A flash-flood event in the western part of Croatia during the night between 24 and 25 September 2010 is analyzed by means of satellite, radar, lightning and rain-gauge measurements and NWP model data. A relatively rare positioning of two Mediterranean cyclones induced a strong convergence zone in the Northern Adriatic. One of the heavy precipitation episodes stroke the town of Pula and the surrounding Istrian inland resulting in 176 mm of rain in less than 7 hours, the highest daily amount ever measured in Pula. Operational 8 km resolution hydrostatic ALADIN model forecast satisfyingly the existence of twin cyclones and precipitation maximum over the sea. The processes that led to the development of the secondary maximum inland were explored using the 2 km resolution non-hydrostatic ALADIN model

D. Mazzocco Drvar

Plačko-Vršnak, D.

Trošić, T

Tudor, M.

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1. INTRODUCTIONDuring the night between 24 and 25 September2010 heavy rain caused flash-flooding of the townof Pula and the surrounding area. In Valbandon,Fažana and Veli vrh 200 mm of rain fell in lessthan 7 hours causing extreme communal difficul-ties. In these extreme event vineyards, olivegroves and roads were flooded, transport and elec-tricity supplies were interrupted. Currents of mud-dy water damaged houses and streets and peoplehad to be evacuated from their homes flooded byup to 1 m of water.This study will show that the flooding was the re-sult of a convective system which developed inthe convergence zone caused by a specific placingof two Mediterranean cyclones. Generally, theMediterranean region is recognized as one of themost cyclogenetic regions in the world (Campinset al., 2000; Alpert et al., 1990). According toHorvath et al. (2008), cyclones that cross theAdriatic are classified in 4 major types: Genoa cy-clones, Adriatic cyclones, mixed Adriatic-Genoaand non-Genoa cyclones that are further dividedinto two sub-types. The major-type cyclones canHrvatski meteoroloπki Ëasopis  Croatian Meteorological Journal, 47, 2012, 35∑43Izvorni znanstveni radOriginal scientific paperFLASH-FLOOD IN PULA IN THE NIGHT BETWEEN 24 AND 25 SEPTEMBER 2010Povodanj u Puli u noći s 24. na 25. rujna 2010. godineDUNJA MAZZOCCO DRVAR, DUNJA PLAČKO-VRŠNAK,MARTINA TUDOR i TANJA TROŠIĆDržavni hidrometeorološki zavodGrič 3, 10000 Zagreb, Hrvatskadunja.mazzocco.drvar@cirus.dhz.hrPrimljeno 24. travnja 2012., u konačnom obliku 6. veljače 2013.Abstract: A flash-flood event in the western part of Croatia during the night between 24 and 25 Sep-tember 2010 is analyzed by means of satellite, radar, lightning and rain-gauge measurements andNWP model data. A relatively rare positioning of two Mediterranean cyclones induced a strong con-vergence zone in the Northern Adriatic. One of the heavy precipitation episodes stroke the town of Pu-la and the surrounding Istrian inland resulting in 176 mm of rain in less than 7 hours, the highest dailyamount ever measured in Pula. Operational 8 km resolution hydrostatic ALADIN model forecast sat-isfyingly the existence of twin cyclones and precipitation maximum over the sea. The processes thatled to the development of the secondary maximum inland were explored using the 2 km resolutionnon-hydrostatic ALADIN model.Key words: flash flood, Istria, convective precipitation, convergence, Mediterranean cyclones, ALADINSažetak: Povodanj u zapadnom dijelu Hrvatske u noći s 24. na 25. rujna 2010. analiziran je u ovomradu pomoću satelitskih i radarskih podataka, podataka o električnom pražnjenju te kišomjera, kao ipomoću numeričkog modela za prognozu vremena. Razmjerno rijedak položaj dvije mediteranske ci-klone uzrokovao je pojavu izražene zone konvergencije u Sjevernom Jadranu. Jedna od epizoda sobilnom oborinom zahvatila je Pulu i njezino zaleđe, pri čemu je palo 176 mm kiše u manje od 7 sati,što je u Puli najveća ikad izmjerena dnevna količina oborine. Operativni hidrostatski ALADIN modelrezolucije 8 km zadovoljavajuće je prognozirao postojanje dvije ciklone, kao i maksimum oborine iz-nad mora. Procesi koji su rezultirali sekundarnim maksimumom oborine nad kopnom razmatrani sunehidrostatskim modelom ALADIN rezolucije 