Flash floods induced by extreme rainfall events represent one of the most life-threatening phenomena in the Mediterranean. While their catastrophic ground effects are well documented by postevent surveys, the extreme rainfall events that generate them are still difficult to observe properly. Being able to collect observations of such events will help scientists to better understand and model these phenomena. The recent flash floods that hit the Liguria region (Italy) between the end of October and beginning of November 2011 give us the opportunity to use the measurements available from a large number of sensors, both ground based and spaceborne, to characterize these events. In this paper, the authors analyze the role of the key ingredients (e.g., unstable air masses, moist low-level jets, steep orography, and a slow-evolving synoptic pattern) for severe rainfall processes over complex orography. For the two Ligurian events, this role has been analyzed through the available observations (e.g., Meteosat Second Generation, Moderate Resolution Imaging Spectroradiometer, the Italian Radar Network mosaic, and the Italian rain gauge network observations). The authors then address the possible role of sea–atmosphere interactions and propose a characterization of these events in terms of their predictability.
The Mediterranean coastal cities, on both southwestern and northeastern sides of Italy, are accustomed to floods and flash floods. The Mediterranean Sea acts as a large heat and moisture reservoir—a source from which convective and baroclinic atmospheric systems get part of their energy. The interaction between convective processes originating on the warm sea and sudden orographic lifting very close to the coast produces heavy rainfalls. It often happens that the rain accumulated in 1 h accounts for the entire monthly average for that location, and the rain accumulated in 1 day can account for the entire yearly average (Altinbilek et al. 1997). The morphology of the Mediterranean basin with numerous small and steep river catchments can turn the intense precipitation into severe devastating flash floods and floods. The large-scale environment propitious to heavy precipitation is relatively well known. However, progress has to be made in understanding the mechanisms that govern the precise location of the precipitation system as well as those that can occasionally produce uncommon amounts of precipitation (Ricard et al. 2012).
To mention a few of these disastrous events, we should recall, for example, the flooding of Genoa in northwesternern Italy in 1970 (Roth et al. 1996; Siccardi 1996), the Vaison-la-Romaine in southern France in 1992 (Massacand et al. 1998; Ducrocq et al. 2008), the Izmir case in western Turkey in 1995 (Komuscu et al. 1998), and the most catastrophic flash flood of Algiers in 2001. Each event hit very small sized catchments (10–50 km2), leaving nearby basins unaffected, thus exhibiting a very fine-grained structure of the rainfall precipitation field. The flooding produced many casualties: the last one, in particular, produced a very large number of victims, on the order of 700, in the Bab el Oued borough of Algiers. The spatiotemporal evolution of the Algiers event was discussed by a number of papers (Argence et al. 2008; Branković et al. 2008; Tripoli et al. 2005): the apparent predictability of the Algiers event suggested a controlling role by large-scale forcing. In fact, the European Centre for Medium-Range Weather Forecasts (ECMWF) Integrated Forecast System (IFS), model version 23r4, with a horizontal spacing of 1.8°, about 200-km resolution, and 40 sigma levels in the vertical, detected the structure of the cyclogenesis of the event. Furthermore, the Office National de la Meteorologie of Algeria issued a flood forecast as early as 5 November 2001. The flooding took place between 9 and 10 November: it was a flash flood phenomenon on a very small creek (drainage area about 15 km2) flowing through the Kasbah city center. In the same period the total rainfall observed at a rain gauge 18 km away was negligible. Closer examination, introduced numerically by Tripoli et al. (2005), revealed that significant mesoscale development led to the actual weather pattern over the city. Wind-induced surface heat exchange (WISHE; Emanuel 1994) over the warm waters north of Tunisia and Libya raised the planetary boundary layer (PBL) humidity and temperature until they matched potential temperature values; therefore, the resulting reduction of the level of free convection (LFC) triggered deep moist convective processes. The rainfall amount was estimated up to 250 mm in 48 h.
The characteristics of the events that hit the small catchments of Liguria in October and November 2011 seem very similar to the historical ones mentioned above, in terms of dynamical and thermodynamical forcings, as well as from a hydrological impact standpoint. The difference is that, for the recent Ligurian events, the rainfall measurements at the surface were very dense and meteorological radar and satellite observations were available as well, while little more than numerical modeling results could be used to explain the fine-grained structure of the rainfall field of the previously mentioned historical events. This is the reason why we devoted this paper to the description of the available observations of the recent Ligurian events: in these cases, the physical hypotheses about the development of the events can be validated, at least partially, based on the available observations.
