Analysis of the Precipitation Regime Over the Ligurian Sea

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Article Analysis of the Precipitation Regime over the Ligurian Sea

Sara Pensieri 1,* ID , Maria Elisabetta Schiano 1, Paola Picco 2, Marco Tizzi 3 and Roberto Bozzano 1 ID

1 National Research Council of Italy, via De Marini 6, 16149 Genoa, Italy; [email protected] (M.E.S.); [email protected] (R.B.) 2 Istituto Idrograﬁco della Marina, Passo dell’Osservatorio 4, 16134 Genoa, Italy; [email protected] 3 Regional Environmental Protection Agency of Liguria, viale Brigate Partigiane 2, 16129 Genoa, Italy; [email protected] * Correspondence: [email protected]; Tel.: +39-010-6475626

Received: 30 March 2018; Accepted: 25 April 2018; Published: 27 April 2018

Abstract: The regions surrounding the North West Mediterranean Sea are often sites of intense precipitation events, particularly during the autumn months. The many casualties and the high economic costs due to these events demand a continuous improvement in forecasting models in support of early warning systems. The main weather conditions that determine episodes of heavy rain over these regions are known, but the high number of processes and interactions taking place at different time and space scales makes it extremely difficult to increase the skill pertaining to their predictability. To deepen the knowledge of the phenomena, both numerical simulations and analysis of historical data sets are required. This paper presents the analysis of a five-year-long time series of rain data collected in the open Ligurian Sea from the fixed platform W1M3A and coastal stations. The analysis aims to characterize the main features of the precipitation over this area and its seasonal and annual variability. Furthermore, the work includes a description of the main atmospheric and oceanic surface parameters measured from the platform during some intense events that occurred in the period 2009–2013 and suggests to what extent offshore observations may contribute to improve the forecast of rainfall events.

Keywords: extreme events; precipitation; ﬂood; marine meteorology; North West Mediterranean sea climate

1. Introduction The Liguria region delimits the North West Mediterranean Sea and it is characterized by the presence of mountains over 2000 m high at only few kilometres away from the coastline that give rise to several catchments with steep slopes and limited extension. Owing to its geographic position and complex orography, the Liguria region is recurrently affected by intense precipitation. Since this area is densely populated, the consequences of these types of events are very dramatic with loss of lives and huge economic damage. Reliable forecasts can mitigate these catastrophic effects, and therefore, there is a constant effort for a continuous improvement in forecasting models in support of the development of early warning systems. The main meteorological causes supporting high precipitations events (HPEs, hereafter) are quite well known: a frontal system or a mesoscale convective system (MCS, hereafter), sometimes combined each other [1,2]. Convective systems are often self-regenerating and persist over the same area for several hours. Some of them show a typical V-shape [3]. The large-scale synoptic patterns

Water 2018, 10, 566; doi:10.3390/w10050566 www.mdpi.com/journal/water Water 2018, 10, 566 2 of 16 determining these situations are equally well known: a deep trough crossing the Mediterranean Sea producing a warm and wet northward flow at low level [2,4]. Orographic forcing, due to the presence of the Alps and Appennines close to the coast is a further important element among mechanisms that generate intense precipitation over this region. A key role in the development of persistent convective systems is also played by the stationary convergence over the Ligurian Sea between the colder north-westerly (i.e., tramontane) winds flowing from the Po Valley through the lowest mountain passes and the warmer and wetter south-easterly (i.e., sirocco) winds which blow from the Tyrrhenian Sea. Nevertheless, the complexity of the processes involved at different spatial and temporal scales is not yet fully resolved and many uncertainties remain in some parameterizations used in models. In order to advance the understanding of the mechanisms determining precipitation phenomena over the Ligurian basin, several comparisons and sensitivity studies have recently been performed on some of the most significant HPEs in the area [5–11]. These investigations make use of the dense network of rain gauges and meteorological stations operating over the land and of satellite data and radar observations. Over the Mediterranean Sea, an operational network of offshore fixed observational stations is lacking, and remote sensing is the principal source of data, especially for measuring the sea surface temperature and the near-surface wind. Khodayar et al. [12] tested the adequacy of the existing Western Mediterranean observing system for improving knowledge of the processes causing high convective precipitation. They concluded that the lack of data at sea introduces relevant uncertainties so that remote sensing data are necessary. However, many satellite products are not always available near the coast and under an overcast sky, a typical condition when extreme precipitation occurs [13]. In addition, remote sensing data are limited to the marine surface whereas it is necessary to extend the investigation into the deep layers since the heat content of the upper ocean strongly contributes to the intensification and translation speed of cyclones, at least in tropical latitudes [14]. The upper ocean heat content can be estimated through in-situ observations provided with mobile or fixed platforms (e.g., drifters, buoys, gliders, moorings). Among them, only eulerian observatories can collect data from the lower atmosphere down to the upper layers of the ocean despite their spatial coverage that, in some cases, could lead to results that cannot be generalized, especially in the studies of local phenomenon such as rainfall. This paper provides an analysis of a five-year-long time series (between 2009 and 2013) of in-situ precipitation and near surface atmospheric and marine data collected on board the W1M3A platform moored in the middle of the Ligurian Sea [15]. The completeness and continuity of the data set give the opportunity to analyse the rain regime of the Ligurian basin and to highlight some interesting aspects. The paper is organized as follows: Section2 describes the dataset and provides an overview of the main weather conditions during the period 2009–2013, Section3 presents the analysis of the time series and a focus on some selected intense events. Discussion and conclusions are given in Section4.

2. Materials and Methods

2.1. Dataset The time series analysed in this study includes hourly meteorological and oceanographic offshore observations acquired in the Ligurian Sea over the period 19 March 2009–31 December 2013 for a total of 1749 days. The measurements were carried out onboard the W1M3A observatory, moored at the centre of the Ligurian basin (Figure1), on a deep sea bed of 1200 m and 80 km far from the coast. It is comprised of a spar buoy speciﬁcally developed for air-sea interaction studies and a close-by subsurface mooring [16]. The position of the observatory, without any shield for winds and waves, is very suitable for detecting early signs of incoming low-pressure systems over the region. Water 2018, 10, x FOR PEER REVIEW 3 of 17

The observatory is equipped with meteorological instruments compliant with the World Meteorological Organisation standards and provides measurements of the main surface parameters: atmospheric pressure, wind speed and direction, air temperature, relative humidity, total shortwave and infrared radiation and precipitation. In order to monitor the physical properties of the water column, oceanographic probes are fixed along the buoy body and on the subsurface mooring. Heat flux at the air‐sea interface is computed using the in‐situ observations from the WM13A and bulk formulae [21–23]. The weather transmitter VAISALA WXT520 equipped with RAINCAP sensor technology acquires the precipitation measurements used in this study. The instrument is an acoustic sensor and measures the impact of individual raindrops to determine the cumulative amount with an accuracy of about 5% and a resolution of 0.01 mmh−1 [24]. The sensor is also capable of distinguishing hailstones from raindrops and it was designed to be immune to flooding clogging, wetting and evaporation losses during rain measurements. This type of instrument was successfully employed to obtain reliable rainfall observations on both land and at sea [25–28]. The instrument is mounted on the central mast of the buoy about 12 m above the mean sea level. The hourly cumulative here used was obtained by integrating the measurements acquired every 5 s, after consistency control was performed. In the analysed time series, only a few very short gaps for some hours occurred, concomitant with periodical routine maintenance activities onboard the observatory. However, since good weather conditions are necessary to carry out these operations, no rain data were lost from the time series. Standardized quality control and quality assurance procedures were applied to near‐real‐time and delayed mode data following the directive established by the worldwide system of long‐term and deep water reference stations OceanSITES as described in Coppola et al. [29]. Hourly precipitation data on land, available through the archive of the Regional Environmental Protection Agency of Liguria (https://www.arpal.gov.it), were used: four stations were selected for the completeness of the time series, their location on the coast and their height above sea level. These stations are: Imperia‐Osservatorio Meteosismico, Savona‐Istituto Nautico, Genova‐Centro Funzionale and La Spezia‐SPEZIA (Figure 1). They are located at a height above sea level of 74 m, 24 m, 30 m and 35 m and their distance from the W1M3A observatory is 71 km, 79 km, 92 km and 64 km, respectively. In all these stations, the precipitation measurements are carried out by using an electronic tipping rain gauge with an accuracy of about 2% and a resolution of 0.2 mmh−1. Climate normal (CLINO 1961–1990) data on the Liguria region are considered in order to analyze the Water 2018, 10, 566 3 of 16 variability of the precipitation on land.

