On October 29, 2024, unprecedented rainfall struck southeastern Spain. In some areas, the volume of water exceeded the annual average in just a few hours. Now, a study led by the University of Valladolid (UVa) and the Spanish State Meteorological Agency (AEMET), in collaboration with researchers from the Spanish National Research Council (CSIC), an agency of the Ministry of Science, Innovation and Universities, quantifies for the first time the alterations in the storm's internal structure caused by climate change. These variations intensified the rainfall rate by 20%, extended the area affected by rainfall exceeding 180 mm by 55%, and increased the total volume of rain in the Júcar River basin by 19%, compared to the pre-industrial era.

In October 2024, the southeastern Iberian Peninsula experienced intense rainfall caused by an isolated high-level depression (DANA), fueled by the influx of very warm, humid air from the Mediterranean Sea. The rains hit the province of Valencia particularly hard, with cases like the municipality of Turís recording rainfall exceeding the annual average (771 mm) in just 15 hours. Furthermore, the accumulation surpassed the highest rainfall ever recorded in Spain in a single hour, at 184 mm.

The new study, published in Nature Communications, with the participation of researchers from the Center for Desertification Research (CIDE, CSIC-UV-GVA) and the Pyrenean Institute of Ecology (IPE-CSIC), uses high-resolution simulations to determine the influence of climate change on the storm's convective dynamics—the process by which rainfall originates after the rapid ascent of warm, humid air from the sea to the upper layers of the atmosphere.

The data show that the surface temperature of the Mediterranean Sea was 1.2°C above normal, resulting in higher atmospheric humidity. As a result, rainfall intensified by 20% for every degree Celsius of sea surface temperature warming; that is, in a context without climate change, rainfall would have been up to one-fifth less intense. This increase even exceeds the Clausius-Clapeyron scale, which explains how for every degree Celsius increase in air temperature, the air can hold approximately 7% more water vapor.

“The sea surface temperature acted as fuel, amplifying the convective potential energy available in the storm with more intense updrafts and changes in cloud microphysical activity,” explains Carlos Calvo, lead author of the study and currently a researcher at CIDE.

From Global to Local

The researchers analyzed the recorded data using a very high spatial resolution ‘pseudo-global warming’ (PGW) approach, which allowed them to assess the contribution of climate change. “We studied different internal components of the storm using high-resolution (1 km) simulations applying this methodology,” Calvo clarifies.

This system works analogously to a digital twin, which, after reconstructing the meteorology that characterized the DANA (isolated high-level depression), applies a forcing process to eliminate the global warming accumulated since the pre-industrial era. This allows researchers to compare both scenarios: the October 2024 storm with climate change, and the reconstruction of the same rainfall without its effects. "This methodology allows us to quantify how global warming has influenced an extreme weather event," he adds.

The models used in the study overcome the limitations of traditional attributions to climate change. These methodologies are based on statistics (surface impact of the meteorological event), with almost exclusively observational data, which prevents analyzing how climate change influences the storm's internal dynamics. Furthermore, "thanks to the high resolution of the simulations, the methodology used allows us to quantify the different components of a convective system and study how climate change influences them," highlights AEMET meteorologist and researcher Juan Jesús González Alemán.

The methodology used in this study demonstrates that nonlinear processes are involved in the impact of climate change on the atmospheric processes responsible for the rapid ascent of warm, humid air (convective system): “This is due to large increases in the release of latent heat and in updrafts caused by small increases in evaporation and water vapor flux,” explains María Luisa Martín, professor and researcher at the University of Valladolid.

“The most interesting aspect of the study is that our experiments allow us to quantify the alterations that occur in the main physical processes involved in an extreme weather event of this nature, even at the microphysical scale of clouds. This approach had never been applied before to an event, being the first time for the Valencia DANA episode, and it allows us to affirm that attributing the magnitude of its torrential rainfall, both in intensity and affected area, to climate change is robustly and physically consistent,” adds Amar Halifa, a researcher at the IPE and the Interdisciplinary Thematic Platform (PTI) Climate and Climate Services of the CSIC.

More Virulent and Complex Storms

The sixth report of the Intergovernmental Panel on Climate Change (IPCC) indicates that global warming recorded during the industrial period resulted in an approximate increase of 1.3°C. This figure represents an increase in the atmosphere's capacity to hold water vapor, which leads to greater global precipitation.

The results of the new study reinforce the IPCC's conclusions by indicating that climate change could intensify the occurrence of flash floods in the western Mediterranean region. In the specific case of the DANA storm, it increased the rainfall rate by 20% and extended the affected area by 55%.

In this context, extreme events in the western Mediterranean could be evolving toward more intense scenarios due to global warming, with the formation of more virulent and complex storms. “The findings highlight the urgent need to implement effective adaptation strategies, including the monitoring and prediction of these phenomena, as well as reviewing urban planning to address increasing hydrometeorological risks in a rapidly warming world,” concludes César Azorín, principal investigator at the Climate, Atmosphere and Ocean Laboratory (Climatoc-Lab) of CIDE and co-author of the study.

The study led by UVa and AEMET, in collaboration with CSIC, also included the participation of the Complutense University of Madrid (UCM), ETH Zurich (Andreas F. Prein) and the Institute of Atmospheric Sciences and Climate, ISAC-CNR (Mario Marcello Miglietta).

NATURE