2 km.Ključne riječi: povodanj, Istra, konvektivna oborina, konvergencija, mediteranske ciklone, ALADINHMC-47.qxp_HMC-41  21/01/14  09:36  Page 35exist simultaneously belonging to the theoreticalcategory of twin or eyeglass cyclones, a phenome-non which appears and is documented worldwide.A case of cyclogenesis in the mid-Mediterranean,in the vicinity of the Alps, was simulated byMesinger and Strickler (1982) who called it alsoan »eyeglasses cyclogenesis« literally translatingthe local Italian expression »ciclogenesi ad oc-chiali«. Further investigations of the twin cy-clones show the orographic influence on the sepa-ration of centres in the Adriatic and the Tyrrhenianarea, which is usually evident at levels below 700hPa. (Brzović and Jurčec, 1997). Nevertheless, themost common twins are the simultaneous Genoaand Adriatic cyclones (Brzović, 1999), thereforeclassified in Horvath et al. (2008), as the 4th typeof Mediterranean cyclones. Depending on the sea-son and general synoptic conditions, the paths ofthe cyclones in the Mediterranean basin are differ-ent. On their way through this complex geogra-phy, they can produce a range of extreme weatherphenomena such as heavy orographic precipita-tion, thunderstorms, supercells and mesoscaleconvective systems (Ivančan-Picek et al., 2003).The local intensity of the precipitation is also verymuch dependent on the available moisture. Theheaviest rain events take place when the cyclonepath is in such position that it produces the con-vergence of moist air supplying a large quantity ofprecipitable water (Lionello et al., 2006). In this particular case the convergence line abovethe North Adriatic was the result of the interactionof the flows driven by the two cyclones placed inTyrrhenian and Ligurian Sea, respectively. Re-mote sensing data including satellite, radar andlightning observations give the opportunity tomonitor the relevant development. Finally, theALADIN/HR model results show how well a nu-merical model can capture the details of such anevent.2. SYNOPTIC CONDITIONSThe large-scale situation preceding the event wascharacterized with a deep upper-level troughstretching from western Scandinavia to southernFrance with the secondary trough located in the36 Hrvatski meteoroloπki Ëasopis  Croatian Meteorological Journal, 47, 2012Figure 1. DWD surface analysis for 25 September 2010, 00 UTCSlika 1. DWD prizemna analiza za 25. rujan 2010., 00 UTCHMC-47.qxp_HMC-41  21/01/14  09:36  Page 36Mid-Mediterranean. In the evening of 24 Sep-tember, a well developed cyclone moved east-wards over the Tyrrhenian Sea. Six hours later,around midnight of 25 September, the upper-level trough moved toward the east. At the rearside of the western trough, in the strong norther-ly flow, the cold air was penetrating over Franceinto the West Mediterranean. Due to large pres-sure gradients in the mid-levels at the front sideof the trough, southerly and south-westerlywinds over northern Italy and northern Adriaticwere rather strong. The area of northern Adriaticand Istrian peninsula was hence undergoing theadvection of warm and humid air while in thelarge area of Mid-Mediterranean cold advectionprevailed as the clear sign of the ongoing cyclo-genesis. The secondary trough over the CentralMediterranean did not move significantly main-taining a persistent southerly and south-easterlyflow. Associated with the two upper-level sys-tems, at the surface level two low-pressure cen-tres emerged, one in Tyrrhenian Sea and the oth-er in the Genoa Bay, as depicted in the surfaceanalysis chart (Fig. 1). In Figure 2, the Airmass RGB composite showsthe convective clouds affecting the large areaaround the fully developed southern cyclone at 18UTC on 24th September 2010. This particularsatellite composite, based upon data from infraredand water-vapour channels from Meteosat SecondGeneration, nicely resolves airmass characteris-tics in cloud-free areas and reveals cloud height incloudy areas. On the other hand, the reddish areaover the Northern Sea and France shows the intru-sion of dry stratospheric air with high potentialvorticity to the Alpine region where the cyclogen-esis was going on. Pronounced positive vorticityadvection (PVA) maxima both in 300 and 500 hPatriggered and maintained strong convection in theunstable air over the Western Alps and the Mid-Mediterranean. The PVA fields were derived fromthe ECMWF (Europen Centre for Medium-rangeWeather Forecast) forecast fields.37D. Mazzocco Drvar et al.: Flash-flood in Pula in the night from 24th to 25th September 2010Figure 2. METEOSAT 9 Airmass RGB image for 24 September 2010, 18 UTC overlaid with ECMWF forecast PVAat 500 hPa (orange), PVA at 300 hPa (red) and isotachs at 300 hPa (yellow)Slika 2. METEOSAT 9 Airmass RGB za 24. rujan 2010., 18 UTC zajedno sa prognostičkim poljem ECMWF - PVAna 500 hPa (narančasto), PVA na 300 hPa (crveno) i izotahe na 300 hPa (žuto)HMC-47.qxp_HMC-41  21/01/14  09:36  Page 373. MESOSCALE INVESTIGATIONIn the afternoon of the 24 September scatteredshowers in northern Adriatic basin started. Due topersisting warm and moist flow, showers intensi-fied and spread to the whole area. According tothe Lisca radar data, the only measurements avail-able over the sea, the hourly precipitation amountsexceeded 50 millimetres. Since there are no radarsalong the Adriatic coast these were the only dataavailable for the analysis of the development innorthern Adriatic. Radar echoes in Figure 3. showthat until 22:00 UTC most of the convective activ-ity took place over the sea.Since the low-level synoptic conditions werecharacterized by the south-easterly flow at thefront side of a cyclone moving eastwards throughthe Tyrrhenian Sea, the formation of the secondcyclone in gulf of Genoa caused the strong con-vergence along the western coast of Istrian penin-sula creating the favourable environment for en-hanced convection (Holton, 1992). The conver-gence line is present in low troposphere up to 850hPa and one at 925 hPa is depicted in Figure 4. During the night the entire system started movingeastwards and at 01:40 UTC convective activitydeveloped within the cloud system near the tip ofIstria. The intense convective system protrudedinto the Istrian land, continued its path along thetrajectory perpendicular to the convergence lineand in the next 6 hours caused severe damagesalong the way. Particularly, the highest 24-hourlyrainfall of 198.9 mm was measured in Vodnjan, avillage northeast of Pula, 170.2 mm in Juršići, 8kilometers further north and 162 mm in Gologori-ca, a settlement deep inland. In Pula, the largesttown of the region, the measured rain-gauge pre-cipitation reached 176 mm (Fig. 5). This is thehighest daily amount on record. Moreover, 120mm of that total amount was recorded in only 3hours, between 01:00 and 04:00 UTC.Besides the strong large-scale horizontal conver-gence, a favourable environmental vertical windshear, within the flow veering from south-easterlyat the ground level to south-westerly at the top ofthe troposphere, sustained the convection. Thearea was therefore affected by the continuous ad-vection of moisture in the warm sector of the sys-tem. Total precipitable water reached valuesgreater than 30 mm over the coastal areas andgreater than 35 mm over the sea and also over thehorn of Istria giving the indication for the high38 Hrvatski meteoroloπki Ëasopis  Croatian Meteorological Journal, 47, 2012Figure 3. Lisca Radar vertically integrated liquid inmm for 24 September 2010 at 17:00 UTC (left), 22:00UTC (right) and 25 September 2010 01:40 UTC (botomleft).Slika 3. Oboriva voda u vertikalnom stupcu izmjerenaradarom na Lisci u mm za 24. rujan 2010. u 17:00 UTC(lijevo), 22:00 UTC (desno) i 25. rujan 2010. u 01:40UTC (dolje lijevo).HMC-47.qxp_HMC-41  21/01/14  09:36  Page 38precipitation amounts. Similar values of precip-itable water were measured in vertical soundingsin San Pietro Capofiume, Udine and Zadar sta-tions at 00 UTC on 25th September 2010.4. PRECIPITATION FORECASTINGA large-scale analysis of synoptic situation wasmade using data from Deutcher Wetterdienst andmodel data from the European Centre for Medium-Range Weather Forecasts (ECMWF), while themesoscale analysis was based on the ALADIN(Aire Limitèe Adaptation Dynamique developmèntInterNational) forecast model. The capability of theALADIN model to predict heavy precipitationevents over the eastern side of the Alps