Figure 1. Map of the study area showingshowing the positionposition of thethe W1M3AW1M3A observatory and the four main coastal cities of the Liguria region. coastal cities of the Liguria region.

The W1M3A platform is part of the Italian [17] and European [18] networks of fixed open ocean observatories. It is also one of the three monitoring systems of the Mediterranean Sea forming the Mediterranean Moored Multi-sensor Array network [19] devoted to fulfill the requirements of operational oceanography in terms of acquisition and distribution of essential ocean climate variables [20] in near real-time. The observatory is equipped with meteorological instruments compliant with the World Meteorological Organisation standards and provides measurements of the main surface parameters: atmospheric pressure, wind speed and direction, air temperature, relative humidity, total shortwave and infrared radiation and precipitation. In order to monitor the physical properties of the water column, oceanographic probes are fixed along the buoy body and on the subsurface mooring. Heat flux at the air-sea interface is computed using the in-situ observations from the WM13A and bulk formulae [21–23]. The weather transmitter VAISALA WXT520 equipped with RAINCAP sensor technology acquires the precipitation measurements used in this study. The instrument is an acoustic sensor and measures the impact of individual raindrops to determine the cumulative amount with an accuracy of about 5% and a resolution of 0.01 mmh−1 [24]. The sensor is also capable of distinguishing hailstones from raindrops and it was designed to be immune to flooding clogging, wetting and evaporation losses during rain measurements. This type of instrument was successfully employed to obtain reliable rainfall observations on both land and at sea [25–28]. The instrument is mounted on the central mast of the buoy about 12 m above the mean sea level. The hourly cumulative here used was obtained by integrating the measurements acquired every 5 s, after consistency control was performed. In the analysed time series, only a few very short gaps for some hours occurred, concomitant with periodical routine maintenance activities onboard the observatory. However, since good weather conditions are necessary to carry out these operations, no rain data were lost from the time series. Standardized quality control and quality assurance procedures were applied to near-real-time and delayed mode data following the directive established by the worldwide system of long-term and deep water reference stations OceanSITES as described in Coppola et al. [29]. Hourly precipitation data on land, available through the archive of the Regional Environmental Protection Agency of Liguria (https://www.arpal.gov.it), were used: four stations were selected for the completeness of the time series, their location on the coast and their height above sea level. These stations are: Imperia-Osservatorio Meteosismico, Savona-Istituto Nautico, Genova-Centro Water 2018, 10, 566 4 of 16

Funzionale and La Spezia-SPEZIA (Figure1). They are located at a height above sea level of 74 m, 24 m, 30 m and 35 m and their distance from the W1M3A observatory is 71 km, 79 km, 92 km and 64 km, respectively. In all these stations, the precipitation measurements are carried out by using an electronic tipping rain gauge with an accuracy of about 2% and a resolution of 0.2 mmh−1. Climate normal (CLINO 1961–1990) data on the Liguria region are considered in order to analyze the variability of the precipitation on land.

2.2. Precipitation over the Liguria Region during the Period 2009–2013 The Liguria region, with its characteristic arched shape open to the south and the mountain ridge behind it, presents a rather complex rain regime. It is in the transitional zone between the typical Mediterranean pluviometric regime, with an accentuated winter peak, characteristic of the peninsular and insular regions of Italy, and the sub-continental and continental one, with the maximum in summer, characteristic of the Prealps and the Alpine arc. The amount of precipitation increases proceeding from the coast towards the inland of the region and from west to east. In fact, along the Western part (from Imperia to Genoa), the annual cumulative amount may be less than 800 mm, whereas in the Eastern part (from Genoa to La Spezia), the annual cumulative amount can exceed 1000 mm. Events of exceptional intensity are frequent from the inland of Genoa to the extreme Levante [30]. These are mainly due to convective systems that develop over the sea and affect the coastal areas, especially in autumn. The selected period shows a high annual and seasonal variability, with long periods of drought and frequent intense events and thus offers a wide and varied case series. The beginning of 2009 is characterized by an intense low pressure activity with diffuse precipitations and sometimes snowfalls even on the coast. Starting from May, there are no significant rain events in the region over a long period. From mid-September the weather worsens, causing strong instability with some episodes of intense rain and thunderstorms. Two HPEs take place in October, from 9 to 10 and from 20 to 21, affecting the central and eastern Liguria with acumulation greater than 60 mm and peaks over 120 mmh−1, recorded on 9 in Fiorino, in the inland of Genoa at 290 above sea level (asl). The very perturbed weather conditions protract until mid-March 2010 with frequent continental cold air intrusions often causing snowfall at low altitudes. After a relatively wet spring, typically summer conditions are established. However, from mid-August, several Atlantic low-pressure systems enter the Mediterranean basin causing a long period of precipitation, snowfalls and thunderstorms. A first short yet very intense event occurs on 13 August when the development of MCS affects Genoa and the Eastern Liguria with rain peaks greater than 70 mmh−1 in Portovenere, near La Spezia (40 asl). A further V-shape MCS occurs on 7 and 8 September in the same areas with hourly accumulation greater than 200 mm. The most dramatic event happens on 4 October, when an MCS hits the coast between Genoa and Savona with more than 400 mm of rain in less than 6 h. The rain is very frequent until the first half of January 2011, when a milder period begins and lasts until mid spring. Summer starts under very perturbed conditions, characterized by frequent thunderstorms, sometimes very intense, between June and July. Particularly, from 8 to 9 June, intense rain affects Eastern Liguria with hourly accumulation of about 100 mm. From the end of July, typical summer conditions occur, with temperatures above average and a rain deficit. High temperatures continue until the early winter months, whereas intense and very localized rainy events already occur by the end of October. In less than ten days, on 25 October and on 5 November, two flash floods hit close to La Spezia and Genoa, due to a V-shape MCS developed over the sea near the coast with accumulation of more than 400 mm. With the exception of February, when polar air strongly lowers the temperature and causes snow, the winter and the summer of 2012 are characterized by mild weather. Some thunderstorms occur only in June. The autumn is exceptionally rainy, particularly in November. The most important event takes place on 10 and 11 November with more than 200 mm of rain in Eastern Liguria and a flood in Tuscany. Water 2018, 10, 566 5 of 16

