Future intensification of severe multi-hazard supercells in a semi-arid environment of Southern Europe

Severe convective storms capable of producing extreme surface impacts simultaneously (such as large hailstones, strong winds, and heavy rainfall) present a major challenge to numerical weather prediction and risk assessment. On 6 July 2023, the Ebro Valley (Aragon, Spain) was impacted by a series of two high-impact supercells. The first supercell exhibited an extraordinary multi-hazard nature, producing a 50-km swath of giant hail (≥ 10 cm), a tornado, and a downburst with estimated gusts exceeding 200 km/h. The second supercell triggered flash flooding in the city of Zaragoza (population > 700,000) and large hail (> 5 cm) to the south of the city.

This study applies a sub-km-scale pseudo-global warming storyline approach to this compound high-impact event to quantify how anthropogenic climate change will alter its physical drivers and destructive potential. We downscale global climate perspectives to the storm scale by comparing a factual simulation with a future scenario (SSP5-8.5). We perturb initial and boundary conditions using climate change deltas derived from each CMIP6 climate model. We focus on the physical understanding of cascading hazards: specifically, whether future warming enables a transition towards storms that sustain giant hail growth while simultaneously enhancing precipitation efficiency (flash flood risk) and downdraft intensity. Our results aim to demonstrate how event-based storylines can unravel the interactions between thermodynamic changes and storm dynamics, revealing if such unprecedented multi-hazard supercells will become the new reality for extremes in semi-arid regions, thereby amplifying their destructive potential.