The first flash flood took place on 25 October; hereinafter, we will refer to OCT25 to indicate this event. An anomalous intense rainstorm ripped through the region and inflicted serious damage on the Cinque Terre coastal towns of Monterosso and Vernazza, on the eastern Ligurian complex orography area (Fig. 1, bottom right). The Brugnato rain gauge station, in the center of the event (10 km from the coast over a 500-m hill), registered up to 470 mm of rain in 6 h—a third of the average annual rainfall, with a peak of 150 mm in 1 h. Thirteen people lost their lives.
Nine days later, on 4 November, the city of Genoa, the capital of the Liguria region, located at the meridional edge of the local Apennines range (Fig. 1, bottom left), was gutted by a torrential rainfall event with up to 450 mm (7 km inland the coast over a 300-m hill) of rain in 5 h. Six people were killed. Hereinafter, we will refer to NOV4 to indicate this event. The large-scale features of these events were well predicted by the Liguria Region Meteo-Hydrological Center (Silvestro et al. 2012).
The goal of this paper is to gain a deeper understanding of these events according to the works of Doswell et al. (1996), Miglietta and Rotunno (2010), Romero et al. (2000), Rotunno and Ferretti (2001), and Yu et al. (2007)—all in agreement on the key conditions for severe flood events over complex orography: (i) conditionally or potentially unstable air masses; (ii) moist low-level jets that impinge the first foothills; (iii) steep orography that helps to release the conditional instability associated with the low-level jet; and (iv) a slowly evolving synoptic pattern that slows the advance of the heavy precipitation system, hence increasing their persistency, or maintains the same favorable environment for heavy precipitation.
Section 2 is devoted to characterizing both events at the synoptic scale, to understanding the mesoscale forcings through the use of remote sensing observational datasets [Meteosat Second Generation (MSG), Moderate Resolution Imaging Spectroradiometer (MODIS), and the Italian radar network mosaic], to analyzing the rainfall patterns, and to addressing the possible role of sea–atmosphere interactions. Section 3 focuses on the characterization of the predictability of the two events, based on the approach of Molini et al. (2011). Section 4 presents the final discussion and conclusions.
2. Event on 25 October
a. Synoptic scale
The severe weather event that struck eastern Liguria on 25 October was associated with a large depression positioned off Ireland’s western shore since the day before. A deep trough extending almost from the Arctic Circle to northern Africa entered the Mediterranean late on 24 October, while a high pressure system (1035 hPa) was centered between northeastern Europe and the southern Balkans, with a maximum near the Baltic countries, blocking the cyclonic structure from moving eastward [Fig. 2a, condition (iv)].
Early on 25 October, a cold front was evident in the western Mediterranean, as depicted by the red–blue–green (RGB) Air Mass Product (Fig. 2b), which is a combination of data from the Spinning Enhanced Visible and Infrared Imager (SEVIRI) WV6.2, WV7.3, IR9.7, and IR10.8 channels and thus usable day and night (Lensky and Rosenfeld 2008). This synoptic configuration stimulated the second condition of midlatitude meteorological extremes, that is, intensive advection of warm and moist air of subtropical origin over the Ligurian Sea area [Fig. 2c, condition (ii)], inducing two different weather regimes over western and eastern Liguria. Western Liguria was mainly affected by widespread, stratiform precipitation, while deep moist convection played the main role in eastern Liguria. Thus, the atmospheric scenario was a southwesterly flow on the storm’s right wing (warm front), channeled between Italy and the Sardinia/Corsica east coast and moistened by warm Mediterranean water, that transported a significant amount of water vapor into eastern Liguria (Fig. 2c).
Radiosonde data, shown in Fig. 3 (Barcelona, Palma de Mallorca, Nimes, Pratica di Mare, Ajaccio, and Milan), provide a quantification of the thermodynamical atmospheric structure at 0000 UTC 4 November. All of the stations are located in the Western Mediterranean, appreciably near the places where the severe event occurred. Relative humidity values in the lower troposphere were around 70% and temperature gradient around 6 K (1000 m)−1, thus confirming the presence of potentially unstable air masses, condition (ii), during 25 October.