During 2013, wintry phenomena, such as snow and frost, last until mid-spring, whereas summer is hot and stable. Spring and autumn are characterized by an exceptional sequence of very heavy rainfall episodes and strong and localized thunderstorms. A dramatic ﬂood hits eastern Sardinia on 18 November due to the development of a Mediterranean cyclone. During 2013, the occurrence of southernWater 2018, gales 10, x FOR increases PEER REVIEW and becomes comparable with the northern ones, which, on the contrary,5 of 17 are usually prevailing. TheThe data data of of the the four four selected selected coastal coastal stations stations well well indicate indicate the the precipitation precipitation variability variability of of the the examinedexamined period period (Figure (Figure2 ).2). Compared Compared to to the the climatologic climatologic values values of of the the CLINO CLINO 1961–1990 1961–1990 archive, archive, thethe accumulation accumulation in in the the summer summer months months is generally is generally lower, lower, whereas whereas that inthat the in winter the winter months months is often is signiﬁcantlyoften significantly greater. greater. In some In some cases, cases, this is this due is todue a singleto a single episode episode of high of high intensity, intensity, as happens as happens in Novemberin November 2011 2011 in Genoa, in Genoa, whereas whereas in others, in others, to an to increase an increase in the in numberthe number of rainy of rainy days. days.

FigureFigure 2. 2.( a(a)) Time Time series series of of cumulative cumulative and and rainy rainy days days in in the the coastal coastal station station of of Genoa Genoa over over the the period period JanuaryJanuary 2009–January 2009–January 2014;2014; ((bb)) Time Time series series of of cumulative cumulative and and rainy rainy days days in in the the coastal coastal station station of of ImperiaImperia over over the the period period January January 2009–January 2009–January 2014; 2014; (c) ( Timec) Time series series of cumulative of cumulative and and rainy rainy days days in the in coastalthe coastal station station of La Speziaof La overSpezia the over period the January period 2009–JanuaryJanuary 2009–January 2014; (d) Time 2014; series (d) Time of cumulative series of andcumulative rainy days and in rainy the coastal days stationin the coastal of Savona station over of the Savona period over January the 2009–Januaryperiod January 2014. 2009–January 2014. 3. Results 3. Results 3.1. Precipitation Variability 3.1. ThePrecipitation data of the Variability observatory for the ﬁrst three months of 2009 is missing; over the years 2009–2013, precipitationThe data events of the at seaobservatory occur for 686for days,the first representing three months 39% of of the 2009 total is number missing; of analyzedover the days.years The2009–2013, values of precipitation the annual cumulative events at measuredsea occur atfor sea 686 are days, in agreement representing with the39% measurements of the total number obtained of inanalyzed the four selecteddays. The coastal values sites, of particularlythe annual cumulative with those observed measured in Genoa.at sea are The in year agreement with the with greatest the measurements obtained in the four selected coastal sites, particularly with those observed in Genoa. The year with the greatest amount of precipitation is 2013 with a cumulative amount of 1386.2 mm, whereas the lowest values are recorded in 2011 and 2012, i.e., 886.9 mm and 857.1 mm, respectively. The analysis of the monthly distributions shows that 2010 is the year with the greatest number of rainy days, with a total of 166, whereas the month with the most rain is March 2013, with 21 days. Only in August 2011 no rain occurs. Daily accumulation less than 1 mm is observed for 331 days (48% of total rainy days) whereas only for 19 days it is greater than 50 mm. Among these days, seven events exceed the threshold of 100 mmday−1 and for two of them, the cumulative is greater than 200

Water 2018, 10, 566 6 of 16 amount of precipitation is 2013 with a cumulative amount of 1386.2 mm, whereas the lowest values are recorded in 2011 and 2012, i.e., 886.9 mm and 857.1 mm, respectively. The analysis of the monthly distributions shows that 2010 is the year with the greatest number of rainy days, with a total of 166, whereas the month with the most rain is March 2013, with 21 days. Only in August 2011 no rain occurs. Daily accumulation less than 1 mm is observed for 331 days (48%Water of2018 total, 10, x rainy FOR PEER days) REVIEW whereas only for 19 days it is greater than 50 mm. Among these days,6 of 17 seven events exceed the threshold of 100 mmday−1 and for two of them, the cumulative is greater than 200mmday mmday−1, both−1, both in November. in November. The The hourly hourly distribution distribution of of rainy rainy data data shows thatthat thethe precipitation precipitation mainlymainly occurs occurs in thein the central central hours, hours, 11–12 11–12 UTC, whereasUTC, whereas the average the hourlyaverage cumulative hourly cumulative has a maximum has a inmaximum the early morningin the early and morning the greatest and the intensity greatest is recordedintensity beforeis recorded sunrise before and sunrise after sunset. and after sunset. PrecipitationPrecipitation can can taketake placeplace for several consecutive consecutive days, days, even even if if not not in in a acontinuous continuous manner. manner. In Inmost most cases cases (48%), (48%), the the consecutive consecutive days days are are two two and and the therainy rainy period period extends extends beyond beyond the week the week only onlytwice, twice, from from 24 September 24 September to 4 toOctober 4 October 2012 2012 and and from from 4 to 4 13 to March 13 March 2013. 2013. On Onthe thecontrary, contrary, the themaximum maximum number number of ofconsecutive consecutive days days without without rain rain is is 46, 46, from from 16 16 July to 2929 AugustAugust 2009. 2009. Another Another longlong dry dry period period isis fromfrom 29 July to to 2 2 September September 2011 2011 (38 (38 days). days). In In almost almost half half of ofthe the cases cases (43%), (43%), the therainy rainy hours hours in ina single a single day day are are less less than than 3 3and and do do not not exceed exceed 5 5 h for 67% ofof thethe time,time, whereas whereas the the daysdays with with more more than than 12 12 h h of of rain rain are are 56 56 (8% (8% of of total total rainy rainy days) days) (Figure (Figure3a). 3a). Only Only on on 24 24 April April 2010 2010 doesdoes the the precipitation precipitation phenomenonphenomenon last throughout the course course of of the the 24 24 h. h. However, However, the the number number of ofrainy rainy hours hours per per day day does does not not affect affect the the daily daily cumulative cumulative since since the the total total precipitation precipitation is less is less than than 16.5 16.5mmday mmday−1. Almost−1. Almost half of half the of 3371 the 3371hourly hourly rain data rain (equal data (equal to 44.5% to 44.5% of total of rainy total rainydays) days)concern concern events eventswith an with intensity an intensity lower than lower 2 mmh than− 21 (Figure mmh− 13b).(Figure Rain3 intensityb). Rain greater intensity than greater 10 mmh than−1 occurs 10 mmh in 224−1 occurscases: inamong 224 cases: these, among in 13 cases these, it inexceeds 13 cases 30 itmmh exceeds−1 and 30 in mmh only− two1 and events in only rainfall two eventsis greater rainfall than is50 greatermmh−1 than (on 18 50 November mmh−1 (on 2013 18 November at 04 UTC 2013 and at on 04 23 UTC December and on 232010 December at 02 UTC). 2010 Taking at 02 UTC). into account Taking intoonly account precipitation only precipitation at sea greater at than sea greater 2 mmh than−1, the 2 monthly mmh−1, distribution the monthly of distribution the hourly of rain the intensity hourly raindata intensity shows that data phenomena shows that of phenomena extreme and of extremeconvective and rain convective are more rain frequent are more between frequent October between and OctoberDecember and (Figure December 3c). (Figure3c).

