Introduction

Floods and landslides represent two of the most dangerous natural hazards, significantly affecting society and having strong negative impacts on the human–environment system (Petrucci et al. 2019; Tesselaar et al. 2020; Pollock and Wartman 2020). A damaging hydrogeological event (DHE) (Petrucci 2013) is defined as the simultaneous occurrence of phenomena such as floods and landslides, triggered by rainfall (Petrucci and Coscarelli 2023), causing damage to people (Petrucci 2022a; 2022b) and properties, significant economic losses in agriculture and huge damage to transport assets (Argyroudis et al. 2019). As evidenced also in the 6th IPCC report, DHEs are increasing worldwide (IPCC 2023). For example, in the CRED’s Emergency Events Database, which contains the world’s most comprehensive data on the occurrence and effects of more than 24,000 technological and natural hazard-related disasters from 1900 to the present day, an increase of about 150% and 50% of floods and landslides, respectively, has been detected between the 20-year period 1980–1999 and 2000–2019 (CRED and UNDRR 2020). In particular, in the last 20 years, DHEs have produced damages estimated to be more than 650 billion USD, affecting millions of people around the world (CRED and UNDRR 2020). These damages can be related to a higher exposure due to urbanization or asset value increase, an increase in vulnerability due to a lack of maintenance of protection structures, or fading of preparedness of administration and affected parties (Kreibich et al. 2017). There is a large body of literature, performed in different areas of the world, which relates the increasing number of DHEs with an increase in the frequency and the intensity of extreme rainfall (e.g., Alexander et al. 2006; Min et al. 2011; Westra 2013). In fact, both climate model simulations and physical theory point to a modification of the rainfall patterns as a consequence of global warming. Consequently, more extreme rainfall events and prolonged droughts should be expected in the coming decades. This supports the "wet-gets-wetter and dry-gets-drier" theory, where already moist regions will likely become even wetter, and dry regions will experience further aridity (Diodato et al. 2019). Within this context, particularly important is the role played by extreme cyclones (Nissen et al. 2014). In fact, in the Mediterranean basin and especially in Italy, a large portion of autumn and winter precipitation is linked to the Mediterranean cyclones that can severely impact weather events, causing windstorms, storm surges, landslides and flooding (Lionello et al. 2012). The results of global climate model simulations evidence a strong reduction in the number of winter cyclones in the Mediterranean basin. At the same time, as regards the intensity, different results have been obtained: some models suggest a decrease in the frequency of the most intense systems, while other models show more extreme events or an increase in the intensity of extreme cyclones (Nissen et al. 2014). To study the effect of these changes on rainfall, several analyses have been proposed on daily values with different methodologies, such as indices, percentiles, thresholds, and extreme value theory (Petrucci et al. 2017). In particular, the analysis performed in southern Italy evidenced an increase in daily rainfall intensity and a tendency toward higher frequencies of heavy and extreme rainfall (Caloiero et al. 2014; Buttafuoco and Caloiero 2014).

Nowadays, the multitude of unofficial information sources and the diffusion of news by unconfirmed sources as social media (Cheong and Babcock 2021) and the numerous websites of meteorologist amateurs seem to make “spectacular” each storm, and tend to bias the general perception of the intensity of the events, overestimating the damage from recent events compared to older ones. Newspapers, the most used damage data sources in the literature surveying the impact of natural hazards (Papagiannaki et al. 2013; Leal et al. 2018; Brázdil et al. 2019), in the past would take 24 h to report an event, and media coverage was ensured only for major events. Oppositely, nowadays Internet allows an almost real-time relay of information from the site of a disaster, and even an almost ordinary rain can be amplified to becoming a sort of “deluge”. This require an unbiased procedure to assess DHE severity based on the comparison of the event, in terms of both triggering rain and damage, to the events that affected the same area in the past.

