Flooding in Emilia-Romagna: scientific analysis by CIMA Research Foundation

Researchers at CIMA Research Foundation have analyzed the meteorological elements that led to the severe flooding in Emilia-Romagna: a persistent depression vortex generated exceptional rainfall, amplified by the region's orography and a marine thermal anomaly, which increased evaporation and available moisture.

Just over a year after the last flood, Emilia-Romagna has once again been struck by intense rainfall that caused rivers to overflow and widespread flooding, forcing many residents to evacuate their homes and halting rail traffic.

Monitoring carried out by researchers at CIMA Research Foundation highlights the factors that contributed to this situation: a dangerous and synergistic combination of elements, some of which originated in previous months.

This flood is the result of a low-pressure vortex that arrived from northern Europe and stalled over eastern Europe, prevented from moving eastward by a high-pressure system over Russia. The depression then moved retrograde toward northern Italy, gathering moisture as it passed over the Adriatic Sea.
The position and persistence of the depression vortex were key to understanding the event's dynamics: it remained stationary over the area for the past 48 hours, generating continuous and exceptionally intense rainfall. Figure 1 shows how precipitation intensified near the Apennine mountains due to orographic lifting (the figure shows cumulative precipitation for September 19th). The rain gauge warning layer over the Tuscan-Emilian Apennines clearly highlights this [fig.1].

Figure 1

An important indicator for understanding the intensity of the rainfall is the "columnar liquid water content," which refers to the total amount of water that would result if all the moisture in an air column above a certain point were condensed into liquid. Figure 2 shows areas with a columnar liquid water content of up to 40 mm (equivalent to an accumulation of 40 liters of water per square meter).

Figure 2

This information is supported by the radiosonde data from San Pietro Capofiume on the afternoon of September 18th (Figure 3), showing a saturated air column up to over 8,000 meters (indicated by the convergence of the two dark lines, representing air temperature and dew point temperature) with a clear and significant easterly airflow at all atmospheric levels (shown on the right of the figure).

Further contributing to the event's intensity is the thermal anomaly of the sea. On September 17, 2024, when the low-pressure system retrograded from the northern Adriatic toward the Ligurian Sea, sea surface temperatures were recorded to be 1.25-1.5°C above the 1990-2020 average. This warming amplified evaporation processes, increasing the amount of moisture available for precipitation. In fact, the sea temperature in the areas from which the vortex drew part of its water vapor is above the 90th percentile, a significant anomaly compared to historical climatology. In short: warmer seas lead to increased evaporation, resulting in more water in the atmosphere. This is a process that urges reflection on the cascading effects of the climate crisis on various natural phenomena.

CIMA Research Foundation experts are continuing to monitor the situation, working to predict its evolution.

Today, the low-pressure vortex is expected to gradually weaken, and due to the influence of an African anticyclone moving into central and southern Italy, the vortex will shift eastward, leading to an improvement in weather conditions for the entire weekend. Therefore, rainfall is expected to decrease starting today, with clearer skies becoming more widespread throughout the weekend.

The complexity of atmospheric phenomena like this one reminds us of the importance of constant and accurate monitoring. Understanding the dynamics behind extreme events is crucial for improving forecasting and mitigation capabilities, as well as raising awareness of the urgency to act in response to climate change, whose role is increasingly evident across different contexts. In this light, the data collected not only help manage the current emergency but also provide valuable information for developing future strategies to reduce risk and protect exposed communities.

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