FigureFigure 3. 3.( (aa)) Histogram Histogram ofof rainyrainy hours per day day at at sea; sea; (b (b) )Histogram Histogram of of hourly hourly rain rain intensity intensity at atsea; sea; (c) (cHistogram) Histogram of of monthly monthly rain rain intensity intensity at at sea. sea.

Among these high precipitation events, the phenomena occurring in the month of November Among these high precipitation events, the phenomena occurring in the month of November strongly contribute to the large cumulative measured in autumn. Indeed, 267 mm out of the 398 mm strongly contribute to the large cumulative measured in autumn. Indeed, 267 mm out of the 398 mm in November 2009 are measured in less than 24 h, as well as the cumulative amount of 258 mm in in November 2009 are measured in less than 24 h, as well as the cumulative amount of 258 mm in November 2013, which is mainly due to the 214 mm of precipitation that occurs on 18, and 5 November 2013, which is mainly due to the 214 mm of precipitation that occurs on 18, and 5 November November 2011, with 162 mmday−1 contributing to the 267 mm monthly cumulative. Intense 2011, with 162 mmday−1 contributing to the 267 mm monthly cumulative. Intense precipitation is less precipitation is less frequent in the other months of the year when the cumulative amount is the frequent in the other months of the year when the cumulative amount is the result of less intense but result of less intense but more frequent and persistent episodes (Figure 4). more frequent and persistent episodes (Figure4).

Water 2018, 10, 566 7 of 16 Water 2018, 10, x FOR PEER REVIEW 7 of 17

Figure 4. (a) Monthly cumulative for the years 2009–2013 at sea; (b) Number of rainy hours for each Figure 4. (a) Monthly cumulative for the years 2009–2013 at sea; (b) Number of rainy hours for each month for the years 2009–2013 at sea. month for the years 2009–2013 at sea. 3.2. High Precipitation Events 3.2. High Precipitation Events In the analyzed period, all thirteen episodes with hourly precipitation exceeding 30 mmh−1 In the analyzed period, all thirteen episodes with hourly precipitation exceeding 30 mmh−1 occur occur in autumn, mainly during the night (Table 1). Some of them are short‐lived phenomena in autumn, mainly during the night (Table1). Some of them are short-lived phenomena whereas others whereas others are more persistent but all are due to mesoscale convective systems. are more persistent but all are due to mesoscale convective systems. Although these events show the greatest precipitation data recorded onboard the W1M3A platform, their cumulative is lower than that measured on land, where it is quite common to find Table 1. High precipitation events with hourly precipitation at sea exceeding 30 mmh−1. hourly values above 50 mmh−1 and daily cumulative over 400 mm. Hourly Precipitation Daily Cumulative Date Time (UTC) Notes Table 1. High precipitation events with(mmh hourly−1) precipitation at(mm) sea exceeding 30 mmh−1.

8 NovemberDate 2009Time 03; (UTC) 04; 05 Hourly Precipitation 33.48; 44.38; 44.39 (mmh−1) Daily Cumulative 267.1 (mm) Notes 23 December 2010 19; 20 41.91; 50.35 155.6 8 November 2009 03; 04; 05 33.48; 44.38; 44.39 267.1 25 October 2011 02 32.74 126.6 Floods in Liguria 235 NovemberDecember 2010 2011 19; 12; 20 13 41.91; 32.99; 50.35 33.72 162.9155.6 Floods in Liguria 825 November October 2011 2011 02 01 32.74 47.49 71.8126.6 FloodsFloods in in Liguria Liguria 5 November5 October 2013 2011 12; 13 01 32.99; 33.39 33.72 34.5162.9 Floods in Liguria 188 November November 2011 2013 03;01 04; 05 45.97;47.49 59.75; 42.29 214.771.8 FloodsFloods in in Sardinia Liguria 5 October 2013 01 33.39 34.5 18 November 2013 03; 04; 05 45.97; 59.75; 42.29 214.7 Floods in Sardinia Although these events show the greatest precipitation data recorded onboard the W1M3A platform,Some their of the cumulative events listed is lower in Table than 1 thatare concomitant measured on with land, important where it flooding is quite common on land, toeven find if − hourlynot all valueshit the above Ligurian 50 mmh region.1 and Particularly, daily cumulative on 8 November over 400 mm. 2009 and 5 October 2013, intense precipitationSome of theoccurs events in the listed northwestern in Table1 are coast concomitant of Tuscany with whereas important on 18 flooding November on land, 2013, even a dramatic if not allflood hit thehits Ligurian Sardinia. region. However, Particularly, there is not on 8 always November a correlation 2009 and between 5 October intense 2013, intense episodes precipitation at sea and occurson land. in For the instance, northwestern during coast the flood of Tuscany in western whereas Liguria on 18on November October 2010 2013, that a caused dramatic considerable flood hits Sardinia.damage, no However, significant there evidence is not alwaysfrom the a in correlation‐situ data is between observed: intense the rain episodes starts in at the sea late and afternoon on land. Forand instance, lasts only during few hours the flood with a in peak western of about Liguria 20 mmh on October−1 at 18 2010UTC thatand causeda cumulative considerable less than damage, 27 mm noin 5 significant h. evidence from the in-situ data is observed: the rain starts in the late afternoon and lasts − only fewIn the hours following, with a peakfour ofevents about concomitant 20 mmh 1 at with 18 UTC floods and on a cumulativeland are analyzed: less than they 27 mm have in 5been h. widelyIn the investigated following, in four several events papers concomitant [7,10,31–33,34], with floods but on none land of are these analyzed: works theyinclude have direct been rainfall widely investigatedobservations in at several sea. For papers each [7 ,selected10,31–34 ],high but noneprecipitation of these worksevent, include a short direct description rainfall observationsof the main atmeteorological sea. For each patterns selected causing high precipitation the phenomenon event, and a short the main description marine and of the atmospheric main meteorological parameters patternsmeasured causing by the the W1M3A phenomenon observatory and the are main given. marine and atmospheric parameters measured by the W1M3A observatory are given. 3.2.1. Event on 25 October 2011 3.2.1. Event on 25 October 2011 A low‐pressure system is present over the European Atlantic coasts while a deep trough is over the MediterraneanA low-pressure area system and is moves present East, over greatly the European slowed by Atlantic the presence coasts whileof the a high deep pressure trough is on over the theBalkans. Mediterranean A secondary area minimum and moves develops East, greatly over the slowed Gulf of by Lion. the presence The Liguria of the region high is pressure affected on by thethe warm sector of this perturbation where a frontal convergence line, almost stationary, favours the development of an intense and small‐sized quasi‐stationary V‐shaped mesoscale convective system