In the meantime, a vigorous north wind blowing across the central Ligurian Apennines, prompted by high pressure over the Adriatic Sea [condition (iv)], maintained the moist flow over eastern Liguria for almost the whole day and promoted local uplift favorable for flash flood storms: the resulting prominent convergence line (Wang et al. 2000) at the eastern portion of the Ligurian Sea is depicted in the ASCAT image (Fig. 4) by a solid red curve. Coastal wind stations (Table 1) on the left side of the line showed wind blowing from north-northwest (15–20 km h−1), while stations on the right recorded the wind from south-southeast at a speed around 15–20 km h−1.
At the mesoscale, local steep topography (Cond iii) was the trigger for the onset of an organized and self-regenerating mesoscale convective system (MCS) that caused severe rainfall on Cinque Terre, Brugnato (Vara River), and later the Magra River. Sudden rainfall rates reached 160 mm h−1, 350 mm (3 h) −1, and 450 (6 h) −1, respectively. Several landslides, mudslides, and debris flows together with flash floods affected the small catchments in this area.
The C-band, dual polarization radar of Mt. Settepani (located on the Apennine ridge approximately 100 km southwest from the region of interest) allows one to gain a deeper understanding of the MCS morphology (Fig. 5): the vertical cross sections clearly show the slantwise organization of the system, with its upwind coastal portion mainly associated with warm rain processes (zero isotherm around 4000 m), while the horizontal view depicts the observed V-shaped precipitation system with well-defined boundaries and highly persistent localization, as suggested by the map in Fig. 6, which highlights the period of time (between 0800 and 1500 UTC) when reflectivity was above a 40-dB threshold.
The V-shaped structure is also evident in the enhanced IR10.8 Meteosat MSG image on OCT25 at 1200 UTC (Fig. 7a). This product is used in combination with the severe storm RGB (Fig. 7b) to refine the overshooting top characteristics by cloud top microphysics: in this case, the continental cloud top appears to be thick and composed of small ice particles. This inference is confirmed by another MSG product, cloud top temperature and height (CTTH) in Fig. 7c. The CTTH product confirms the slantwise structure of the V-shaped convective cell, with cloud-top heights ranging from 4000 to 5000 m in the Ligurian Sea portion of the structure up to 15 000 m over the continental part.
c. Precipitation analysis
The 25 October event, mainly affecting Brugnato-Borghetto and Cinque Terre (Fig. 8), was observed by the remotely automated weather station network operated by the National Civil Protection Department. Between 0800 and 1500 UTC rainfall accumulated 500 mm in the areas that were hit most severely. Radar-derived rainfall map shows very pronounced spatial variability of the rainfall processes (Fig. 10). As Fig. 8 shows, Serò di Zignago and Santa Margherita Vara are two villages located about 4 and 6 km, respectively, from the small town of Brugnato in the Vara valley: Vara is a left-bank tributary of the Magra River and drains an area slightly larger than 1700 km2, with a hydrological concentration time (Tc) around 6 h.
Comparison of the rain gauges at these three adjacent locations shows large differences in the hyetographs, confirming the very localized features of the autoregenerating thunderstorm responsible for this event (Fig. 9). There is a factor of 3 difference between the Brugnato and Serò di Zignago (Santa Margherita Vara) rain gauges for d = 1 h and d = 3 h. Differences smoothed over longer durations are analyzed, yet the ratio remains over 1.7 (Table 2), even for d = 24 h.
Conversely, rainfall hit Monterosso and Vernazza, two coastal villages separated by more than 10 km from Brugnato, with approximately the same intensity. The reason has to be found in their locations since they are both aligned along the main direction of the southwesterly moist air jet that fed the thunderstorm triggered by coastal topography, and they, as well as Brugnato, experienced a very similar total of rainfall amount, as the radar-derived rainfall map also suggests (Fig. 10).
3. Event on 4 November
a. Synoptic scale
The heavy rainfall episode that resulted in the deadly flooding in Genoa was the most powerful outburst within the larger system that affected southern Europe from 3 to 8 November. The extratropical macrostorm originated from an extension of the 2011 “Halloween Nor’easter” (Ryan 2011) that brought early heavy snowfall to the central and eastern United States on 31 October. This system, tracked across the Atlantic Ocean and regained strength after combining with the remnants of Tropical Storm Rina (23–28 October, from Yucatan and Cuba), which significantly enhanced its precipitable water content.
