Abstract. Extreme precipitation is a weather phenomenon with tremendous damaging potential for property and human life. As the intensity and frequency of such events is projected to increase in a warming climate, there is an urgent need to advance the existing knowledge on extreme precipitation processes, statistics and impacts across scales. To this end, a working group within the Germany-based project, ClimXtreme, has been established to carry out multidisciplinary analyses of high-impact events. In this work, we provide a comprehensive assessment of the 29 June 2017 heavy precipitation event (HPE) affecting the Berlin metropolitan region (Germany), from the meteorological, impacts and climate perspectives, including climate change attribution. Our analysis showed that this event occurred under the influence of a mid-tropospheric trough over western Europe and two shortwave surface lows over Britain and Poland (Rasmund and Rasmund II), inducing relevant low-level wind convergence along the German–Polish border. Over 11 000 convective cells were triggered, starting early morning 29 June, displacing northwards slowly under the influence of a weak tropospheric flow (10 m s−1 at 500 hPa). The quasi-stationary situation led to totals up to 196 mm d−1, making this event the 29 June most severe in the 1951–2021 climatology, ranked by means of a precipitation-based index. Regarding impacts, it incurred the largest insured losses in the period 2002 to 2017 (EUR 60 million) in the greater Berlin area. We provide further insights on flood attributes (inundation, depth, duration) based on a unique household-level survey data set. The major moisture source for this event was the Alpine–Slovenian region (63 % of identified sources) due to recycling of precipitation falling over that region 1 d earlier. Implementing three different generalised extreme value (GEV) models, we quantified the return periods for this case to be above 100 years for daily aggregated precipitation, and up to 100 and 10 years for 8 and 1 h aggregations, respectively. The conditional attribution demonstrated that warming since the pre-industrial era caused a small but significant increase of 4 % in total precipitation and 10 % for extreme intensities. The possibility that not just greenhouse-gas-induced warming, but also anthropogenic aerosols affected the intensity of precipitation is investigated through aerosol sensitivity experiments. Our multi-disciplinary approach allowed us to relate interconnected aspects of extreme precipitation. For instance, the link between the unique meteorological conditions of this case and its very large return periods, or the extent to which it is attributable to already-observed anthropogenic climate change.

<p>Thunderstorms associated with severe weather phenomena such as heavy rainfall, wind gusts, hail, tornados or lightning frequently cause considerable damage across the globe. In light of climate change, it is generally anticipated that the increase in thermal instability due to an increase in low-level temperature and moisture leads to more and more intense thunderstorms, thus increasing the risk from convective storms. However, previous studies have shown high annual and interannual variability of thunderstorm frequency in Europe related to large-scale atmospheric processes and mechanisms such as blocking or teleconnection patterns.</p> <p>To investigate the relationship between large-scale atmospheric conditions and local-scale thunderstorms and their temporal changes, as well as, their long-term trends, we used cloudto-ground (CG) lightning measurements in western and central Europe from 2001 to 2021 (summer half-year; EUCLID network), representing a spatially and temporally homogeneous dataset. Surprisingly, trend analyses of this dataset show a large contiguous area with significantly decreasing trends from central France to Belgium and the Eifel region in Germany – for both lightning activity and thunderstorm days (defined as at least 5 lightning strokes on a 10 x 10 km² grid). In contrast, significant positive trends occur in northern Spain and some parts of the Balkans. However, in other regions of Europe, the observed trends are not significant due to high annual variability.</p> <p>Further analyses suggest a relation between large-scale flow characteristics and thunderstorm activity in different regions of Europe. For example, based on odds ratio analyses, episodes with negative values of the North Atlantic Oscillation (NAO) Index are associated with decreasing thunderstorm activity in the above mentioned region (France to Germany). We observed that negative NAO phases, which have occurred more frequently over the last decade, may be an explanation for the negative trends in lightning activity and thunderstorm days.</p> <p>In addition, we present a new algorithm for the temporal and spatial clustering of CG lightning based on ST-DBSCAN (Spatial-Temporal Density-Based Spatial Clustering of Applications with Noise). This algorithm also filters lightning in low-density regions and thus allows for the detection of contiguous areas of high lightning activity (clustered convective events, CCE). Initial analysis of the newly generated dataset shows that in recent years the number of CCEs with small spatial extent has increased, whereas the number of CCEs with large spatial extent has decreased.</p>

Abstract. Extreme precipitation is a weather phenomenon with tremendous damaging potential for property and human life. As the intensity and frequency of such events is projected to increase in a warming climate, there is an urgent need to advance the existing knowledge on extreme precipitation processes, statistics and impacts across scales. To this end, a working group within the German-based project ClimXtreme, has been established to carry out multidisciplinary analyses of high-impact events. In this work, we provide a comprehensive assessment of a selected case, affecting the Berlin metropolitan region (Germany) on 29 June 2017, from the meteorological, impacts and climate perspectives, additionally estimating the contribution of climate change to its extremeness. Our analysis shows that this event occurred under the influence of a mid-tropospheric trough over western Europe and two short-wave surface lows over Britain and Poland, inducing relevant low-level wind convergence along the German-Polish border. Several thousand convective cells were triggered in the early morning of 29 June, displacing northwards slowly under the influence of a weak tropospheric flow (10 m s-1 at 500 hPa). A very moist and warm southwesterly flow was present south of the cyclone over Poland, in the presence of moderate Convective Available Potential Energy (CAPE). We identified the soil in the Alpine-Slovenian region as the major moisture source for this case (63 % of identified sources). Maximum precipitation amounted to 196 mm d-1, causing the largest insured losses due to a heavy precipitation event in the period 2002 to 2017 (€60 Mill.) over the area. A household-level survey revealed that the inundation duration was 4 to 12 times larger than other surveyed events in Germany in 2005, 2010 and 2014. The climate analysis showed return periods of over 100 years for daily aggregated precipitation, and up to 100 years and 10 years for 8 h and 1 h aggregations, respectively. The event was the 29th most extreme event in the 1951–2021 climatology in terms of severity and the second with respect to the number of convective cells triggered from 2001 to 2020 over Germany. The conditional attribution demonstrated that warming since the pre-industrial era caused a small, but significant increase of 4 % in total precipitation and 10 % for extreme intensities. The aerosol sensitivity experiments showed that increased anthropogenic aerosols induce larger cloud cover and probability of extreme precipitation (> 150 mm d-1). Our analysis allowed relating interconnected aspects of extreme precipitation. For instance, the link between the unique meteorological conditions of this case and its climate extremeness, or the extent to which this is attributable to already-observed anthropogenic climate change.

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