Water 2018, 10, x FOR PEER REVIEW 8 of 17

[7–10] (Figure 5a). The persistence of the precipitation is the crucial factor leading to the daily elevated cumulative experienced both at sea and on land. The quasi‐stationary V‐shaped convective system hits the small areas of Cinque Terre and Vara Valley near La Spezia causing 13 deaths and extensive damage. On 25 October, in Brugnato, the rain reaches a peak of 143.4 mmh−1 with a daily cumulative of 539 mm, whereas in the nearby La Spezia, the precipitation amount is lower than 40 mmh−1 (Figure 5b). Water 2018, 10, 566 8 of 16 The system originates over the eastern Ligurian Sea, north‐east of the W1M3A observatory, because of the stationary convergence of warmer and wetter south‐easterly winds over the TyrrhenianBalkans. A secondarySea (i.e., sirocco minimum winds) develops and overcolder the northerly Gulf of Lion. winds The flowing Liguria from region Po is valley affected (i.e., by tramontanethe warm sector winds). of thisRain perturbation starts on 25 October where a at frontal 00 UTC convergence and reaches line, a peak almost of 35 stationary, mmh−1 at 02 favours UTC, persistingthe development throughout of an the intense day with and lower small-sized intensity. quasi-stationary The difference between V-shaped the mesoscale sea temperature convective and thesystem air [temperature7–10] (Figure is5a). positive, The persistence presumably of the because precipitation of the ispersistence the crucial of factor northerly leading flow to the blowing daily fromelevated northern cumulative sectors experienced for three days both atbefore sea and the on event. land. The The intrusion quasi-stationary of warmer V-shaped water convective from the Tyrrheniansystem hits theSea small strongly areas affects of Cinque the Terresurface and marine Vara Valley stratification. near La SpeziaThe heat causing flux 13 towards deaths andthe atmosphereextensive damage. increases On at 25 the October, passage in Brugnato,of the baric the minimum, rain reaches accompanied a peak of 143.4 by strong mmh− northwesterly1 with a daily wind,cumulative rain and of 539a consequent mm, whereas drop inof the air nearby temperature, La Spezia, and the then precipitation begins to decrease amount reaching is lower much than lower40 mmh values.−1 (Figure 5b).

Figure 5. (a) Visible RGB combination from Meteosat Second Generation satellite on 25 October 2011 Figure 5. (a) Visible RGB combination from Meteosat Second Generation satellite on 25 October 2011 at 12at 12 UTC; UTC; (b )(b Time) Time series series of precipitationof precipitation collected collected by by the the W1M3A W1M3A observatory, observatory, the the coastal coastal station station of Laof LaSpezia Spezia and and an inlandan inland station station (Brugnato, (Brugnato, 112 112 asl) asl) between between 24 and24 and 26 October26 October 2010. 2010.

3.2.2. Events on 5 and 8 November 2011 The system originates over the eastern Ligurian Sea, north-east of the W1M3A observatory, becauseFew of days the stationaryafter the dramatic convergence high ofprecipitation warmer and event wetter occurred south-easterly in Cinque winds Terre, over a flash the Tyrrhenian flood hits GenoaSea (i.e., causing sirocco 6 winds) casualties. and colderThe synoptic northerly situations winds flowing of the two from events Po valley are very (i.e., similar: tramontane a deep winds). low overRain startsWestern on 25Europe October blocked at 00 UTCby a andstrong reaches high‐ apressure peak of 35field mmh over−1 Easternat 02 UTC, Europe. persisting This throughoutis due to a southernthe day with intense lower flow intensity. of wet The and difference warm air between over the the Tyrrhenian sea temperature Sea, andinduced the air by temperature this cyclonic is positive,circulation presumably and a northern because low of‐ thelevel persistence flow, induced of northerly by baric flow and blowing thermal from gradients northern between sectors forPo Valleythree days and beforeLigurian the Sea, event. leading The intrusion to the consequent of warmer formation water from of thea prefrontal Tyrrhenian convergence Sea strongly line affects over the surfaceLigurian marine Sea (Figure stratification. 6a,b). These The heat conditions, flux towards also owing the atmosphere to the particular increases orography at the passage of Liguria, of the determinebaric minimum, the development accompanied of byquasi strong‐stationary, northwesterly self‐regenerating wind, rain V and‐shaped a consequent convective drop systems of the and air intensetemperature, precipitation and then on begins 4 November to decrease with reaching a peak muchof 180 lower mmh values.−1 at Vicomorasso, an inland village close to Genoa. The event lasts from 3 to 9 November with very intense rainfall, often associated with3.2.2. storms Events at on sea 5 and (Figure 8 November 7c). 2011 Few days after the dramatic high precipitation event occurred in Cinque Terre, a flash flood hits Genoa causing 6 casualties. The synoptic situations of the two events are very similar: a deep low over Western Europe blocked by a strong high-pressure field over Eastern Europe. This is due to a southern intense flow of wet and warm air over the Tyrrhenian Sea, induced by this cyclonic circulation and a northern low-level flow, induced by baric and thermal gradients between Po Valley and Ligurian Sea, leading to the consequent formation of a prefrontal convergence line over the Ligurian Sea (Figure6a,b). These conditions, also owing to the particular orography of Liguria, determine the development of quasi-stationary, self-regenerating V-shaped convective systems and intense precipitation on 4 November with a peak of 180 mmh−1 at Vicomorasso, an inland village close to Genoa. The event lasts from 3 to 9 November with very intense rainfall, often associated with storms at sea (Figure7c). Water 2018, 10, 566 9 of 16 Water 2018, 10, x FOR PEER REVIEW 9 of 17

FigureFigure 6.6. (a(a)) Visible Visible RGB RGB combination combination from from Meteosat Meteosat Second Second Generation Generation satellite satellite on 5 on November 5 November 2011 at2011 12 UTC;at 12 ( bUTC;) Visible (b) RGB Visible combination RGB combination from Meteosat from Second Meteosat Generation Second satellite Generation on 8 November satellite on 2011 8 atNovember 12 UTC; (2011c) Time at 12 series UTC; of ( precipitationc) Time series collected of precipitation by the W1M3A collected observatory, by the W1M3A the coastal observatory, station the of Genoacoastal and station an inland of Genoa station and (Vicomorasso, an inland station 310 asl) (Vicomorasso, between 3 and 310 7 asl) November between 2011. 3 and 7 November Water 2018, 10, x FOR PEER REVIEW 10 of 17 2011.

At the W1M3A site, the rain starts very weakly in the late afternoon of 4 November. The precipitation is more intense on 5 November, with a daily cumulative amount of 162 mm, but without hourly significant peaks, and it mainly originated from a thunderstorm stationary system centred on the Ligurian Sea to the north of Corsica. Apart from weak episodes, the peak rainfall is measured during the night of 8 November, i.e., more than 47 mmh−1. It is worth noting that in this event the V‐shape convective system originates over the northern Ligurian Sea, north‐west of the observatory, which therefore lies in the sirocco sector.

