Precipitation-induced disasters in Emilia Romagna Region – May 2023

During the first twenty days of May, four different heavy precipitation events occurred in Emilia Romagna Region (Central Italy) causing huge damages and victims. In the specific, the events took place on:

1-4 May: at regional level, the cumulative values over 48 hours were the largest ones since 1997 and record-breaking for the Spring season (they represent maximum point scale values for 27 stations over 45 in the central-eastern part of the region); if the 24 hour duration is considered, precipitation percentile associated with a return period larger than 100 years were observed in several stations of Bologna, Ravenna and Forlì-Cesena Provinces. In this regard, on May 1, the maximum values did not exceed 70 mm while the following day, the analysis of rain gauges and radar data returned values largely exceeding 150 mm (a peak of 204,6 mm was observed for the station Le Taverne [Fontanelice, Bologna Province]). On the other side, hourly values rarely exceeded 10mm/h. Due to such heavy rainfall events, river floodings were observed in several basins (Samoggia, Idice, Quaderna, Sillaro, Santerno, Senio, Lamone e Montone, not larger than 400-500 kmq) inducing damages to crops, production plants and settlements while several landslide events occurred along the Apennines mountain chains.

9-10 May: the precipitation events, starting in the afternoon of 9 May, interested a large part of the Emilia Romagna Region; during the first day, the cumulative values did not exceed 50 mm while the following day the attained values were slightly larger than 100 mm in the Ferrara Province. Hourly values close to 20 mm were observed, instead in Bologna province. Such events induced limited damages (e.g. flooding of the basements)

12-14 May: in the first day, a large part of the Region is interested by intense short convective rainfall events and hailing that occurred, in a less intense way, also during the following days. Daily values result not exceptional while values over 15 minutes were larger than 8 mm in multiple locations. Hail events were observed in many locations of Central-Eastern part of the Region.

16-18 May: as occurred on 1-4 May, the precipitation was localized on the hilly and pedecolline territories of Central-Eastern Emilia Romagna; the maximum cumulative values over 48 hours were observed at Monte Albano (260.8 mm), Trebbio (254.8 mm) and San Cassiano sul Lamone (254.6 mm) [Imola, Forlì and Faenza territories, respectively]; for the same rain gauges, values over 24 hours (16 May) were quite close to 200 mm.

The cumulative precipitation recorded in the period 1-17 May represent the record-breaking value for over 65% of the rain gauges in the Central-Eastern part of the Region with average values of about 300-400 mm and peaks up to 600 mm at Trebbio and 563 at Le Taverne [Fontanelice, Bologna Province].

Such brief descriptions about the analysis of observation data are retrieved by the interesting reports released by ARPAE-ER[1] and CIMA[2] in last weeks (in Italian).

Hydrological and geological disasters induced by exceptional rainfall events were observed in several areas and induced significant damages to assets and communities; according to a first provisional assessment, 23 were the flooded rivers with resulting riverbank overtopping and breakings, and 936 the detected landslides; the fatalities were 13 with about 36,000 evacuated people, 622 the closer roads; the damages are estimated to be about 8 billion of euros.

HOW E3CI CAN SUPPORT THE BACK-ANALYSIS OF PRECIPITATION INDUCED EVENTS

Figure 1 reports the evolution of precipitation[3] indicator, as computed in E3CI suite, for the Emilia Romagna Region over the all investigated period 1981- May 2023. The value computed for May 2023 represents, at regional scale, the second highest value (about 8[4]) after that estimated in February 2015 (about 10). Significant impacts were observed also in this case. In terms of evolution, not clear increasing trends are recognizable: on average, the number of months for year larger than 1 were 1.4 in the first decade 1981-1990, 1.1 over the two decades 1991-2000 and 2001-2010 and 1.6 over the final period 2011-2023 (3 monthly values in the most recent 2 years, 16 in 10 years, 56 over the entire time span).

precipitation_2023may_1

The 10 most largest values are also summarized in Figure 2. It is interesting to note that 9 over 10 are occurred in the most recent 10 years while the 10th in July 2002 (it represents the lowest values among the ten). Furthermore, from the analysis of the graph, it clearly emerges how the patterns of extreme precipitations could undergo significant variations in some parts of the year: indeed, three values are observed in February and two in May (2023 and 2019).

precipitation_2023may_2-1024x821
precipitation_2023may_3-1024x438

Figure 3 returns information about the value of the extreme precipitation indicator at NUTS 3 value. It is clear that the events prominently affected the eastern part of the region. The highest value is observed over Bologna Province (16.3) while it represents the largest on the entire ranking also for the Ferrara Province (10.1). Furthermore, it results the highest value computed over May for all the provinces except Parma and Piacenza where values close or below to 0 are returned.

precipitation_2023may_4

The suite of indicators included in E3CI can provide useful information also about the predisposing conditions occurred in the months before May 2023. In the specific, in Figure 4, are represented the values close to/exceeding 1 for droughts, heatwave and extreme precipitations. During the first months, significant rainfall deficit were observed over the Region (as in all the Po River Basin). They were associated with warmer than the usual conditions (in special way, during the Spring and first part of the Summer Season). In November, the indicator related to extreme precipitation returned values slightly larger than 3 while several impacts were observed in the regional territories. After, a second month where the extreme precipitation indicator exceeded 1, was observed in February 2023 while rainfall deficit in Spring 2023 led to a value of drought indicator larger than 1 in April (just before the extreme events recorded in May). This alternation returns a clear picture of precipitation whiplash expected to increase, in next decades, due to anthropogenic greenhouse gas emissions.

[1] Regional Agency for Prevention, Environment and Energy of Emilia-Romagna

[2] International Centre for Environmental Monitoring

[3] Specifically, over 1981-2010, at monthly scale, the 95th percentile of daily precipitation, output of ERA5 reanalysis, for wet days is computed; then, the mean and standard deviation of monthly cumulative values of precipitation exceeding such threshold is computed. Operationally, each month, the value of precipitation over threshold is then standardized by using average and standard deviation computed over 1981-2010 time span. It is worth recalling that the indicators are estimated at grid point scale and after aggregated over the spatial unit of interest. Then, the values greater than 1 identify areas where the amount of precipitation beyond the 95th percentile, exceeds the monthly average of a value larger than the standard deviation. In a nutshell, the anomaly could be assumed more significant than the climatological interannual variability (assuming as proxy, the standard deviation).

[4] It is worth noting that a n value of the indicator represents an overcoming of the monthly average value (computed over 1981-2010) equal to n times the monthly standard deviation (computed over the same time span).