Lightning activity in northern Europe during a stormy winter: disruptions of weather patterns originating in global climate phenomena
Abstrakt
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 degrees x 0.5 degrees 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 Nino event.
Increased updraft strengths, which are believed to accompany the cold-to-warm transition phase of El Nino, 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.