3.2.3. Event on 18 November 2013 This event is concomitant with an extratropical cyclone in the Western Mediterranean Sea. Cold area from North Europe flows over the Mediterranean basin and interacts with the warm moist air. The Figurecyclone 7. movesVisible slowlyRGB combination towards southeast,from Meteosat developing Second Generationviolent storm satellite systems image (Figureon 18 7). Figure 7. Visible RGB combination from Meteosat Second Generation satellite image on 18 November Particularly,November it 2013hits at north 12 UTC.‐eastern Sardinia where several floods occur causing 18 causalities and serious2013 damage at 12 UTC. and it only marginally affects the coast of Liguria 3.2.4.The Event rain on records 10 November of the Ligurian2012 land stations show weak diffuse precipitation over the whole regionAt theslowly W1M3A moving site, thewestward rain starts with very weaklya maximum in the latecumulative afternoon of of 440 November. mm in twelve The precipitation hours at In contrast to the other analysed cases, on 10 November 2012, the precipitation at sea is absent isPortofino, more intense between on 5Genoa November, and La with Spezia. a daily cumulative amount of 162 mm, but without hourly while floods occur on land. A wide trough over the Iberian Peninsula gives rise to a secondary signiﬁcantOn the peaks, contrary, and itin mainly the early originated hours fromof 18 a thunderstormNovember, the stationary sensors systemonboard centred the W1M3A on the minimum in the Gulf of Lion, leading to a strong flow of warm and wet air along the Tyrrhenian Ligurianobservatory Sea record to the north intense of Corsica.rain for a Apart few hours from weak simultaneous episodes, with the peak the increase rainfall is and measured turning during to the coasts. During the night of 10 November, a clear line−1 of convergence between the sirocco winds close thenorth night‐west of of 8 November, the wind, which i.e., more remains than 47under mmh the. threshold It is worth of noting 15 ms that−1. This in this turning event could the V-shape be the to the coast and those from the northwest blowing on the Ligurian Sea activate intense convectivesignal of the system passage originates of a possible over tramontane the northern‐sirocco Ligurian convergence Sea, north-west line, triggering of the observatory, strong convection which self‐regenerating thunderstorm systems that hit Magra valley, particularly on the Tuscan side thereforein the area lies surrounding in the sirocco the sector. observatory. The air temperature fluctuates around the values of the sea (Figure 8). surface, with a remarkable drop during rainfall and concurrent tramontane conditions. The heat While on land the event is of exceptional intensity and causes considerable damage, nothing of flow, even if always directed to the atmosphere, is weak due to the reduced thermal gradient significance appears at sea: the rain starts in the late afternoon and lasts a few hours with a peak less between air and sea, the low wind and the increase in water vapour in the atmosphere. than 20 mmh−1 at 17 UTC and a cumulative less than 53 mm. The wind and pressure data clearly show the propagation of the low‐pressure system that moves eastwards and the development of the convergence line, with the observatory lying in the sector under northwesterly winds. The incoming northwesterly wind produces a sharp drop in temperature and a consequent increase in the heat fluxes towards the atmosphere.

Figure 8. Visible RGB combination from Meteosat Second Generation satellite image on 10 November 2012 at 12 UTC.

4. Discussion and Conclusions In‐situ measurements of rainfall at open sea are really scarce, thus the multiannual measurements collected by the W1M3A observatory provide valuable information to characterize the precipitation regime over the Ligurian Sea.

Water 2018, 10, x FOR PEER REVIEW 10 of 17

Water 2018, 10, 566 10 of 16

3.2.3. Event on 18 November 2013 This event is concomitant with an extratropical cyclone in the Western Mediterranean Sea. Cold area from North Europe flows over the Mediterranean basin and interacts with the warm moist air. The cyclone moves slowly towards southeast, developing violent storm systems (Figure7). Particularly, it hits north-eastern Sardinia where several floods occur causing 18 causalities and serious damage and it only marginally affects the coast of Liguria The rain records of the Ligurian land stations show weak diffuse precipitation over the whole regionFigure slowly 7. movingVisible westwardRGB combination with a maximumfrom Meteosat cumulative Second ofGeneration 40 mm in satellite twelve hoursimage at on Portofino, 18 betweenNovember Genoa 2013 and at La 12 Spezia. UTC. On the contrary, in the early hours of 18 November, the sensors onboard the W1M3A observatory record3.2.4. Event intense on rain 10 November for a few hours 2012 simultaneous with the increase and turning to the north-west of the wind, which remains under the threshold of 15 ms−1. This turning could be the signal of the passage of In contrast to the other analysed cases, on 10 November 2012, the precipitation at sea is absent a possible tramontane-sirocco convergence line, triggering strong convection in the area surrounding while floods occur on land. A wide trough over the Iberian Peninsula gives rise to a secondary the observatory. The air temperature fluctuates around the values of the sea surface, with a remarkable minimum in the Gulf of Lion, leading to a strong flow of warm and wet air along the Tyrrhenian drop during rainfall and concurrent tramontane conditions. The heat flow, even if always directed to coasts. During the night of 10 November, a clear line of convergence between the sirocco winds close the atmosphere, is weak due to the reduced thermal gradient between air and sea, the low wind and to the coast and those from the northwest blowing on the Ligurian Sea activate intense the increase in water vapour in the atmosphere. self‐regenerating thunderstorm systems that hit Magra valley, particularly on the Tuscan side 3.2.4.(Figure Event 8). on 10 November 2012 While on land the event is of exceptional intensity and causes considerable damage, nothing of significanceIn contrast appears to the at other sea: the analysed rain starts cases, in the on 10late November afternoon 2012, and lasts the precipitationa few hours with at sea a peak is absent less whilethan 20 floods mmh occur−1 at 17 on UTC land. and A a wide cumulative trough less over than the 53 Iberian mm. PeninsulaThe wind and gives pressure rise to adata secondary clearly minimumshow the propagation in the Gulf of of Lion, the low leading‐pressure to a strong system flow that of moves warm andeastwards wet air and along the the development Tyrrhenian coasts.of the Duringconvergence the night line, of with 10 November,the observatory a clear lying line in of the convergence sector under between northwesterly the sirocco winds. winds The close incoming to the coastnorthwesterly and those wind from produces the northwest a sharp blowing drop in on temperature the Ligurian and Sea a activate consequent intense increase self-regenerating in the heat thunderstormfluxes towards systems the atmosphere. that hit Magra valley, particularly on the Tuscan side (Figure8).

Figure 8. Visible RGB combination from Meteosat Second Generation satellite image on 10 Figure 8. Visible RGB combination from Meteosat Second Generation satellite image on 10 November 2012 November 2012 at 12 UTC. at 12 UTC.

4. Discussion and Conclusions While on land the event is of exceptional intensity and causes considerable damage, nothing of signiﬁcanceIn‐situ appearsmeasurements at sea: the of rainrainfall starts at in theopen late sea afternoon are really and lasts scarce, a few thus hours the with multiannual a peak less thanmeasurements 20 mmh− 1collectedat 17 UTC by and the aW1M3A cumulative observatory less than provide 53 mm. valuable The wind information and pressure to characterize data clearly showthe precipitation the propagation regime of over the low-pressure the Ligurian systemSea. that moves eastwards and the development of the convergence line, with the observatory lying in the sector under northwesterly winds. The incoming northwesterly wind produces a sharp drop in temperature and a consequent increase in the heat ﬂuxes towards the atmosphere. Water 2018, 10, 566 11 of 16

