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Synoptic and regional meteorological ingredients which induced severe flash floods in Romania
 

Synoptic and regional meteorological ingredients which induced severe flash floods in Romania

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Synoptic and regional meteorological ingredients which induced severe flash floods in Romania

Synoptic and regional meteorological ingredients which induced severe flash floods in Romania

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    Synoptic and regional meteorological ingredients which induced severe flash floods in Romania Synoptic and regional meteorological ingredients which induced severe flash floods in Romania Presentation Transcript

    • Synoptic and regional meteorological ingredients which induced severe flash floods in Romania G. Stancalie, B. Antonescu, , S. Cheval, A. Irimescu and A. Dumitrescu National Meteorological Administration Bucharest, Romania International Disaster and Risk Conference IDRC Davos 2010
    • Contents
      • Introduction
      • Climate characteristics of Romania
      • Understanding the meteorological processes governing flash-flood driven storms
        • The ingredients-based methodology
      • Representative flash flood events in Romania
      • Results
        • Large scale atmospheric patterns associated with the flash flood events in Romania
        • Common factors that yield to cause extreme rainfall events in Romania
        • Factors generating flash flood and /or worsening flooding conditions
      • Conclusions
    • Introduction
      • Flash floods : extreme flood events induced by severe stationary storms.
      • The duration and spatial extension of the area affected by such floods depend on the causative storm and hence on the climatic setting. Most generally, the storms inducing flash floods lead to great local rainfall accumulations over a few hours and affecting limited areas (tens to hundreds of square kilometers).
      • Flash floods are one of the most significant natural hazards and cause serious loss of life and economic damage.
      • In Romania , as well as in many Central and Eastern European countries, floods and flash floods represent the most severe hazard , causing important damages and causalities.
      • This work has been carried out in the framework of the HYDRATE project, which is a currently ongoing STREP-EC funded project that is aiming to improve techniques for flash flood forecasting.
      Torrential rain is key to the onset of flash flooding but the drainage and topography of the surrounding area determines the scale and impact of the event (e.g. steep-sided valleys in the Carpathian mountains, accentuated flooding by acting as funnels for the run-off). FF on the Grinties River Basin – 4/08/2007
    • Climate characteristics of Romania
      • Romania has a continental climate with a non-uniform distribution of precipitation during the year, in both time and space.
      • An important characteristic of the Romanian climate is the wide variability of the meteorological elements and phenomena , due to the influence exerted by the pressure centers governing the weather-state.
      • The distribution in the territory of the main weather elements and phenomena is mostly imposed by the peculiarities of the relief .
      Multiannual precipitation mean (1961–2005)
      • The Romanian topography is dominated by the S-shape of the Carpathian Mountains.
      • The fingerprint of this pattern on the low-level wind is the so-called Romanian Plain Convergence Zone that often forms in Southern Romania.
    • Precipitation regime of Romania Multiannual precipitation mean (1961–2005)
      • Romania receives, on the average, 640 mm of precipitation throughout the year. The plains in the south and west of the country sum-up between 550 and 650mm, and the hill and plateau areas – between 650 and 750mm, whereas the most abundant precipitation exceed 1000 mm, localized in the mountain area .
      • The lowest annual precipitation amounts are recorded in the Dobrogea littoral area and mostly in the east of the Danube Delta - less than 400 mm on the average.
      • Precipitation is characterized through large time and space variability . The multiannual mean display an annual cycle, with a maximum in May-June (mostly with a torrential aspect) and a minimum in February–March , almost all over the territory.
      • The Mediterranean influence materializes through the occurrence in the south- east and south-west of the country of a secondary minimum in October .
      Multiannual mean of maximum precipitation in 24 hours (1961 – 2005)
    • River flows regime in Romania
      • Romania has a dense hydro-graphical network (0.5 km/km 2 ).
      • The stream flow depends on river source, local conditions of its hydrographical basin and climatic factors.