A Mediterranean region such as Calabria, because of the combined effect of its geographical position and orographic system, is affected by several types of meteorological conditions with spatial variability of rainfall characteristics. Particularly, the east side of the region shows low annual rainfall (with a minimum of 500 mm/year) and is frequently hit by short and intense rains generated by warm humid southern air masses with high inter-annual variability (Petrucci and Coscarelli 2023). On this side of the region, the Crotone municipality (Calabria, Italy) and its surroundings have been selected as the study area for this paper. Here, the population density is higher (330 inhabitants per square kilometre) than the regional average (127.9) (ISTAT 2024). During intense rains, this area is strongly damaged by landslides in the innermost sectors, while floods-affected plain areas and the concurrence of sea storms prevent the outflow of floods into the sea, thus increasing damage along the coast (Canale et al. 2020). The widespread outcroppings of impervious clayey soils cause the rapid river outflow creation, while the weakness of some of the slopes contribute with landslides to increase in damage. The port city of Crotone was strongly damaged in 1957, especially its industrial district, and more recently on 1996, when six people were swept away from the city centre by the Esaro River flood.

In this paper, a procedure pointed out in previous papers (Aceto et al. 2016) was used to evaluate the severity of a case study event which affected the Crotone area. Moreover, differently from past studies, in this study the meteorological framework of the event has been described and compared to past similar events to assess its frequency in determining damaging events and supply an outline of meteorological precursors of damaging events in the study area.

In the following section, the methodological approach is described. “The Crotone 2020 case study: describes the analysed event from the meteorological, hydrological, and damaging point of view. The next section presents the discussion, and the final section outlines the main conclusions.

The methodological approach

To characterize the DHE which affected the Calabria region in November 2020, the methodological approach proposed by Caloiero et al. (2014) and revised by Aceto et al. (2016) was applied. Briefly, the approach is based on the assessment of the rainfall score (Rscore) and the damage score (DScore), representing the severity of rain and damage, respectively. DScore is made of three damage indexes: a) the index of damaged area (IDA), that is, the percentage of regional area damaged by the DHE; b) the damage index (DI), that is, a relative evaluation of direct damage which affected six categories of vulnerable elements (roads and railways, houses, public buildings, services, productive activities, and hydraulic works), and c) the number of victims (NoV). Each damaged element has a relatively pre-defined value (0.25, 0.50, 0.75 and 1) and the damage ranges from total destruction (1) to low damage (0.25). The damage affecting each damaged element is the product between the relative value of the element and the level of damage suffered (Petrucci 2013; Petrucci and Gullà 2010). Thus, the damage is summarized and normalized by Dscore, following Eq. (1):

$$D_{score} = {{\left( {\frac{{IDA_{j} }}{{IDA_{\max } }} + \frac{{DI_{j} }}{{DI_{\max } }} + \frac{{NoV_{j} }}{{NoV_{\max } }}} \right)} \mathord{\left/ {\vphantom {{\left( {\frac{{IDA_{j} }}{{IDA_{\max } }} + \frac{{DI_{j} }}{{DI_{\max } }} + \frac{{NoV_{j} }}{{NoV_{\max } }}} \right)} 3}} \right. \kern-0pt} 3}$$
(1)

where IDAj, DIj, and NoVj are the damage indicators of the event j, and IDAmax, DImax, and NoVmax are their maximum values, obtained from the historical series of regional DHEs.

The rainfall score (Rscore) is calculated considering the return period (T) of the maximum daily rainfall evaluated in each gauge of the affected area during the DHE. Given a series of return period classes, the Rscore is calculated following Eq. (2), by evaluating the percentage of gauges falling within the mentioned classes:

$$R_{score} = c_{0} \cdot \frac{{\sum\limits_{i = 1}^{n} {i \cdot P\left( T \right)_{i} } }}{{\sum\limits_{i = 1}^{n} {P\left( T \right)_{i} } }},$$
(2)

where n is the number classes, i is a return period class, P(T)i is the percentage of gauges falling within each i class, and c0 is a graphic scale factor.

