Thunderstorms are a prominent weather element in Europe particularly during summer months. Apart from lightning and thunder, they may be associated with various other phenomena, such as heavy rain, hail, strong wind gusts, and sometimes even tornadoes. In southern Germany, these weather events account for the major share of insured losses due to natural hazards. Impressive examples are the Reutlingen hail storm of 28 July 2013 (Kunz et al., 2017) and the Braunsbach flash flood of 29 May 2016 (Piper et al., 2016).
Thunderstorm frequency strongly depends on the region. Simultaneously, it is subject to large year-to-year fluctuations. At the Institute of Meteorology and Climate Research (IMK-TRO) and the Center for Disaster Management and Risk Reduction Technology (CEDIM) at Karlsruhe Institute of Technology, scientists performed statistical analyzes of high-resolution lightning data for the summer half-year, in order to quantify the spatial and temporal variability and to better understand the underlying processes (Piper and Kunz, 2017). A relatively long reference period of 14 years (2001-2014) and a high accuracy of the data, which have been provided by Blitzinformationsdienst Siemens, led to reliable and robust results.
In Germany, most thunderstorm days occur along the northern Alpine range and over the Alpine foreland, with the highest values observed west and east of the town of Garmisch-Partenkirchen. Other prominent maxima are located between the river Neckar and the Swabian Jura, over the Ore Mountains, and over the Bavarian Forest. Thunderstorm activity is weakest along the coasts of the North and Baltic Seas, particularly around the city of Kiel.
With additional consideration of Austria, Switzerland, Benelux, and France, the results show a continuous increase of the mean number of thunderstorm days from the northwest to the southeast. Superimposed on this large-scale trend are several distinct regional structures caused by the individual mountain ranges. Most thunderstorm days are observed in some parts of the southern Austrian Alps and in the area between the Swiss canton of Ticino and the Italian city of Turin. However, some deep inner-alpine valleys, such as the upper Rhône valley, are characterized by an extremely weak thunderstorm activity.
This spatial pattern is governed by three factors: variable distance to marine areas, local orographic features, and regional differences in the abundance of low-level moisture. Sea water tends to inhibit thunderstorm formation during summer, since it cools, and thus stabilizes, the lower atmospheric layers. In mountainous areas, conversely, various lifting mechanisms are present forcing the air to ascend and, therefore, promoting thunderstorm genesis. In those alpine areas, however, where the local orography prevents sufficient moisture supply, only sporadic thunderstorms are observed.
The diurnal and seasonal courses of lightning activity are subject to major spatial differences as well. In most regions, lightning incidence culminates in the afternoon or evening with the maximum varying within the time range between 15:00 and 20:00 local time. Nighttime thunderstorms play a major role in several regions, such as along the northern Alpine range in Bavaria, whereas they have to be considered as highly unusual in some other areas. The peak season for thunderstorm activity is given by the months June, July, and August with a maximum in July in most places. A notable exception from this pattern is observed over the Mediterranean, where most lightning flashes occur in September. During autumn, sea water cools more slowly than air and, thus, represents a source of instability.
Furthermore, thunderstorm activity is characterized by a strong multiannual variability. The scientists showed that this long-term temporal behavior can partly be attributed to the natural climate variability. For this purpose, they investigated the impact of various atmospheric teleconnections, such as the North Atlantic Oscillation (NAO), on the spatial distribution of thunderstorm activity. For example, less thunderstorm days are generally observed in periods, when the NAO index is in its positive phase. This effect can be explained by high-pressure ridges prevailing during positive North Atlantic Oscillation phases and inhibiting thunderstorm initiation due to a lack of large-scale lifting.
Piper, D. und M. Kunz (2017): Spatio-temporal variability of lightning activity in Europe and the relation to the North Atlantic Oscillation teleconnection pattern. Nat. Hazards Earth Syst. Sci., 17, 1319–1336.
Kunz, M., U. Blahak, J. Handwerker, M. Schmidberger, H. Punge, S. Mohr, E. Fluck, und K. M. Bedka (2017): The severe hailstorm in SW Germany on 28 July 2013: Characteristics, impacts, and meteorological conditions. Quart. J. Roy. Meteor. Soc., eingereicht.
Piper, D., M. Kunz, F. Ehmele, S. Mohr, B. Mühr, A. Kron, und J. Daniell (2016): Exceptional sequence of severe thunderstorms and related flash floods in May and June 2016 in Germany. Part I: Meteorological background. Nat. Hazards Earth Syst. Sci., 16, 2835–2850.