      • The river stream flow shows important seasonal variations (Diaconu & Serban, 1994, Stanescu et al., 1998)
      • The seasonal peak flow of rivers in Romania, like most of European rivers at mid-latitude (Dettinger and Diaz, 2000), are observed during spring , representing on average 40-50% of the annual river flow. In the summer months the flow is between 15 and 20% for hill and plain zones.
    • The Romanian weather and rain gauge stations network
    • The Romanian radar network
      • Doppler radars generate an impressive suite of more than 70 products, including both base and derived products.
      • Available products for flash flood issues:
        • Base products: R (Reflectivity), V (Velocity), W (spectrum Width);
        • Elevations:=0.5, 1.5, 2.4, 3.4, etc;
        • Reflectivity range: 5 to >75 dBZ;
        • Approx. 6 minutes between scans;
        • Polar products: 1 degree, 1 km;
      • The Romanian Meteorological and Hydrological Forecasting Services are during a transition period, towards a modern integrated systems :
      • The SIMIN (National Integrated Meteorological System) and DESWAT (DEStructive WATers Abatement and Control of Water Disasters), based on 9 Doppler radars ( 5 new WSR-98D S-band, based on the technology developed in the US NEXRAD network and 4 Doppler radars in C band -EEC and Gematronik), 8 lightning detectors , meteorological and hydrological automatic stations, meteorological and hydrological models, warning dissemination systems.
      • Despite the fact that meteorological and hydrological services are managed by two different institutions, the cooperation and integration of data and methods are at the base of the SIMIN-DESWAT systems .
      • Romania has fully integrated three types of radar equipment into one integrated network (SIMIN).
      • Three types of national radar mosaics are produced every 10 minutes: first tilt base reflectivity, echo top and composite reflectivity.
      • The radar systems met the criteria imposed by the EUMETNET and OPERA programs for harmonizing and improving the exchange of the data from operational weather radars in Europe.
    • Understanding the meteorological processes governing flash-flood driven storms
      • The sample of flash flood events for Romania region (continental climate), provided the conditioning data to identify the generating synoptic fields over the Atlantic and Europe:
        • vertically-integrated moisture flux or
        • relative humidity at 925 hPa ;
        • low-level winds ;
        • sea-level pressure, at 700 hPa height.
      • Based on these samples, conditional mean fields or composite maps were estimated.
      • The selected synoptic regimes were analysed to identify interactions between low-level and upper-level flow fields, local topographic barriers and regional moisture and energy supply in producing heavy precipitation s leading to flash floods.
    • Understanding the meteorological processes governing flash-flood driven storms (cont.)
      • The different synoptic regimes were extracted from the "HYDRATE ” weather classifications (Anquetin et al., 2010, in preparation) ;
      • The 7 «HYDRATE» weather classes is a combination of the 30 classes of the Grosswetterlagen system (GWL), [Baur et al. (1944), Hess and Brezowsky (1952, 1969,1977), Gerstengabe et al. (1999)] used by the German Weather Service ;
      • These weather regimes can be viewed as readily identifiable large-scale circulation patterns involving the whole of Europe and the North-East Atlantic , with their primary focus on central Europe.
    • The ingredients-based methodology (Doswell et al., 1996)
      • Analysis of the synoptic and mesoscale conditions prevailing to heavy local rainfall leading to flash flood, for the selected cases in Romania.
      • The sample of flash flood events for Romania region
      • The ECMWF model - to analysis to investigate synoptic forcing for the selected severe flash flood cases that occurred in Romania.
      • D ata from Doppler radar and satellite imagery were used to investigate the structure and the evolution of convective system that produced flash floods.
      • The ALADIN limited area model - to investigate the mechanism responsible for the deep convection development, taking into account the presence of the Carpathian Mountains and Black Sea that induce different mesoscale circulations in Romania.
      • The sample of flash flood events for Romania region
    • Representative flash flood events in Romania
      • More than 150 flash flood events have been observed in the last 30 years in Romania;
      • 30 representative FF (1973-2009) have been selected and analyzed.