The combinations of rainfall and damage severity levels, measured using Rscore and Dscore, allow to classify DHE into types according to their severity (Table 1):

Table 1 Damaging hydrogeological events classification based on rainfall and damage indexes (from Aceto et al. 2016, modified)
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Plotting the historical DHEs which affected a region on a damage/rainfall diagram, the events will fall in one of four sectors corresponding to the four severity levels described in Table 1. This chart can be used to plot, and thus classify in terms of severity, all the current and future DHE affecting the same area used to calibrate the diagram.

For a more detailed description of the methodology, interested readers can refer to Aceto et al. (2016). The flowchart in Fig. 1 shows the synthesis of the procedure for the classification of the DHEs evidencing the different variables considered, and the indices and the scores evaluated.

Fig. 1
figure 1

Flowchart explaining the procedure for the classification of the DHES

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The Crotone 2020 case study

Torrential rain over the weekend 20th to 22nd November 2020 caused severe flooding and damage in Calabria which strongly affected Crotone town and its surroundings, on the east side of the region (Fig. 2).

Fig. 2
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The Calabria region and its position in the Mediterranean Basin

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Calabria, the southernmost peninsular region of Italy (Fig. 2), is frequently affected by DHEs due to its geological and geomorphological configuration and exposed to Mediterranean climate, with warm dry summers and rainy autumn–winter seasons. Tectonic stresses and climatic conditions worsen the rock strength and expose the slopes to landsliding. The river system is mainly constituted of ephemeral streams, almost dry during summer and affected by severe flash floods in the autumn–winter. In this regional framework, to classify the Crotone 2020 DHE, the severity chart proposed in Aceto et al. (2016) was applied. The chart was constructed using historical rainfall data collected from the Multi-Risk Functional Centre of the Regional Agency for Environmental Protection of Calabria (ARPACAL), the damage database resulting from the monitoring which has been conducted since 1970 by researchers at the CNR-IRPI of Cosenza (Research Institute for Geo-Hydrological Protection), and from several papers concerning the impact of either landslides (Petrucci et al. 2023) or floods (Sardou and Petrucci 2023). In particular, the rainfall database presents high spatial and temporal resolution of the data. Since 1916, precipitation amounts (in mm) were collected in several stations distributed across the region with a daily temporal aggregation; from 1989, temporal aggregations of 5, 20, or 30 min were also collected, while, currently, all the stations are characterized by a temporal aggregation of 1 min (Morbidelli et al. 2020).

Synoptic overview

From November 19th, a robust anticyclone on the Atlantic favoured the descent of a trough towards the Western Mediterranean, originating a low pressure at ground level on the leeward of Alps, since the morning of November 20th. The latter, following the further arrival of the same trough towards south, travelled the Tyrrhenian Sea, reaching the Strait of Sicily on November 21st afternoon, where it took on sub-tropical features. The low pressure continued its path by locating the minimum even further south on the morning of November 22nd, with the trough elongated with respect to the main flow (Fig. 3).

Fig. 3
figure 3

Initialization of the ECMWF model—High Resolution Forecast (ECMWF 2020) related to: a run on November 20th, at 01:00 CET, parameters: geopotential height at 500 hPa and temperature at 850 hPa; b run on November 20th, at 13:00 CET, parameters: mean sea level pressure and wind speed at 850 hPa; c run on November 21st, at 13:00 CET, parameters: mean sea level pressure and wind speed at 850 hPa and d parameters: geopotential height at 500 hPa and temperature at 850 hPa

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On Calabria, rainfall started on November 20th late morning, with an intensification around 18:00–18:30 CET on Crotone area. Since then, the southern winds conveyed air masses with moderate moisture content that extended over the Central Mediterranean, like a long and narrow tongue, from the Gulf of Sirte (Libya) to the Crotone area and the Gulf of Taranto. As a result, the air mass drawn from south assumed total column water values up to about 27 mm (Fig. 4). These values persisted also on November 21st and 22nd. The event can be schematically divided into three phases as described in the following.

