The project SEVERE investigates the physics, processes and scale dependency of very extreme precipitation events. It is part of the ClimXtreme Module A, which is dedicated to understand the enhancing or limiting factors affecting meteorological extreme events in a changing climate.

Very extreme precipitation events with very long return periods (e.g. 100 years) can potentially cause large damages, especially when followed by regional or large scale flooding. This is crucial in a warming climate since, atmospheric physics shows that warmer air contains more water than colder air (as described in the Clausius-Clapeyron equation). Hence a larger water content in the air masses brings an increased potential for precipitation extremes. However, this effect is not the only factor since the future development depends as well on the large and regional scale evaporation, atmospheric stability conditions and large-dynamics dynamics.

A solid knowledge of the expected precipitation return values is also needed to determine the construction requirements for infrastructures (dykes, bridges, roads, etc.) that can help prevent the impacts or for insurance companies to estimate a maximum risk potential. The period, for which reliable observations exist (~ 50 years), is too short to derive robust estimates on longer time-scales. Therefore, SEVERE will use the data from existing large ensembles of regional climate simulations from German and International projects (MiKlip, CMIP-5/6, CORDEX).

The project is structured into three phases:

1. The characterization of intensity, extension and duration of observed extreme precipitation events over Europe with respect to their temporal and spatial distribution.

2. Evaluation of the potential of climate simulations to reproduce the relevant features of extreme precipitation as well as the large and regional scale processes. Special emphasis is given to the model resolution dependency, ranging from very high-resolution mesoscale models (grid size ~3 km) to the resolution of global climate models (~100km). With this aim, we downscale the global data to the highest resolution with the regional climate model CCLM for selected events.

3. The results will then be applied to the existing large ensembles of climate simulations to identify a sufficient number of very extreme precipitation events. For the assessment of the role of the different modes of climate variability, the simulations of the MiKlip decadal prediction system will be used, which includes approx. 15.000 simulation years. To estimate the future trends, the regional climate projections from EURO-CORDEX will be used.

Fig. 1: Schematic project overview