      • The selected f lash flood situations represent a large spectrum of mesoscale possibilities -from heavy precipitation supercells to mesoscale convective systems.
      S patial distribution of the 30 representative FF (1973-2009)
    • Examples of selected flash flood cases Casimncea 28 August 2004 Distribution of pp (mm) 28/08/2004 - 22:30 UTC 29/08/2004 - 17:30 UTC Dofteana 28 July 2004 Distribution of pp (mm) in 5 hours on 28 July 2004 Distribution of pp amounts (mm) 03:30 - 12:30 UTC 7 May 2005 07 May 2005 – Nehoiu basin Distribution of pp (mm) for 11 hours between 2 and 3 July 2005 03 July 2005 - Cartalu and Topol og basins
    • Examples of selected flash flood cases 23 August 2005 - Sarat and Feernic basins Distribution of pp (mm) for 5 hours on 23 August 2005 20 August 2006 – Ilisua Basin D istribution of pp (mm) for 2 hours on 20 June 2006 Distribution of pp (mm) in 6 hours in 4 August 2007 Distribution of pp 05/09/2007 05:30 – 06/09/2007 05:30 05.09.2007 Tecucel basin
    • Case study: F lash flood in Tecucel basin - 05.09.2007
      • Tecucel River
      • Tributary of Barlad River , the only importing affluent of the Siret River on the left side, from Moldavian central plateau.
      • The basin is located on the southern slopes of Covurlui Plateau, at nearby 200 m altitude .
      • It flows towards south and has junction with Barlad River after 28 km, in the Tecuci town , at an altitude of 39 m.
    • Flash flood in Tecucel basin - 05.09.2007
      • Climate characteristics
      • Climate of the basins area is characterized as temperate continental pattern;
      • Monthly temperature means from - 3.5 o C (January) to +20 o C (July);
      • Annual temperature mean 9.0 o C and with annual mean precipitation: 400 –500 mm.
      • Hydrological characteristics
      • The runoff characteristics in Tecucel hydrometric station are:
        • mean discharge (1968-2006): 0.224 m3/s
        • specific runoff: 2.0 l.s/km2
        • minimum discharge: 0.011 m3/s (06.08.1987)
        • maximum discharge: 121 m3/s (5.09.2007). Until this flood, the maximum discharge was recorded in 1996: 79.9m3/s
        • maximum possible discharge:
          •  0.1% - 205 m3/s;  0.5% - 145 m3/s;
          •  1% - 120 m3/s;  5% - 65 m3/s;
          •  10% - 45 m3/s
      Tecucel basin
    • Flash flood in Tecucel basin - 05.09.2007 Synoptic conditions
      • The analysis of 500 hPa from GFS model showed a long wave through and cut-off structure with pressure values of 552 hPa, situated in the Western part of Romania. At this level the flow was mainly from the south over Romania.
      • At lower levels, the central and Eastern part of Romania was characterized by the presence of warm, moist air and in the Western part cold air was advancing toward East.
      Geopotential and temperature at 500 hPa (a) and equivalent potential temperature and sea level pressure at 850 hPa (b) from the GFS model. a b
    • Flash flood in Tecucel basin - 05.09.2007 Synoptic conditions Sea level pressure and 10 m wind at 06 UTC from the analysis of ALADIN model (a) and from the corresponding analysis of DWD (b). The blue arrow show the location of the flash-flood event. The frontal analysis shows at 06 UTC a warm front over the central and western part of Romania, and a cold front advancing rapidly from the south-west . The analysis from ALADIN model the 850 hPa shows the existence of an area with high values of relative humidity in the south-east Romania. The low level jet from the south brings the humidity necessary for maintaining the convection. a b Relative humidity and wind direction at 850 hPa at 09 UTC from the analysis of ALADIN model
    • Flash flood in Tecucel basin - 05.09.2007 Storm scale conditions
      • The cloud cover over the western part of Romania associated with warm front can be observed in the METEOSAT visible image at 06 UTC.
