The UMBRELLE project explored how case studies of high impact observed extreme precipitation events might respond to climate change, through comprehensive analyses of the mechanisms controlling their changes. On 1 October 2020, the extra-tropical storm Alex hit western France, bringing heavy rainfall and flooding. It initiated a south-westerly atmospheric flow in the Mediterranean Sea, which led to the development of a Mediterranean heavy precipitation event in the French and Italian Alpine regions. This Mediterranean event was of rare intensity, with record-breaking precipitation that caused severe flooding and landslides, killing at least 10 people, and causing economic losses of about 1 billion euros. This sequence of the mid-latitude storm Alex and the intense convective Mediterranean heavy precipitation event in 2020 was therefore highly relevant to UMBRELLE and was selected for its uniqueness, the diversity in the nature of the two successive events, and their societal impacts.
Key achievements of UMBRELLE include the realistic simulation the observed event in October 2020, using the CNRM-AROME model. The simulations accurately reproduced the characteristics and impacts of extreme rainfall, providing valuable insights into the underlying mechanisms. This was achieved by the kilometer-scale spatial resolution that enables processes of deep atmospheric convection to be explicitly resolved. The accuracy of the simulated precipitation was established by comparison of high quality and high resolution observations merging rainfall estimates from radars and gauges.
Besides, UMBRELLE assessed the impacts of a warmer world on the intense Mediterranean heavy precipitation event observed in 2020. By conducting sensitivity experiments with different sea surface temperature conditions, the project demonstrated the intensifying effect of warmer sea surface temperatures on extreme precipitation, as well as the potential shift in local precipitation patterns. A plausible worst case scenario of the observed extreme precipitation event occurring in 2022 instead of 2020, with warmer sea surface temperatures, was tested and contextualized with idealized experiments. This research highlighted the limited impact of the changes in storm Alex compared to regional Mediterranean warming in explaining extreme precipitation changes during the Mediterranean event. Results showed that warmer sea surface temperature lead to increased humidity and instability advected by the low-level atmospheric flow over the mountainous region where deep convection is enhanced locally. This storyline shows a milder precipitation-related risk in the French Alpine region, but increased damage in Italy, information that is highly relevant for stakeholders and climate-related risk assessment.
Another aspect of the research concerned the larger scales of the atmospheric circulation, based on a large ensemble of state-of-the-art Earth System Models. UMBRELLE demonstrated the need for a seasonal focus when assessing the climate drivers of changes in extreme precipitation in the Mediterranean basin. An innovative methodology was developed to identify plausible future pathways for the atmospheric circulation over Europe from the different responses given by the ensemble of models, and to test these circulation changes on the observed extreme precipitation event.
In terms of exploitation and dissemination, the project has actively communicated its findings through publications in international peer-reviewed journals, presentations at conferences and workshops, and engagement with the wider public. These efforts have contributed to advancing scientific knowledge in the field of extreme precipitation and have practical implications for adaptation and resilience strategies in the face of climate change.