The development of severe rainfall was the result of the complex combination the aforementioned meteorological conditions for flash floods. Figure 11a presents the 500-hPa height analysis at 0000 UTC 4 November, just a few hours in advance of the onset of the precipitation period over Liguria. An upper-level cold low centered northwest of Ireland and extending southward over the Iberian Peninsula (Fig. 11b) resulted in diffluent southwesterly flow over the Ligurian Apennines ridge at 500 hPa, while the main flux at lower levels was southeasterly. At the same time, a strong pressure ridge centered over the Ukraine and eastern Balkans acted as a block to the eastward motion of the cyclonic structure [condition (iv)].
This synoptic pattern resulted in the onset of an intense south-southeasterly very moist flow [Fig. 11c, condition (ii)] that triggered, together with the local topography [condition (iii)], a series of severe rainfall episodes beginning in late afternoon 3 November (in southeastern France). In the early morning of 4 November, the western boroughs of Genoa were hit by a series of organized self-regenerating thundercells caused by the convergence of moist flow within a small area (less than 10 km2). The hypothesized presence of Doswell’s condition (i) is supported by skew T–logp diagrams at six radiosonde stations (Barcelona, Palma de Mallorca, Nimes, Pratica di Mare, Ajaccio, and Milan; Fig. 12) located in or near the frontal system at 0000 UTC 4 November 2011: relative humidity values are ~70%–75% in the lower troposphere, and the temperature gradient ~6 K (1000 m)−1.
As in the case on 25 October, pattern on 4 November was also associated at the mesoscale with a V-shaped, isolated, and self-regenerating convective cell triggered in the Gulf of Genoa during the night of 4 November (0100–0200 UTC). The cell was again produced by the interaction of cold air, coming from north-northwest in the central-western part of the Gulf of Genoa, with warm moist air coming from the southeast, resulting in a mechanism of cold pool–wind shear interaction (Moncrieff and Changhai 1999), driven by local convergence at low-level flow separation lines (Wang et al. 2000)—well depicted by the Advanced Scatterometer (ASCAT) image (Fig. 13). As on 25 October, coastal wind stations to the left of the convergence line showed the wind blowing from north-northwest (15–20 km h−1), while for stations on to the right the wind is from south-southeast at ~15–20 km h−1.
The Italian radar network observed the structure, but unfortunately, the closest radar located at Mt. Settepani, in Liguria, was not operating owing to technical problems. The radar that actually observed the structure is located near Turin [Bric della Croce, L’Agenzia Regionale per la Protezione dell’Ambiente (ARPA) Regione Piemonte], more than 100 km from the center of the storm. The low quality of the observation is mainly due to the long-distance combined with attenuation and orographic beam blocking/shadowing. The reflectivity pattern over Genoa suffered strong attenuation, but it is nevertheless possible to quantify its persistence: Fig. 14 shows a structure persistence over Genoa city basins (Bisagno and Rio Fereggiano) weaker than that exhibited by the 25 October cell. This cell started meandering along the eastern coast (from 0300 to 0900 UTC) of Liguria and finally stalled over the western portion of the Genoese hills (Fig. 15a), generating the dramatic flash flood of the Rio Fereggiano. Once again, the very high rainfall amount is associated with a slantwise structure, with the cloud-top continental portion appearing to be thick, comprised of small ice particles (Fig. 15b), while the upwind one is dominated by warm rain processes, as during the 25 October event. When compared with the convective structure responsible for the 25 October event, this self-regenerating convective cell appears to be less developed vertically (11 000–12 000 m) and less coherent (Fig. 15c).
c. Precipitation analysis
The precipitation analysis of this event relies on the Liguria National Civil Protection Department rain gauge network, the Liguria Region Hydro-Meteorological Observatory (OMIRL), and on real-time semiprofessional stations belonging to the Liguria Meteorological Association (LIMET): the total number of sensors is about 200, with an average regional density of about one per 40 km2. The most severe rainfall hit the mideastern part of Genoa—in particular, the borough of Quezzi, which is a densely populated borough built on the left bank of the Bisagno creek, a small catchment that drains a total area of 90 km2 (Fig. 16). One of its inlets, Rio Fereggiano (5 km2 area), crosses Quezzi, and its flood was responsible for six casualties. Here, two rain gauges are considered (Fig. 17): one belonging to the LIMET network, located on the Rio Fereggiano, and the other belonging to the OMIRL official network, located 2 km away on the Bisagno.