4. Discussion and Conclusions In-situ measurements of rainfall at open sea are really scarce, thus the multiannual measurements collected by the W1M3A observatory provide valuable information to characterize the precipitation regime over the Ligurian Sea. Results show that the number of rainy days at sea is greater and the rainfall intensity is lower than on land, although the average annual precipitation at sea is quantitatively similar to that recorded in the coastal stations of Genoa, La Spezia, Savona and Imperia, particularly to the one in Genoa. Despite the high inter-annual variability, the analysed data confirm that precipitation occurs mainly in the period from October to March and is absent during summer. The wet period may be roughly divided in two parts: precipitation from January to March is usually more persistent but often very weak, whereas the heaviest occurs more frequently in autumn. Extreme events mainly happen between October and November, and a single event, though short lived, can be enough to drastically increase both monthly and annual cumulative amounts. This distribution is in agreement with the seasonal behaviour of the air-sea heat fluxes at the mid latitudes—and in particular in the Mediterranean Sea—where in autumn the air temperature rapidly drops, favoured by North-Atlantic low systems, and the heat release from the sea surface to the atmosphere increases [4,35]. The time series of heat fluxes and ocean heat content confirm this trend with the maximum flux toward the atmosphere in winter months and the maximum heat content of the sea at the end of summer. Notwithstanding the similar annual trends of precipitation, heat fluxes and ocean heat content, a clear correlation at any temporal scale is not particularly evident. This is not surprising, since the source of moisture that triggers the convective processes may be also remote [3]. Both variables are key ingredients in the development of heavy precipitation that should be triggered by upper levels and larger scale features (e.g., an incoming tropopause dynamic anomaly, the presence of a jet-streak left exit), and/or by local effects (e.g., the formation of a convergence line at lower levels) [11,14]. Great local values of heat fluxes and water content play a crucial role over land, where orography can represent a trigger, such as during summer heat storms. The intensity of precipitation is also poorly linked to the lowest baric values recorded at sea. As an example, the lowest pressure value (980.8 hPa) of the entire data set occurs on 9 November 2010 with a daily cumulative less than 10.5 mm. Analogously, on 2 February 2013, the pressure drops to 986.5 hPa but the cumulative is only 3.5 mm (with snow occurring over land). On the other hand, on 23 December 2010 with a pressure minimum of 987 hPa, the rain exceeds 156 mmday−1 with a peak of 50.35 mmh−1. Rainfall peaks may also occur, with mean daily pressure over 1010 hPa. In any case, all extreme events from 2009–2013 occur simultaneously due to a remarkable drop in pressure values in the preceding 24/12 h. The analysis of the average of hourly rainy data shows that, at sea, the maxima of the cumulative occur in the early morning of the diurnal minimum of pressure, whereas on land they are found in the late afternoon. Yang and Slingo [36] found signals of propagation of the phase of the diurnal cycle of convection, suggesting that this could be due to land-sea breeze circulation. The diurnal cycles alter the flow of energy, momentum and water and are therefore fundamental in climate models [37]. Local sea surface temperature and rain intensity do not indicate a clear relationship at any temporal scale. On the contrary, Rebora et al. [7], by overlaying radar-derived accumulation rain maps to images of sea surface temperature anomalies for the two events that occurred in autumn 2011, suggest that both storms may have originated and developed in an area of positive sea surface temperature anomaly close to the position of the W1M3A observatory. In fact, during October and November 2011, the vertical profile of the sea temperature and salinity recorded at the W1M3A observatory (Figure9) provide evidence of a strong mesoscale dynamic linked to the Ligurian front displacement [38,39], confirming the presence of mesoscale structures. These can last for several days and can be detected by means of satellite sea surface temperature and altimetry [40,41]. Water 2018, 10, x FOR PEER REVIEW 12 of 17 Water 2018, 10, 566 12 of 16 Water 2018, 10, x FOR PEER REVIEW 12 of 17

Figure 9. (a) Time series of sea temperature between 21 October and 10 November 2011 acquired by the W1M3A observatory; (b) Time series of salinity between 21 October and 10 November 2011 Figure 9. (a) Time series of sea temperature between 21 October and 10 November 2011 acquired by Figureacquired 9. (bya) Timethe W1M3A series of observatory. sea temperature Dash between‐dotted 21lines October mark the and time 10 November of the maximum 2011 acquired precipitation by the the W1M3A observatory; (b) Time series of salinity between 21 October and 10 November 2011 W1M3Aon land. observatory; (b) Time series of salinity between 21 October and 10 November 2011 acquired acquired by the W1M3A observatory. Dash‐dotted lines mark the time of the maximum precipitation by the W1M3A observatory. Dash-dotted lines mark the time of the maximum precipitation on land. onIt shouldland. be also noted that the air temperature during the event on 4 November is greater than during the event on 25 October, probably because of the south‐easterly wind, as recorded at the It should be also noted that the air temperature during the event on 4 November is greater than W1M3AIt should observatory. be also noted Consequently, that the air although temperature the temperature during the eventof the onsea 4 is November still elevated, is greater the thermal than during the event on 25 October, probably because of the south-easterly wind, as recorded at the W1M3A duringgradient the between event on the 25 seaOctober, and atmosphere probably because is lower of (Figurethe south 10a)‐easterly and thewind, heat as flowrecorded toward at thethe observatory. Consequently, although the temperature of the sea is still elevated, the thermal gradient W1M3Aatmosphere observatory. is smaller Consequently, (Figure 10b). Thisalthough suggests the temperature that the lower of thecorrelation sea is still evidenced elevated, by the Rebora thermal et between the sea and atmosphere is lower (Figure 10a) and the heat ﬂow toward the atmosphere is gradiental. [7] between between thethe radarsea and‐derived atmosphere accumulation is lower rain (Figure map 10a)and andthe theimage heat of flow the towardsea surface the smaller (Figure 10b). This suggests that the lower correlation evidenced by Rebora et al. [7] between atmospheretemperature is anomaly smaller (Figurefor the second 10b). This event suggests could be that also the due lower to the correlation minor heat evidenced release from by Rebora the sea et to the radar-derived accumulation rain map and the image of the sea surface temperature anomaly for al.the [7] atmosphere. between the radar‐derived accumulation rain map and the image of the sea surface temperaturethe second event anomaly could for be the also second due to event the minor could heatbe also release due to from the the minor sea heat to the release atmosphere. from the sea to the atmosphere.

Figure 10. (a) Time series of sea temperature and air temperature between 21 October and 10 Figure 10. (a) Time series of sea temperature and air temperature between 21 October and 10 November November 2011 acquired by the W1M3A observatory; (b) Time series of heat fluxes between 21 2011 acquired by the W1M3A observatory; (b) Time series of heat ﬂuxes between 21 October and FigureOctober 10. and (a) 10Time November series of 2011 sea temperaturecomputed using and airthe temperaturein‐situ data betweenof the W1M3A 21 October observatory. and 10 10 November 2011 computed using the in-situ data of the W1M3A observatory. Dash-dotted lines NovemberDash‐dotted 2011 lines acquired mark the by time the ofW1M3A the maximum observatory; precipitation (b) Time on seriesland. of heat fluxes between 21 mark the time of the maximum precipitation on land. October and 10 November 2011 computed using the in‐situ data of the W1M3A observatory. Dash‐dotted lines mark the time of the maximum precipitation on land.

Water 2018, 10, 566 13 of 16 Water 2018, 10, x FOR PEER REVIEW 13 of 17

The greatest amount of precipitation is recorded when the wind turns to north north-north-east‐north‐east at sea: taking into account the position of the W1M3A observatory, thesethese are winds blowing from thethe landland and are more prevalentprevalent fromfrom OctoberOctober toto AprilApril (Figure(Figure 1111).).

Figure 11. (a) Monthly occurrences of hourly wind direction; (b) Monthly cumulative at sea Figure 11. (a) Monthly occurrences of hourly wind direction; (b) Monthly cumulative at sea according toaccording wind direction. to wind direction.

