Comment on nhess-2022-96

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.