The sensors provide further proof of the localization of the severe rainfall (Fig. 17), discussed in Fig. 14: despite their proximity, these two sensors observed quite dissimilar rainfall amounts. Differences are displayed in Table 3. LIMET exceeds OMIRL by almost 30%. The return period of the event evaluated with the OMIRL observations is O(50 yr), while the return period of the event with LIMET observations exceeds 200 yr, as computed by means of the two-component extreme value (TCEV) distribution and for a level of confidence α = 0.05 (Boni et al. 2006). Rain gauges 5–10 km away observed nearly no rain.
4. Events on 25 October and 4 November: SST role
While the role of the tropical North Atlantic sea surface temperature (SST) in driving tropical storm activity has been discussed and assessed extensively in the literature (Landsea 1999; Trenberth 2005), a similar potential role has not been explored in detail in the case of midlatitude storms over the Mediterranean area. It is, however, well understood that a warmer SST increases air–sea surface heat fluxes, which in turn moisten and destabilize the marine atmospheric boundary layer, resulting in an increase of the available energy and moisture for atmospheric convection and, thus, precipitation. In this context, a SST analysis is undertaken to gain a deeper understanding of the spatiotemporal properties of these events and the possible role of sea–atmosphere interactions in triggering and driving torrential events.
The 25 October SST anomaly (SSTA) scenario is shown in Fig. 18. The left panel shows the Global 1-km Sea Surface Temperature (G1SST) product (Chao et al. 2009) produced daily by the Jet Propulsion Laboratory Regional Ocean Modeling System (JPL ROMS) group (http://ourocean.jpl.nasa.gov/SST/#), while the right panel displays the Italian Research Council Mediterranean Sea Surface Temperature L4 (CNR MED SST L4), produced and distributed in near real time in the framework of the Global Monitoring for Environment and Security (GMES) MyOcean project (http://www.myocean.eu.org/) by the Institute of Atmospheric Sciences and Climate–Satellite Oceanography Group (ISAC-GOS).
Both anomaly products are computed using the CNR daily pentad mean climatology sea surface temperature, which is based on the Advanced Very High Resolution Radiometer (AVHRR) Pathfinder version 5 dataset over the 1985–2004 time period (Marullo et al. 2007). Although the use of these products is limited by the lack of input SST data from microwave sensors when cloud cover is significant, they are a useful tool for a qualitative description of a very likely SSTA scenario.
Both panels show a positive anomaly of temperature in the central part of the Ligurian Sea: certainly, the two datasets give different SSTA patterns, which can be attributed to different data fusion techniques adopted (Chao et al. 2009; Buongiorno Nardelli et al. 2013) and to the fact that, for G1SST over the Mediterranean Sea, the Geostationary Operational Environmental Satellite (GOES) Imager observational data are not available. However, for both products a major anticyclonic eddy structure and several minor structures distributed in the central part of the basin are evident.
Figure 18 also shows a map of SSTA overlaid with the footprint of the radar-derived rainfall accumulation map.
The main positive SST anomaly located in the northeastern part of the basin, from which the flash-flood-producing storm seems to have originated, broadened both in north–south and east–west directions as it approached the coast (Fig. 18) and possibly fed the storm according to the WISHE mechanism (see section 3).
Along the same lines, the maps of SST anomalies for 4 November are shown in Fig. 19: a positive temperature anomaly in the central part of the Ligurian basin is evident in both panels, with several structures characterized by positive values distributed in the central part of the basin from which the finger convection responsible for the 4 November scenario seems to originate. The link between anomaly and rainfall origin is still present, but it appears somewhat weaker than on 25 October. We must recall that the radar of the Italian radar network closest to the precipitation field was only partly operational during the first part of this event. For this reason, the rainfall pattern was very likely underestimated, and the localization of its starting point is more uncertain than in the previous case.
5. Events on 25 October and 4 November: Predictability analysis
Midlatitude severe events affecting Mediterranean regions can be classified into two categories (Molini et al. 2009): events mainly long lived (lifetime > 12 h) and widespread (area > 50 × 50 km2, hereinafter referred to as T1) and events characterized by smaller space–time extent (hereinafter T2). A quasi-equilibrium environment is a common feature for T1 events while, in general, localized and intense storms belong to the T2 group (Molini et al. 2011).