Fig. 4
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Spatial distribution of the “Total Column Water” parameter of November 20th, 2020 at 19:00 CET (EUMeTrain 2020)

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1) November 20th afternoon–November 21st early morning: rain on Basilicata and storm in the Crotone area

In the first phase, the most affected zones were the Ionian side of the Basilicata region and the Crotone area (red in Fig. 5a), due to a convergence area at low altitude (1000 hPa) and a contextual divergence at medium/high altitudes (up to 300 hPa) extending from the Basilicata up to the Crotone area (Fig. 5b). This configuration caused extensive vertical upward motions and abundant rainfall.

Fig. 5
figure 5

a Rainfall distribution between November 20th at 22:00 CET and November 21st at 03:00 CET (Dipartimento della Protezione Civile 2020); b convergence (concentric red lines, blue lines for divergence) at low altitude (1000 hPa) and divergence (concentric yellow lines, light blue lines for convergence) at an altitude of 300 hPa at 4:00 CET on November 21st (EUMeTrain 2020)

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From November 20th evening, many rain cells originated from the Gulf of Squillace and/or the mid Ionian area, due to the convergence between the synoptic sirocco winds and the land breezes from Catanzaro hinterland to the Gulf of Squillace (Fig. 6a). Probably, these breezes were favoured by the formation of a relative minimum leeward of the Calabrian and Sicilian Apennines, which, starting from the evening, gradually settled on the lower Ionian (Fig. 6c). Slight instability conditions, with CAPE values lower than 1000 J/kg, were also present (Fig. 6b).

Fig. 6
figure 6

a Wind distribution at 10 m a.s.l. of November 20th at 22:00 CET, based on the run of November 20th at 4:00 CET, model MOLOCH of CNR (ISAC CNR 2020); b CAPE distribution at 07:00 CET on November 21st, based on the run of November 21st, at 4:00 CET (ISAC CNR 2020); c mean sea level pressure distribution (black lines) at 10:00 CET of November 21st (EUMeTrain 2020)

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The surface temperature of the sea adjacent to the Catanzaro area showed strong positive anomalies (up to + 1.8° C/ + 2.2 °C—E.U. Copernicus Marine Service Information 2020) and favoured convective motions. These motions originated convective cells moving northwards due to currents having a southern component at the altitude of the isobaric surface of 700 hPa. The cells amplified their intensity by interacting with the orography, causing downpour, such as that which occurred in Crotone between local 5:45 and 6:45 CET (Fig. 7a). This happened in a context still characterized by a large convergence area at low altitude (1000 hPa) and a divergence at medium and high altitudes (up to 300 hPa), mainly stretching on the Gulf of Taranto (Fig. 7b).

Fig. 7
figure 7

a Rainfall distribution between 3:00 and 9:00 CET of November 21st (Dipartimento della Protezione Civile 2020); b convergence (concentric red lines, light blue lines for divergence) at low altitude (1000 hPa) and divergence (concentric yellow lines, pale blue lines for convergence) at an altitude of 700 hPa at 7:00 CET of November 21st (EUMeTrain 2020)

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The divergence at the medium altitudes (700 hPa) was favoured by the currents at that altitude, which tended to fan out (directional divergence) and accelerate (divergence due to speed) (Fig. 8a). The convergence zone at low altitude was caused by the confluence of the synoptic winds from north-east and south, combined with the deceleration caused by regional orography (convergence due to speed) (Fig. 8b).

Fig. 8
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a Wind vectors and divergence (yellow lines) at 700 hPa and b wind vectors at 10 m a.s.l. and convergence (red lines) at 1000 hPa, for November 21st at 13:00 CET (EUMeTrain 2020)

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2) November 21st early morning–evening: rain in the Crotone area and Ionian coast

On November 21st, the storm also hit the lower Ionian territory of Cosenza province, as shown by the accumulated rain 07:00–13:00 CET and 13:00–19:00 CET (red in Fig. 9a, b). In fact, from 07:00 CET to the afternoon, the situation was characterized by the shift towards the south, of the convergence areas at low altitude (1000 hPa) and of divergence at medium–high altitudes (initially up to 500 hPa, decreasing in the afternoon up to 600 hPa). This situation involved the Gulf of Taranto, the Ionian side of the Basilicata region and Cosenza province, Crotone, and the Gulf of Squillace (Fig. 9c, d).

