Event definition

Heavy precipitation is an episode of abnormally high rain or snow (95th percentile). The definition of « extreme » is a statistical concept that varies depending on location, season, and length of the historical record. The mechanisms (perturbation, air mass, water content, instability, interaction with local forcings, persistence, etc.) that generate a heavy/extreme event can be very different among different regions. Same amount of heavy/extreme precipitation over different areas can lead to different responses at ground (in terms of floods). In the case presented here the key factor for the event to be extreme is for sure persistence of a very common perturbation for the area during the fall period.

Objectives

XAIDA aims at developing an Artificial Intelligence based method to detect such kinds of events and in combination with high resolution convection permitting modeling dataset at emulating the costly dynamical models in representing them.  By means of the AI methodology XAIDA will try to better understand the physical mechanisms/key factors which drive the most extreme events among the whole population of precipitating events over Europe. 

XAIDA also aims to understand if such events observed in the present day period can be projected in a future climate scenario with the AI methodology and if the method can be extended to directly connect the climate variables to the observed hazard (flood) both in present and future climate conditions. 

XAIDA also aims to develop an AI based method to attribute such events to climate change; the identifications of their driving mechanisms/ingredients would allow to eventually perform such attribution trough key-variables in addition to precipitation.

Study results

Coppola et al (2024) used the Climameter analogues method to find differences between the present (2001–2023) and the past (1979–2001) climate in the region [0°E 22°E 34°N 48°N]. They conclude this was a very exceptional event in terms of meteorological conditions. Mediterranean depressions similar to that causing multiple floods in Italy in October 2024, show increased precipitation (up to 10 mm/day, or up to 25% more precipitation) and produce stronger winds (up to 6 km/h, representing a 10% increase in wind strength) in the present compared to the past. This was a very exceptional event in terms of meteorological conditions. Natural variability alone cannot explain the changes in precipitation and wind associated with this very exceptional meteorological conditions

Fantini (2019) used a multimodel approach to study the impact of flood hazard over Iraly, involving three steps: i) Hydrological Modeling, using multiple sources of precipitation data to simulate water discharge over a region or river basin for extended periods; ii) Hydrodynamic Modeling, in which the discharge data was then used in a hydrodynamic model to create flood extent maps and, if needed, additional details like flood depth or flow speed; iii) Extreme Value Analysis, using statistical methods to estimate return periods for extreme flood events, even with shorter observation records. The model chain containsn regional, hydrological and hydraulic models. To provide high-quality precipitation data for the hydrological model, they created a new dataset called GRIPHO(GRidded Italian Precipitation Hourly Observations). 

Both a boundary simulation using ERA-Interim data and a future scenario simulation driven by HadGEM (RCP8.5) matched observations for precipitation and temperature, showing good results for both average and extreme conditions. Projections suggest that while average precipitation may decrease in some areas, extreme precipitation is expected to increase overall.

Hydrological simulations using observed and climate model data revealed that the CHyM model performed well against discharge station data. For the future projections, mean discharge is expected to increase in northern Italy in winter and decrease in summer, reflecting seasonal rainfall shifts in the Po River basin. Maximum yearly discharges may rise by 50% in winter and autumn in many regions. Furthermore, 100-year flood discharges could more than double across Italy by the century’s end and extreme flood events (e.g., 100-year floods) may increase by over 500% in some rivers, including the Po. Preliminary flood maps for various return periods align with regional agency products but are based on a consistent, scientifically robust method. These findings underscore significant flood risks in Italy due to climate change, highlighting the urgency of improved planning and mitigation strategies.

References

Delrieu, G., Nicol, J., Yates, E., Kirstetter, P., Creutin, J., Anquetin, S., Obled, C., Saulnier, G., Ducrocq, V., Gaume, E., Payrastre, O., Andrieu, H., Ayral, P., Bouvier, C., Neppel, L., Livet, M., Lang, M., du-Châtelet, J. P., Walpersdorf, A., & Wobrock, W. (2005). The Catastrophic Flash-Flood Event of 8–9 September 2002 in the Gard Region, France: A First Case Study for the Cévennes–Vivarais Mediterranean Hydrometeorological Observatory, Journal of Hydrometeorology, 6(1), 34-52.

Huet, P., Martin, X., Prime, J.-L., Foin, P., Laurain, C., and Cannard, P.: Retour d’exp erience des crues de spetembre 2002 dans les departements du Gard, de l’Herault, du Vaucluse, des Bouches-du-Rhone, de l’Ardeche et de la Drome. (In French), Tech. rep., Ministere de l’ecologie et du developpement durable, Republique Francaise, 2003

Fumière Q, Déqué M, Nuissier O, Somot S, Alias A, Caillaud C, Laurantin O, Seity Y (2019) Extreme rainfall in Mediterranean France during the fall: added-value of the CNRM-AROME Convection-Permitting Regional Climate Model. Clim Dyn. https:// doi.org/10.1007/s00382-019-04898-8)

Tabary P, Dupuy P, L’Henaff G et al (2012) A 10-year (1997–2006) reanalysis of quantitative precipitation estimation over France: methodology and first results. IAHS Publ. 351:255–260