Separating dynamic and thermodynamic contributions in Mediterranean extreme precipitation (in a storyline approach)

Extreme precipitation in the Mediterranean basin emerges from a complex interaction between large-scale circulation, moisture transport and mesoscale dynamics, making the most damaging events difficult to sample in conventional climate simulations. This work presents a storyline-based framework to explore  very rare and  extreme rainfall under present and future climate conditions. 

We apply ensemble boosting to the fully coupled CESM2 model to generate alternative realizations of the most intense precipitation events affecting the Southern Alps and the Spanish Mediterranean coast. Starting from a 35 member parent ensemble of CESM2, these occurrences are identified and resimulated through boosted ensembles, resulting in a large sets of dynamically consistent trajectories that preserve the synoptic evolution of the original event while sampling its internal variability by perturbing the initial conditions. Comparisons with ERA5 reanalysis and available observations are performed to assess the realism of the simulated circulation patterns and precipitation characteristics associated with these extreme events. 

Preliminary results demonstrate that ensemble boosting successfully reproduces the temporal evolution of reference precipitation extremes, with many boosted members closely matching the timing and peak intensity of the parent events. In several cases, individual boosted realizations exceed the peak intensity of the reference simulation, revealing physically consistent more intense scenarios within the same large-scale setup. The amplification potential depends strongly on the perturbation lead time: short lead starts tend to cluster near the reference intensity, whereas longer lead times display a broader ensemble spread and occasionally generate substantially stronger or delayed rainfall peaks. 

In a second step, a conditional attribution methodology is applied in which the large-scale circulation is constrained while the thermodynamic background is modified to represent different climate states. This allows us to isolate the thermodynamic contribution of climate change to extreme precipitation intensity, providing physically interpretable estimates of how much more intense these events become in a warmer climate. 

By bridging weather-scale event evolution with climate-scale statistics, this approach provides new insight into the physical limits of Mediterranean extreme precipitation and offers a robust basis for assessing future extreme rainfall scenarios.