Fig. 9
figure 9

(a-b) Rainfall distribution between 07:00 and 13:00 CET a and between 13:00 and 19:00 CET b of November 21st (Dipartimento della Protezione Civile 2020); c convergence (concentric red lines, blue lines for divergence) at low altitude (1000 hPa) and divergence (concentric yellow lines, light blue lines for convergence) at medium altitudes (700 hPa) at 13:00 CET of November 21st; d profiles of equivalent potential temperature, convergence (red lines) and divergence (blue lines) along the section in (c) at 13:00 CET of November 21.st (EUMeTrain 2020)

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3) November 22nd night: intense storm in the northern Crotone area

After a phase of stagnation, at 4:00 CET of 22nd, the concomitant convergence at low altitude (1000 hPa) and divergence at medium altitudes (up to 600 hPa with minimums at 750 hPa), between the lower Ionian area of Cosenza and the Crotone area, were particularly intense. The equivalent potential temperature and the omega parameter along the cross section (Fig. 10) highlighted the vertical upward motions that were intense up to the heights of the isobaric surfaces of 600/700 hPa (Fig. 10a).

Fig. 10
figure 10

a Equivalent potential temperature profiles and omega parameter (red lines) along the section in b at 4:00 CET of November 22nd (EUMeTrain 2020)

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Rainfall analysis

The rainfall event was localized on the Ionian side of Calabria and marginally involved the Tyrrhenian side of the region. Starting from the hourly rainfall data provided by the Multi-Risk Functional Centre of the ARPACAL, the maximum rainfall values with duration of 1 h, 3 h, 6 h, and 12 h for the period between the 20th and 22nd of November 2020 were detected and mapped (Fig. 11). The spatial distributions of the different temporal aggregations looked very similar, with the highest rainfall values in the province of Crotone.

Fig. 11
figure 11

Spatial distributions of the maximum rainfall values with duration of 1 h, 3 h, 6 h, and 12 h evaluated between the 20th and 22nd of November 2020

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The city of Crotone recorded the maximum rainfall with durations of 1 h (73.6 mm), 9 h (186 mm), and 12 h (212.6 mm). The 6-h maximum rainfall was recorded in three stations near Crotone City (> 100 mm). Similarly, for the duration of 9 h, eight stations along the Ionian coast reached values higher than 100 mm, while for the duration of 12 h, seven rain gauges near Crotone City recorded more than 150 mm (Fig. 11).

Besides, the maximum daily rainfall was evaluated and mapped (Fig. 12). Results evidenced that in three stations, rainfall reached values higher than 200 mm in 24 h, with a maximum value of 265 mm with the Crotone rain gauge.

Fig. 12
figure 12

Spatial distributions of the maximum daily rainfall values and values of the daily return period evaluated between the 20th and 22nd of November 2020

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Moreover, to point out the frequency of similar rainfall events in the Calabria region, the return period of the maximum daily rainfall values between the 20th and 22nd of November 2020 was evaluated in each rain gauge (Fig. 12). For one rain gauge located in Crotone City, the return period of maximum daily rainfall was evaluated as greater than 1000 years. Nevertheless, in the remaining area involved, daily rainfall was not extraordinary, except for one gauge on the northeast sector of Crotone province, where the return period was assessed as greater than 200 years.

Damage analysis

Bad weather and torrential rain over the weekend 20th to 22nd November 2020 caused severe flooding and damage mainly in the provinces of Crotone and Cosenza (Fig. 13). The most affected area was the City of Crotone (Fig. 14). The town neighbourhoods at the sea level, such as Marinella, were flooded by water and mud 1-m high that submerged cars and entered into basements of houses. The seafront way was submerged by debris carried by sea waves, and the shops in the city centre, closed due to the pandemic COVID-19, were inundated and suffered huge damage to stored goods. The railway, running parallel to the coast and perpendicular to the main river network, was inundated by water and mud: more than ten trains were cancelled or replaced by bus services. Traffic along the State Road N.106, running along the sea coast, suffered delays due to debris and mud that fell on the roadway from slopes (Fig. 13).

