulses to the return strokes. We can speculate that this observed short totalduration of the prestroke activity can be linked only to the low height of the cloud base. However, bycombining the height of the initiation of the prestroke activity obtained from LMA data (~3–4 km) and theaverage duration of the PBP-RS time separation (2.5ms), we get very fast average speed of propagation of theprestroke discharges in analyzed thunderstorm, starting by in-cloud PBPs and ending by the stepped leader.The duration of these processes (~2–4ms) obtained from LMA data is consistent with the duration of theprestroke pulse sequences from broadband records. This confirms that the propagation velocities of theprestroke processes were 1 × 106 to 2 × 106m/s, much faster than typical velocities of stepped leaders prior toa typical CG flash which are on the order of 105m/s [Heavner et al., 2002]. Very fast prestroke processes areprobably indicative of strong negative charge sources, when lower positive charge is either entirely or partlyconsumed by the initial negative in-cloud leader [Nag and Rakov, 2009] and is incapable to slow thepropagation down by 1 order of magnitude to the typical stepped leader speeds. This charge configurationcan be possibly explained by unusually small liquid water content in the summer thundercloud, which wouldresult in a lower conductivity of hydrometeors inside the cloud, a higher corona discharge threshold, andstronger electric fields [Griffiths et al., 1974; Heavner et al., 2002]. Fast prestroke processes are again typical forwinter thunderstorms [Brook, 1992]. When we take into account our results and the maximum daytimetemperature (22°C) and the minimum nighttime temperature (15°C) measured at the weather station inAvignon/Caumont (15–50 km from the lightning locations) on 11 October 2012, we can state that we haveobserved a very unusual thunderstorm exhibiting winter signatures in summer.

The interpulse intervals between the PBPs were reported in the past by Nag and Rakov [2009] for the Floridaseaside. They obtained the average interpulse interval of 65μs. The mean value of the interpulse intervalcalculated from our data set is 41μs. Considering the large spread of obtained values in Figure 2a, our resultsare consistent with the previously published data. Fluctuations of the interpulse intervals within theindividual trains of are too large for finding any general trends in their evolution. Combining the estimatedaverage propagation speed of the prestroke processes on the order of 106m/s with the wide distribution ofmeasured interpulse intervals between the PBPs, we obtain typical step lengths of tens to hundreds ofmeters. The upper part of our distribution is in a rough agreement with the results of Stolzenburg et al. [2013]who found that each large PBP can be caused by a current surge that is a few hundreds of meters long.

We have shown that the pulse amplitude is typically decreasing by ~40% during the B parts of BIL typetrains. Such a quantitative analysis has been done for the first time. The observed decrease of the pulseamplitudes during the sequences is more than twice faster, and the interpulse intervals are several timeslonger than the values obtained using the same method by Kolmašová and Santolík [2013] for intracloudunipolar pulse sequences connected with K changes and dart-stepped leaders preceding subsequentstrokes. Assuming the same order of magnitude of the propagation speed (106m/s) of in-cloud PBPs before

Table 1. Overview of Previous Observations

Location PBP/RS Ratio (%) PBP-RS Time Separation (ms) Reference

Albany, NY, USA (winter) 70 2.75 Brook [1992]Srí Lanka 16.5 11.9 Gomes et al. [1998]Uppsala, Sweden 101.4 13.8 Gomes et al. [1998]Johor, Malaysia 27.8 57.6 Baharudin et al. [2012a]Florida, USA 29.4 22.7 Baharudin et al. [2012a]Hokuriku, Japan (winter) 47 (BIL) 3.6 (BIL) Wu et al. [2013]

45 (BL) 1.2 (BL)Florida, USA 20 43.8 Marshall et al. [2014]Austria 51 53.1 Marshall et al. [2014]Dakota, USA 22 N/A Marshall et al. [2014]Rustrel, France 25 (BIL) 3.2 (BIL) This study

18 (BL) 1.8 (BL)

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the first stroke and in-cloud processes before the subsequent strokes, these results can indicate thatthe detailed charge structure in the thundercloud leading to the initiation of the first stroke differsfrom finer charge structures existing inside the cloud during the initiation of subsequent strokes. Forthe subsequent strokes, these hypothetical periodic charge structures might have a characteristicdimension of 10m [Kolmašová and Santolík, 2013] while for first strokes we would obtain several tensto hundreds of meters.

Finally, we took advantage of the fact that we have observed both types of prestroke sequences during onethunderstorm, and we have tried to find a difference in their properties. We can exclude an instrumentaleffect causing the lack of the “I” part in the records, since we observed the same pattern in both the close anddistant recordings done by different instruments. The BL type sequences have been found on average by~44% shorter than the BIL type sequences. Figure 2f can indicate, similarly as Figure 7 ofWu et al. [2013] thatthe BIL and BL types might occur in different stages of the thunderstorm, but in the first 40min of ourrecordings, the two types aremixed together. Unlike observationsmade byWu et al. [2013] andMarshall et al.[2014] who reported that they never observed strokes with both large values of the PBP-RS separation(characteristic for the BIL sequences) and the PBP/RS ratio, our Figure 2g does not show any clear relationbetween these two properties. In summary, we have not found any substantial differences in the propertiesof both sequence types other than the PBP-RS separation. Therefore, we cannot verify the hypothesis madeby Wu et al. [2013] that long BIL sequences are composed of a weak initial B part followed by a horizontallypropagating in-cloud leader during the I part of the sequence. However, our data set is insufficient forderiving any general conclusions from the comparison of BIL and BL type sequences; an extended data set ofsequences recorded in different conditions is needed for further analysis.

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AcknowledgmentsThis work was supported by the ASCR(grant M100421206) and by GACRgrant 14-31899S. The HYMEX specialobservation period atmospheric electri-city field campaign was sponsored bygrants MISTRALS/HYMEX and ANR-11-BS56-0005IODA-MED. Data analyzed inthis paper are on http://bleska.ufa.cas.cz.We thank Laboratoire Souterrain a BasBruit, UNS/UAPV/CNRS, Rustrel, France,and its director S. Gaffet, for usage oftheir facility at La Grande Montagne forour receiving station. We are grateful toA. Cavaillou from LSBB, and J. Vojta andF. Hruska from IAP for their technicalassistance. We thank M. Fullekrug fromthe University of Bath, UK, for providingus with MÉTÉORAGE data.

The Editor thanks two anonymousreviewers for their assistance inevaluating this paper.

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