duringMAP (Mesoscale Alpine Program) was verified bycomparing the HRID (High Resolution IsentropicDiagnosis) vertical time cross-sections based onvertical-sounding measurements and the AL-ADIN/LACE prognostic pseudo-TEMPs (Ivančan-Picek et al., 2003). This study is concerned mainlywith the analysis of meteorological fields.Forecast of the event was based on the meso-scalehydrostatic ALADIN/HR model that runs at theMeteorological and Hydrological Service of Croa-tia, 2 times a day (starting from 00 and 12 UTCanalyses) with horizontal resolution of 8 km on 37levels in the vertical (Ivatek-Šahdan and Tudor,2004) up to 72 hours in advance. Model versionused for operational 8 km resolution forecast usesprognostic treatment of TKE and prognostic con-densates (cloud water and ice, rain and snow) butdiagnostic convective precipitation parameteriza-tion (Geleyn et al, 1994). The analysis shown hereis based on the model run initialized with the 00UTC analyses on 24 September 2010.Surface pressure forecast (Fig. 5) was in goodagreement with the analyses in terms of positionand intensity of the two lows (Fig. 1). Thus thefrontal systems and the large-scale convergencewest of Istria were predicted correctly. The modelpredicted heavy rain in the area of northern Adri-atic both stratiform and convective (Fig. 6). The39D. Mazzocco Drvar et al.: Flash-flood in Pula in the night from 24th to 25th September 2010Figure 4. Horizontal wind at 925 hPa ALADIN/HR model analysis in 8 km resolution for 00 UTC on 25 September 2010Slika 4. Horizontalni vjetar na 925 hPa plohi, analiza ALADIN/HR modela na 8 km rezoluciji za 25. rujan 2010. u00 UTCHMC-47.qxp_HMC-41  21/01/14  09:36  Page 3940 Hrvatski meteoroloπki Ëasopis  Croatian Meteorological Journal, 47, 2012Figure 5. 24 hours rain-gauge precipitation accumulation measured on 25 September 2010 in Istrian areaSlika 5. 24-satna akumulirana oborina izmjerena na kišomjernim postajama 25. rujna 2010 na području IstreFigure 6. Forecast 3-hourly accumulated resolved (left) and convective (right) precipitation forecast by ALADIN/HRmodel valid at 03 UTC on 25 September 2010.Slika 6. Prognostička 3-satna akumulirana razlučena (lijevo) i konvektivna (desno) oborina iz modela ALADIN/HR u03 UTC za 25. rujan 2010.HMC-47.qxp_HMC-41  21/01/14  09:36  Page 4041D. Mazzocco Drvar et al.: Flash-flood in Pula in the night from 24th to 25th September 2010Figure 7. Hourly accumulated resolved precipitation (left) and convective precipitation (right) forecast by non-hydro-static ALADIN/HR model in 2 km resolution  valid at 02 UTC (upper), 03 UTC (middle) and 04 UTC (lower) on 25September 2010.Slika 7. Prognostička satna akumulirana razlučena oborina (lijevo) i konvektivna oborina (desno) dobivena nehidro-statskom verzijom modela ALADIN/HR na 2 km rezoluciji za 25. rujan 2010. u 02 UTC (gornji red), 03 UTC (srednjired) i 04 UTC (donji red).HMC-47.qxp_HMC-41  21/01/14  09:36  Page 41maximum over the sea was forecast correctly, butwith the delay of 3 hours.Nevertheless, there was no indication of the sec-ondary precipitation maximum over Istria, whichcaused the flash-flood. The processes that lead tothe development of the secondary rainfall maxi-mum were further explored using the 2 km resolu-tion non-hydrostatic ALADIN model. This modelversion employs prognostic deep convection pa-rametrization that enables memory, advection anddiffusion of prognostic variables that describe theconvection processes: updraft and downdraft ver-tical velocities and mesh fractions (Gerard, 2007).The model set-up is described in detail in Tudorand Ivatek-Šahdan (2010). It can be noted that the precipitation is mostlyconvective, especially the inland maximum (Fig-ure 7). Maximum over the sea corresponds nicelyand is advected towards the east with the conver-gence line.5. CONCLUSIONA relatively rare positioning of the two Mediter-ranean cyclones induced a strong convergencezone in northern Adriatic. Intense convective ac-tivity developed during the night between 24 and25 September. 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FLASH-FLOOD IN PULA IN THE NIGHT BETWEEN 24 AND 25 SEPTEMBER 2010

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