Differences between the two groups are found not only in their spatiotemporal length scales, but also in the role that large- or local-scale forcing play on their outbreak, which can be determined by considering the convective heating time scale τCH. This time scale provides a measure of the rate at which convective available potential energy (CAPE) is consumed by convective heating. Usually, midlatitude severe events are triggered in a quasi-equilibrium environment (Emanuel 1994, 2000). This means that the CAPE growth rate due to large-scale forcing almost balances its consumption by local convection. In this case, convective time scales τCH are typically small compared to time scales of forcing changes; thus, large-scale forcing determines the statistical properties of convection and the spatiotemporal behavior of the corresponding severe rainfall events, making them more predictable (Done et al. 2006; Molini et al. 2011). On the contrary, a nonequilibrium environment requires heavy rainfall to originate from a weaker synoptic forcing. Therefore, in a nonequilibrium case, convection is controlled by local triggering modalities, for example, the existence of a strong convective inhibition condition, thus with a low degree of predictability.
Done et al. (2006) describes how to calculate τCH from rain depths and CAPE, according to the following formula:
where iR is the rainfall intensity (mm h−1), Lv the latent heat of vaporization, g the acceleration due to gravity, cp the specific heat of air at constant pressure, and T0 and ρ0 reference values of temperature and density.
Molini et al. (2011) found that quasi-equilibrium environments typically show τCH less than 6 h, while values larger than that characterize nonequilibrium configurations. Done et al. (2006) argue that a typical synoptic time scale would be a day or more. Over land, changes in forcing associated with the diurnal cycle are likely to be relevant, so a shorter threshold time scale of ~6 h is adopted (Done et al. 2006; Molini et al. 2011).
Hourly τCH calculations were carried out using the hourly rain depth provided by the Italian National Civil Protection real-time rain gauge network and 3-hourly CAPE estimates at 0.7° horizontal resolution retrieved from the ECMWF Interim Re-Analysis (ERA-Interim) database, the latest ECMWF global atmospheric reanalyses available for the period 1989 to present (Simmons et al. 2007). The 3-hourly CAPE field was linearly interpolated in space and time to match the geographical coordinates of the rain gauges that actually observed the event and their hourly sampling.
The τCH values for 4 November remain small during its most intense phase (from 0900 to 1300 UTC; Fig. 20, lower panel); they indicate this event to be a T1. The increase in the last part is due to the rainfall on the lee side of the Apennines ridge, loosely related to the event that hit the city in the morning. The 25 October scenario is a T1 event as well (Fig. 20, upper panel). Heavy rainfall started around 0900 UTC while the observed maxima were observed between 1300 and 1400 UTC (at the Cinque Terre and Brugnato stations).
The T1 classification of both scenarios is also supportive of the importance of SST anomalies for these events and, thus, of their feeding according to a WISHE mechanism: WISHE involves a positive feedback between the circulation and heat fluxes from the sea surface, with stronger circulation giving rise to larger surface fluxes of heat, which are then quickly redistributed aloft by convection, in turn strengthening the circulation. This emphasizes the role of the surface fluxes as the principal rate-limiting process, while convection serves only to redistribute heat, thus corresponding to the quasi-equilibrium vision for both 25 October and 4 November.
6. Discussion and conclusions
The 25 October and 4 November flash flood events show similarities from several points of view. Both are characterized by a short duration (approximately 6 h in their most intense part), and in both cases the total rainfall amount significantly exceeds the value of multicentennial return period (Boni et al. 2006). The large-scale features of the two can be assumed to be analogous: a synoptic low pressure system originated in the western Atlantic and tracked into the Mediterranean and encountered a robust block exerted by a stationary high-pressure structure located over eastern Europe.
In both events the creation of positive vertical vorticity in the low and middle troposphere by wind shear is clear from the synoptic maps (reanalyses from the NOAA/National Centers for Environmental Prediction model corresponding to 1200 UTC on corresponding days) at different levels shown in Fig. 21. The surface wind direction over Liguria on 25 October (4 November) was from the south-southwest (south), while at midlevels it was from the west-southwest (southwest); in either situation and in a similar fashion, therefore, we note the very moist air flow landward and how it rotates clockwise and intensifies with height in the low to middle layers of atmosphere. The effect of such a configuration contributed to the long persistence of autoregenerating V-shaped heavy rainfall structures over small areas, less than 50 × 50 km2, which in turn were responsible for flash floods that affected small- and medium-sized catchments.
