Exceptional sequence of severe thunderstorms and related flash floods in May and June 2016 in Germany – Part 1: Meteorological background

Abstract

Abstract. During a 15-day episode from 26 May to 9 June 2016, Germany was affected by an exceptionally large number of severe thunderstorms. Heavy rainfall, related flash floods and creek flooding, hail, and tornadoes caused substantial losses running into billions of euros (EUR). This paper analyzes the key features of the severe thunderstorm episode using extreme value statistics, an aggregated precipitation severity index, and two different objective weathertype classification schemes. It is shown that the thunderstorm episode was caused by the interaction of high moisture content, low thermal stability, weak wind speed, and large-scale lifting by surface lows, persisting over almost 2 weeks due to atmospheric blocking.For the long-term assessment of the recent thunderstorm episode, we draw comparisons to a 55-year period regarding clusters of convective days with variable length (2-15 days) based on precipitation severity, convection-favoring weather patterns, and compound events with low stability and weak flow. It is found that clusters with more than 8 consecutive convective days are very rare. For example, a 10-day cluster with convective weather patterns prevailing during the recent thunderstorm episode has a probability of less than 1 %. IntroductionBetween the end of May and mid-June 2016, Germany and large parts of central and southern Europe were affected by an exceptionally large number of severe convective storms and related extremes such as heavy rainfall, hail, and tornadoes (Fig. 1). Rain totals exceeding 100 mm within a few hours at several locations in Germany triggered various flash floods and floods mainly in small catchments. In the town of Braunsbach in the federal state of Baden-Württemberg, for example, a severe flash flood on 29 May with a height of up to 3.5 m caused serious damage to more than 80 buildings, of which five were completely lost (Daniell et al., 2016). Only 3 days later on 1 June, extreme rain in the district of Rottal-Inn in the south of Bavaria evoked a sudden and dramatic rise in the levels of several creeks such as the Simbach, where the height increased from 20 cm to more than 5 m within only 12 h. Subsequently, the village Simbach am Inn experienced the largest flooding in history. Some of the thunderstorms during the 2 weeks also produced hail with diameters between 0.5 and 5 cm. A total of 12 tornadoes in 8 days with intensities between F0 and F1 on the Fujita intensity scale, were recorded and confirmed by the European Severe Weather Database (ESWD; Dotzek et al., 2009).The severe thunderstorms caused substantial damage to buildings, infrastructures, transportation networks, and crops. A large number of roads and railroads were blocked or severely damaged, and some villages experienced power outPublished by Copernicus Publications on behalf of the European Geosciences Union.

Representation of Model Error in Convective‐Scale Data Assimilation: Additive Noise, Relaxation Methods, and Combinations

“…It can be clearly seen that the first week was predominantly strong forcing and the second week was predominantly weak forcing. This is in good agreement with Piper et al () that found particularly low atmospheric stability in the second week.…”

Spatiotemporal variability of lightning activity in Europe and the relation to the North Atlantic Oscillation teleconnection pattern

“…Information about both polarity and current strength is neglected, and flashes rather than strokes are studied with the grouping procedure performed internally by EUCLID. Since the LF operational range implies a significantly lower detection efficiency of cloud-to-cloud (IC) lightning (Pohjola and Mäkelä, 2013;Schulz et al, 2014), only cloud-to-ground (CG) flashes are taken into account.…”

“…In addition, as shown by Piper et al (2016), days with large-scale weather types favorable for convection frequently form clusters of several days; i.e., the incidence of such weather types implies a considerable probability of a longer-lasting convective situation. This persistent behaviour amplifies the number of the corresponding favorable flow patterns in those years, when they frequently set in, and hence the interannual variability of convective activity.…”

“…Among the damage caused by lightning strikes, convectionrelated weather phenomena such as strong wind gusts, heavy rain, hail, and tornadoes often lead to major economic losses and pose a significant threat to human life (e.g., Kunz and Puskeiler, 2010;Peyraud, 2013;Piper et al, 2016). In several European countries and regions such as Switzerland and southern Germany, the largest share of losses by natural hazards is related to severe convective storms (e.g., Punge et al, 2014;Puskeiler et al, 2016).…”

Tailoring Climate Parameters to Information Needs for Local Adaptation to Climate Change

“…Climate change is evident in Baden-Wuerttemberg with respect to daily minimum and maximum temperatures, including heat, as well as heavy precipitation events [40][41][42]. Baden-Wuerttemberg is highly exposed to extreme meteorological events, including hot and dry summers [43,44], hailstorms [45], and heavy precipitation events [46], and several municipalities in Baden-Wuerttemberg have experienced negative effects of such events during years past. For the future, climate model simulations project a significant warming on seasonal and annual scales, an increase in the frequency, intensity, and duration of heat waves [47,48], and in mean winter precipitation, and a slight decrease in summertime mean precipitation; extreme precipitation events are expected to become more frequent [32].…”