These winds carry cold air masses that reach the warm sea surface and increase the instability of These winds carry cold air masses that reach the warm sea surface and increase the instability of the marine atmospheric boundary layer. Over the land, the greatest cumulative amounts are the marine atmospheric boundary layer. Over the land, the greatest cumulative amounts are measured measured when the wind at the observatory is blowing from south‐south‐west (Figure 12a,b,d). This when the wind at the observatory is blowing from south-south-west (Figure 12a,b,d). This well well indicates the role of the surrounding orography: the Alps and the Apennines close to the coast indicateson the one the hand role protect of the surrounding the coastline orography:from the perturbations the Alps and coming the Apennines from North close‐Western to the coastEurope, on theon the one other, hand support protect the the coastline lifting of from the unstable the perturbations wet and warm coming low from level North-Western air masses coming Europe, from on the other,sea and support leading the to liftingconvective of the precipitations. unstable wet In and La warm Spezia, low the level distribution air masses is less coming clear, from and theintense sea andepisodes leading can totake convective place also precipitations. when a north‐westerly In La Spezia, wind is the blowing distribution at the isobservatory less clear, location and intense due episodesto the channelling can take placeeffect also of the when valley a north-westerly of Magra river wind (Figure is blowing 12c). This at characterization the observatory is location also found due towhen the channellinganalysing the effect most of the intense valley ofhourly Magra rainfall river (Figure data 12andc). Thiscan characterizationbe partly explained is also by found the whenconvergence analysing between the most north intense‐westerly hourly and rainfall south data‐easterly and canflows, be partlywhich explained is the crucial by the factor convergence in many betweenintense events: north-westerly when the and convergence south-easterly line is flows, to the which east of is the the observatory, crucial factor north in many‐westerly intense winds events: are whenlikely thethere convergence and precipitation line is to is the more east probable of the observatory, in eastern north-westerly Liguria, as shown winds for are the likely event there on and 11 precipitationNovember 2012. is more probable in eastern Liguria, as shown for the event on 11 November 2012. Wind speed and direction and heat fluxesfluxes in thethe openopen seasea providedprovided byby thethe W1M3AW1M3A observingobserving system demonstrate that in-situin‐situ measurements are useful to better understand the evolution of convective systems with particular attention to convergence lines whose position could virtually trigger violent storms storms on on land. land. In In fact, fact, although although precipitation precipitation is one is one of the of the best best-known‐known terms terms of the of theMediterranean Mediterranean water water budget, budget, there there are are still still large large uncertainties uncertainties owing owing to to the the lack lack of offshore observational data data [42]. [42]. This This limitation limitation forces forces us to us use to models use models or satellite or satellite-derived‐derived products products to obtain to obtainestimates estimates of precipitation of precipitation and its and variability its variability over over sea sea[43], [43 the], the validations validations of of which which are are mainly performed usingusing measurements measurements on on land. land. Hence, Hence, any any multiannual multiannual continuous continuous data-set data of‐ in-situset of rainfallin‐situ measurementsrainfall measurements at sea can at improve sea can model improve forecasting model offorecasting precipitation of andprecipitation reduce the and uncertainties reduce the of theuncertainties water cycle of over the thewater oceans. cycle This over is particularlythe oceans. trueThis for is theparticularly Ligurian basin,true for which the isLigurian characterized basin, bywhich a particular is characterized orography by anda particular is often theorography site of HPEs and is producing often the floods site of on HPEs the coast.producing floods on the coast.

Water 2018, 10, 566 14 of 16 Water 2018, 10, x FOR PEER REVIEW 14 of 17

Figure 12. (a) Cumulative amounts at Genoa according to wind direction at sea; (b) Cumulative Figure 12. (a) Cumulative amounts at Genoa according to wind direction at sea; (b) Cumulative amounts at Imperia according to wind direction at sea; (c) Cumulative amounts at La Spezia amounts at Imperia according to wind direction at sea; (c) Cumulative amounts at La Spezia according according to wind direction at sea; (d) Cumulative amounts at Savona according to wind direction at to wind direction at sea; (d) Cumulative amounts at Savona according to wind direction at sea. sea.

Author Contributions: S.P. and R.B. were the responsible of the data collection at sea. S.P., R.B. and M.E.S. Author Contributions: S.P. and R.B. were the responsible of the data collection at sea. S.P., R.B. and M.E.S. performedperformed the analysis.analysis. M.T. M.T. and P.P. contributedcontributed toto thethe analysisanalysis ofof thethe meteorologicalmeteorological andand oceanographicoceanographic conditionsconditions duringduring thethe selectedselected highhigh precipitationprecipitation events.events. AllAll authorsauthors contributedcontributed equallyequally toto writingwriting thethe paper.paper. Funding:Funding: ThisThis research research was was partially partially funded funded by the Europeanby the European Union Seventh Union Framework Seventh ProgrammeFramework FP7/2013-2017 Programme grantFP7/2013 number‐2017 312463grant number and by 312463 the Italian and Ministryby the Italian of Education, Ministry of University Education, and University Research and through Research the Flagshipthrough Project RITMARE. the Flagship Project RITMARE. Conﬂicts of Interest: The authors declare no conﬂict of interest. Conflicts of Interest: The authors declare no conflict of interest. Appendix A Appendix A The data used for the analysis are freely available from the portal of the Copernicus Marine The data used for the analysis are freely available from the portal of the Copernicus Marine Environment Monitoring Service (http://marine.copernicus.eu) and from the web site of the Regional Environment Monitoring Service (http://marine.copernicus.eu) and from the web site of the Environmental Protection Agency of Liguria (https://www.arpal.gov.it). The CLINO 1961–1990 Regional Environmental Protection Agency of Liguria (https://www.arpal.gov.it). The CLINO dataset is available from the Italian Air Force Meteorological Service (https://www.meteoam.it). 1961–1990 dataset is available from the Italian Air Force Meteorological Service Satellite images are available from the European Organisation for the Exploitation of Meteorological (https://www.meteoam.it). Satellite images are available from the European Organisation for the Satellites (https://www.www.eumetsat.int). Exploitation of Meteorological Satellites (https://www. www.eumetsat.int). References References 1. Nuissier, O.; Ducrocq, V.; Ricard, D.; Lebeaupin, C.; Anquetin, S. A numerical study of three catastrophic 1. Nuissier, O.; Ducrocq, V.; Ricard, D.; Lebeaupin, C.; Anquetin, S. A numerical study of three catastrophic precipitating events over southern France. I: Numerical framework and synoptic ingredients. Q. J. R. precipitating events over southern France. I: Numerical framework and synoptic ingredients. Q. J. R. Meteorol. Soc. 2008, 134, 111–130. [CrossRef] Meteorol. Soc. 2008, 134, 111–130, doi:10.1002/qj.200. 2. Ducrocq, V.; Braud, I.; Davolio, S.; Ferretti, R.; Flamant, C.; Jansa, A.; Kalthoff, N.; Richard, E.; Taupier-Letage, I.; 2. Ducrocq, V.; Braud, I.; Davolio, S.; Ferretti, R.; Flamant, C.; Jansa, A.; Kalthoff, N.; Richard, E.; Ayral, P.A.; et al. The field campaign dedicated to heavy precipitation and flash flooding in the Northwestern Taupier‐Letage, I.; Ayral, P.A.; et al. The field campaign dedicated to heavy precipitation and flash Mediterranean. Bull. Am. Meteorol. Soc. 2014, 95, 1083–1100. [CrossRef] flooding in the Northwestern Mediterranean. Bull. Am. Meteorol. Soc. 2014, 95, 1083–1100, doi:10.1175/BAMSD‐12‐00244.1. 3. Duffourg, F.; Ducrocq, V. Origin of the moisture feeding the Heavy Precipitating Systems over Southeastern France. Nat. Hazards Earth Syst. Sci. 2011, 11, 1163–1178, doi:10.5194/nhess‐11‐1163‐2011.

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