Following these considerations, large-scale forcing played a leading role in enhancing heavy rains. The consequent quasi-equilibrium configuration is confirmed by the analysis of the convective time scale: in both cases, the value of τCH remains below the threshold of 6 h for most of the event as the production of CAPE by large-scale processes is nearly balanced by its consumption by convective phenomena, and thus, CAPE values stay small. Moreover, low values of CAPE were measured by numerous radiosondes at nearby stations.
The local factor that, together with the strong southwesterly (southeasterly) circulation, triggered precipitation was represented for the 25 October (4 November) case study by the steep coastal topography, which forced moist air to rise suddenly and then condense into rain, that is, the well-known orographic lifting mechanism that usually causes flash floods in Italian shoreline towns and nearby inland boroughs. Miglietta and Rotunno (2009) developed a conceptual model for large convective orographic rainfall based on three nondimensional numbers: the ratio of mountain height to the level of free convection hm/LFC, the slope parameter hm/a (with a ridge half width), and the ratio of an advective time scale τa = a/U to a convective growth time scale τc = ht/(CAPE)1/2 (the time that convective elements take to grow, covering the tropopause height ht and producing rain at the surface). For the two Ligurian events, the following scales can be estimated: hm ≈ 500 m, LFC ≈ 1000 m (estimated using Lawrence 2005), a ≈ 10 000 m, U ≈ 8 m s−1 (from ASCAT data products), ht ≈ 10 000 m (average value for 25 October and 4 November from the CTTH product), and CAPE ≈ 500 J kg−1 (from ERA-Interim reanalysis). Consequently, both events respect the prerequisites for large convective orographic rainfall:
corresponding to a regime where the orographic trigger is significant and the peak is located near the top of the ridge (see Fig. 2 in Miglietta and Rotunno 2010), as indeed observed for both events.
Another very interesting aspect deals with the measure of sea surface temperature and, specifically, its anomaly with respect to climatological values. In particular, the sea surface positive anomaly is a concomitant factor that possibly contributes to the exceptional severity of the rainfall processes, according to the WISHE mechanism. Furthermore, in both cases, if SSTA patterns and the radar-derived rainfall accumulation map are overlaid, there are considerable similarities: V-shaped precipitation patterns seem to spread out from the higher anomalies sectors. This feature is more evident for the 25 October study case since radar products did not suffer any gap in the time series, as unfortunately happened during 4 November.
Some studies discuss the effect of SST on torrential Mediterranean rain events (Pastor et al. 2001; Lebeaupin et al. 2006). These studies state that SST plays a key role in the recharge of moisture and heat and contributes to increased conditional convective instability. However, this fact remains to be verified, and further research is needed for fully defining the role of SST in controlling Ligurian intense events. For the two Ligurian events here presented, it is planned to run high-resolution numerical models to clarify whether the presence of a positive anomaly of sea surface temperature can be a factor that is significant in the process of triggering and driving Ligurian torrential rain events. Future work will be devoted to undertaking a modeling study of the 4 November NOV4 event to gain a deeper understanding of the physical processes associated with these prototypal Mediterranean storm events.
This work is supported by Italian Civil Protection Department and by Regione Liguria. We acknowledge Regione Liguria and Regione Piemonte for providing us with the data of the regional meteorological observation networks. We acknowledge the Italian Civil Protection Department for providing us with the Italian Radar Network data. We acknowledge the LIMET association for providing us with the data from their meteorological observation network. We acknowledge the Institute of Atmospheric Sciences and Climate–Satellite Oceanography Group (ISAC-GOS) for providing us with the CNR MED sea surface temperature data. We are very grateful to the meteorologists and the hydrologists of the Meteo-Hydrologic Centre of Liguria Region for many useful discussions. The portion of work carried out by Simone Tanelli was performed at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with National Aeronautics and Space Administration; support from the Precipitation Measurement Missions program is gratefully acknowledged. Nicola Rebora and Antonio Parodi would like to acknowledge the support by the FP7 DRIHM (Distributed Research Infrastructure for Hydro-Meteorology, 2011–2015) project (Contract 283568). The authors also thank Garvin Cummings for help in revising this paper.