Fig. 13
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Calabrian municipalities damaged and coloured according to the phenomenon/s causing damage, according to the legend

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Fig. 14
figure 14

a Crotone city centre (source: Il Crotonese 25/11/2020); b Isola Capo Rizzuto: a hole opened in a square (source: www.corrieredellacalabria.it); c Crotone city centre (source: Il Quotidiano del Sud); d and e Melissa: a bridge of the provincial road collapsed (source: ANSA.it)

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The Crotone Mayor activated the municipal team for emergency management and invited people to stay at home, also using official social media of the municipality. Fire brigades and rescue services carried out over 400 interventions using rubber boats, to rescue about 200 people, after their cars were trapped in water and mud coming from ravines and torrents. Several people underwent precautionary evacuation, especially after the Esaro River broke levees near the Crotone urban area. 13 evacuated people were in quarantine, being positive to COVID 19 test. They were temporarily sheltered partly in a local school and partly in a hotel that were available to host them.

Electric power interruptions left more than 70 houses without electricity supply. Drinkable water supply too suffered interruptions, due to infiltrations in the aqueducts caused by small landslides and debris that made water turbid. Agriculture suffered huge damage, especially to fennel and olive growing with a consistent loss of product, as declared by local associations of farmers.

In some municipalities, such as Strongoli, Cirò Marina and Mirto Crosia, several roads were temporarily flooded; water and mud invaded houses’ base floors and caused extensive damage to agriculture. Moving to north, in the Melissa village, a bridge collapsed without damage to people (Fig. 14). The large municipality of Corigliano-Rossano was affected by small landslides in the hilly sectors, while in its seaside, the hamlets of Schiavonea and S. Angelo were flooded by 20 cm of water, and 40 people were evacuated by fire brigades using rubber boats.

On the west side of the region, near the village of Paola, strong winds fell trees and rolled over a truck on a high viaduct of the state road. Moving to north, in Belvedere Marittimo, a 75-year-old man was killed by the chimney of his house that was hurled by the wind. Data collected by the Multi-Risk Functional Centre of the ARPACAL, on the west side of the region reported wind gust between 28.4 m/s (Paola) and 25.8 m/s (Belvedere Marittimo), while on the east side values between 27.1 m/s (Monasterace) and 21,7 m/s (Crotone).

In the entire area involved (about 850 km2), around ten provincial roads were temporarily interrupted by water and mud on the roadway; further, three provincial roads were interrupted due to trees hurled by wind and some municipal roads, and the highest mountainous sectors were interrupted due to snow.

The event severity assessment

With the aim of evaluating the severity of the event, the chart proposed by Aceto et al. (2016) was applied. This chart was evaluated starting from the historical database of the damaging hydrogeological events in Calabria by selecting 21 of the most severe events which occurred in the region since the 1920s. As a result, the November 2020 DHE has been classified as an extraordinary event (Fig. 15). In fact, although this event affected a relatively restricted regional sector, with rainfall lasting few days, it presented high Rscore but low Dscore values, because, fortunately, no casualties were recorded. Generally, extraordinary events occur in every season, but mainly in autumn, as the one studied, when the descent of Atlantic troughs in the Mediterranean and the consequent formation of minimum surfaces capable of conveying air with high humidity from the south enhance the convective phenomena.

Fig. 15
figure 15

Chart classifying the analysed event according to its magnitude. Dscore and Rscore are described in Sect. "The methodological approach". Blue points indicates past regional events

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Discussion

The analysis of the Crotone 2020 case study increases our knowledge of damaging hydrogeological events in a frequently affected Mediterranean region as Calabria. The application of a definite methodological criterion based on both triggering rain and consequent damage allowed us to assess the event severity based on the history of regional DHEs, thus reducing the bias that can be related to the absence of the terms of comparison.

