Abstract. In this study, we use World Wide Lightning Location Network data and investigate properties of more than 90 000 lightning strokes which hit northern Europe during an unusually stormy winter in 2014/15. Thunderstorm days with at least two strokes hitting an area of 0.5∘ × 0.5∘ occurred 5–13 times per month in the stormiest regions. Such frequency of thunderstorm days is about 5 times higher than the mean annual number calculated for the same region over winter months in 2008–2017. The number of individual winter lightning strokes was about 4 times larger than the long-term median calculated over the last decade. In the colder months of December, January and February, the mean energy of detected strokes was 2 orders of magnitude larger than the global mean stroke energy of 1 kJ. We show for the first time that winter superbolts with radiated electromagnetic energies above 1 MJ appeared at night and in the morning hours, while the diurnal distribution of all detected lightning was nearly uniform. We also show that the superbolts were often single stroke flashes and that their subsequent strokes never reached megajoule energies. The lightning strokes were concentrated above the ocean close to the western coastal areas. All these lightning characteristics presume anomalously efficient winter thundercloud charging in the eastern North Atlantic, especially at the sea–land boundary. We found that the resulting unusual production of lightning could not be explained solely by an anomalously warm sea surface caused by a positive phase of the North Atlantic Oscillation and by a starting super El Niño event. Increased updraft strengths, which are believed to accompany the cold-to-warm transition phase of El Niño, might have acted as another charging driver. We speculate that a combination of both these large-scale climatic events might have been needed to produce the observed enormous amount of winter lightning in winter 2014/15.

Abstract. In this study, we use the World Wide Lightning Location Network data and investigate properties of more than ninety thousand lightning strokes which hit Northern Europe during an unusually stormy winter 2014/2015. Thunderstorm days with at least two strokes hitting an area of 0.5° × 0.5° occurred 5–13 times per month in the stormiest regions. Such frequency of thunderstorm days is about five times higher than a mean annual number calculated for the same region over winter months in 2008–2017. The number of individual winter lightning strokes was about four times larger than the long-term median calculated over the last decade. In colder months of December, January and February, the mean energy of detected strokes was by two order of magnitude larger than the global mean stroke energy of 1 kJ. We show for the first time that winter superbolts with radiated electromagnetic energies above one mega joule appeared at night and in the morning hours, while the diurnal distribution of all detected lightning was nearly uniform. We also show that the superbolts were often single stroke flashes and that their subsequent strokes never reached MJ energies. The lightning strokes were concentrated above the ocean close to the western coastal areas. All these lightning characteristics favors a hypothesis that the intense winter lightning activity might have originated in an anomalously warm sea surface in the eastern North Atlantic which made the thundercloud charging more efficient. The increase of the sea surface temperature and resulting unusual production of lightning in winter 2014/2015 might have been caused by a starting super El Nino event, by a positive phase of the North Atlantic Oscillation or by a combination of both these large-scale climatic events.

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<p>We study evolution of lightning activity accompanying rapid intensity changes of tropical cyclones worldwide. We use a dataset of 400 tropical cyclones occurring between 2012 and 2017. We use the cyclones tracks from the International Best Track Archive for Clime Stewardship. The lightning data are provided by the World Wide Lightning Location Network (WWLLN). We inspect the lightning activity and median stroke energies accompanying rapid intensifications (RI) of cyclones, defined as increases of the wind speed by more than 30 kt in 24 hours, and their rapid weakenings (RW), defined as decreases of the wind speed by more than 40 kt in 24 hours.</p> <p>In an area of radial wind maximum (RWM), we observe a stroke density of 15.1 strokes/(100 km)<sup>2</sup>/hour for RI and 21.8 strokes/(100 km)<sup>2</sup>/hour for RW, respectively, which is much higher than average RWM density 7.9 strokes/(100 km)<sup>2</sup>/hour over the duration of the cyclone. A median stroke energy is 0.3 kJ during RI and 0.7 kJ during RW. It means that during rapid intensification of cyclones, there are less strokes with slightly higher energies and during rapid weakening there are more strokes with slightly lower energies. When analyzing the cyclones in both hemispheres separately, we obtain 0.3 kJ for RI and 0.6 kJ for RW in the northern hemisphere, and 0.8 kJ for RI and 0.9 kJ for RW in the southern hemisphere. The difference in the stroke density during RI and RW was observed larger in the northern hemisphere (19.7 vs 34.1 strokes/(100 km)<sup>2</sup>/hour), when in the southern hemisphere the stroke density is much lower and differs less (4.4 strokes/(100 km)<sup>2</sup>/hour for RI and 5.1 strokes/(100 km)<sup>2</sup>/hour for RW).</p> <p> </p>

<p>We used the World Wide Lightning Location Network and cyclones tracks from the International Best Track Archive for Climate Stewardship to study properties of lightning strokes occurring in tropical cyclones. We studied 429 cyclones occurring from 2012 to 2017 in both hemispheres with more than 11 million lightning strokes found within a distance of 600 km from the cyclone eye. For purposes of our study, we divided the cyclones into 6 basins: Indian Ocean, North Atlantic, Northeast Pacific and Northwest Pacific in the northern hemisphere and Indian Ocean and Southern Pacific in the southern hemisphere. We found differences in the numbers, energies and multiplicities of lightning strokes occurring in the cyclones in the northern and southern hemispheres. We calculated the median stroke energy for each cyclone. We used Saffir-Simpson scale for classifying the intensity of tropical cyclones and found a tendency of decreasing median stroke energies with an increasing cyclone intensity. We compared the evolution of lightning activity accompanying the cyclones with the evolution of their central pressure and wind speed to examine the possibility of using the lightning activity for prediction of cyclone intensity changes. In the northern hemisphere, there was on average about 28 thousands of strokes per cyclone with a median energy of 1.7 kJ, while in the southern hemisphere, there was on average 24 thousands of strokes per cyclone with a median energy of 2.7 kJ. The difference in multiplicity is not really noticeable with an average of 1.39 strokes per flash in the northern hemisphere and 1.34 strokes per flash in the southern hemisphere. In our dataset, we found 28 strokes with an energy over 1 MJ (superbolts), which occurred in a short period during the winter 2013-14, which was the winter exhibiting the largest SOI (Southern oscillation index).</p><p> </p>