Actually, a judgement of event severity simply based on the severity of triggering rain can be not significant, because the amount of rain able to cause damage strictly depends on both the climate of the affected area and the rainfall regime to which terrains are customized. On the other side, the severity assessment simply based on damage can be biased by the presence/absence/density of the elements exposed at risk. These two factors require that the event severity assessment be based on local climatic and anthropogenic conditions. Nevertheless, this is also the main limitation of the methodological approach, because it can be applied in other areas only after recreating the classification chart by using climatic and anthropogenic data of the new study area.

In the present paper, a step forward in completing the framework of the event was performed by analysing the meteorological situation leading to intense damaging rain and comparing it with similar case studies available in literature (Petrucci and Polemio 2009). The synoptic situation that gave rise to the studied event is similar to that of the 25th–27th October 1921 event: the presence of a trough, which, due to the anticyclone on the Atlantic area, elongates on the western Mediterranean basin (in tear-off or cut-off) and generating the formation of a low pressure on the ground on Tyrrhenian sea, which quickly descends southwards on the North African coasts (as in the 1921 event) or Tunisia (as in the 2020 event). Similar dynamics, with differential southern trajectories, also occurred for the events of 1st–5th November 1953, 7th–11th November 1953, 13th–14th November 1959, and 29th September–2nd October 1971.

The highlighted similarities pose a series of new questions that could be investigated in the future, as for example the definition of a stricter classification of meteorological framework allowing to recognize in advance an upcoming damaging event characterized by a certain expected severity in a certain area.

Moreover, a further factor to investigate could be the absence of fatalities in the Crotone 2020 event, while past DHE killed several persons, as the six people killed in October 1996. This can be explained by the pandemic occurrence and also the role of social media (Banikalef 2018), which literature recently recognized as a good tool for information diffusion and post-event data gathering (Alizadeh 2022) contributing to the so-called citizen science (See 2019).

Conclusions

In this paper, the DHE hitting the Calabria between the 20th and 22nd of November 2020 was first analysed from a synoptic point of view and then considering the rainfall and the damage that occurred.

The meteorological situation causing the event is quite common in Calabria during the autumn season, when the descent of Atlantic troughs in the Mediterranean and the consequent formation of minimum surfaces capable of conveying air with high humidity from the south enhance the convective phenomena. DHEs characterized by the same framework have been found in recent Calabrian history (autumnal events occurred in 1921, 1953, 1959, and 1971).

As regards rainfall, the event is different from past cases considering the limited area affected by severe rainfall. To summarize this feature, it can be noted that only in one gauge the return period of daily rain was very high (1000 years), and this indicates the concentration of intense rain on a quite restricted area.

Concerning damage, mainly effects on communication lines and agriculture were detected. The long series of DHEs which affected the Crotone area, and particularly the one that in 1996 killed six people, certainly favoured precautionary behaviours in part of the population, which was strongly and repeatedly advised by local authorities and also by social media; however, numerous rescue operations were also carried out by emergency services. Moreover, during the event, the population was under some form of lockdown due to the COVID-19 pandemic, and this reduced the number of people at risk from the heavy rainfall, having been asked or ordered to stay at home by the government.

The paper shows the necessity of a systematic approach to assess DHE’s severity, also as a support for public administrations to have a synoptic view of the impact of the events finalized to the distribution of damage recovery funds. Moreover, the knowledge of the meteorological situations that can cause heavy rainfall and damage is paramount for civil protection purpose, especially in regions, such as Calabria, very frequently affected by DHEs. Finally, the paper evidenced that the wide dissemination of weather warnings by the authorities, through both conventional channels (e.g. televisions) and new forms of communication (e.g. social media) can be considered a good approach to reduce the effects on the population in an event that was nonetheless rainfall remarkable.

In this context, future efforts should be aimed at creating a high-resolution imagery archive, provided by geostationary weather satellites, aimed at comparing forecasts with the actual ground impacts of events in terms of both rainfall and damage.