      • Can be also observed in the eastern part of Romania the first convective developments associated with the convergence line.
    • Flash flood in Tecucel basin - 05.09.2007 Storm scale conditions 200 mm in 6h - estimated from radar data 235 mm in 6h - measured by rain gauges. 3 hours accumulated precipitation estimation (08:02-11:02 UTC) Radar loops Reflectivity and radial velocity at 0.5° elevation angle from the WSR-98D radar at 5 September 2007, 06:59 UTC (upper part) and 10:30 UTC (down)
    • Flash flood in Tecucel basin - 05.09.2007
      • Tecuci hydrometric station:
      • Max. discharge = 121m 3 /s
      • H = 604 cm (121 cm over the danger level)
      Rainfall and discharge data Distribution of precipitation amounts registered between 05/09/2007 05:30 – 06/09/2007 05:30
    • Flash flood in Tecucel basin - 05.09.2007 Damages
      • 3 casualties;
      • more the 1000 people were evacuated;
      • 200 houses and many streets flooded;
      • 225 ha agricultural land flooded.
    • Results
      • Flash-flood situations represent a spectrum of synoptic and mesoscale possibilities.
      • The main synoptic regimes associated with the flash flood events in Romania, according to the "HYDRATE ” weather classifications :
        • #1. Zonal West
        • #2. Mixed
        • #5. Meridional Northeast and East
        • #6. Meridional Southeast and South
      • Heavy precipitation events in the Romanian region typically occur downstream of a significant cyclone aloft, often exhibiting “cut-off” cyclone characteristics .
      • Mediterranean and Black Sea proximity provide a source of moisture for Romanian territory and the shape of the Carpathian Mountains provides local circulations that help the forcing.
    • Large scale atmospheric patterns associated with the flash flood events in Romania Composite map with wind at 700 hPa (m s-1) and vertical velocity at 700 hPa (.10-2 Pa s-1) for the flash-flood event in class 1 and class 2, according to HYDRATE classification. Class 1 and 2 . In the western part of Romania, due to the opening to the west of the Carpathian Mountains and also to the high altitudes, the precipitation accumulations increase over the western and northern slopes. Class 1 composite maps - characterized by a westerly flow at 700 hPa , Class 2 – a north-westerly flow at 700 hPa.
    • Large scale atmospheric patterns associated with the flash flood events in Romania (cont.) Class 1 and 2 . The wind at 925 hPa and relative humidity at the same level, indicate that for class 1 and 2 , moist air is advected toward eastern part of Carpathian Mountains. Thus, the flash-flood cases from these two classes are manly generated by orography . Composite map with wind at 925 hPa (m s-1) and relative humidity at 925 hPa (•10-2 Pa s-1) for the flash-flood event in class 2 according to HYDRATE classification.
    • Large scale atmospheric patterns associated with the flash flood events in Romania (cont.) Class 5 is characterized by instability over entire Romania , with a low pressure system located over Black Sea , and a high pressure system over Baltic Sea . Also, in the Western region of Romania there is a influence region of the easterly flow at 700 hPa , with a relative humidly between 70-80%. The precipitations in this case are observed over the northern-part of Romania . Composite map with wind at 700 hPa (m s-1) and vertical velocity at 700 hPa (•10-2 Pa s-1) for the flash-flood event in class 5 according to HYDRATE classification.
    • Large scale atmospheric patterns associated with the flash flood events in Romania (cont.) Class 6 is characterized by a high pressure system over Mediterranean Sea and low pressure over the western and central Europe , with a south-westerly flow at 700 hPa over the north-western Romania , and positive values for the vertical velocity at 700 hPa over the extern north-western Romania. The area of precipitation is concentrated over north-western Romania , where the relative humidy is between 60-70% (right). Composite map with wind at 925 hPa (m s-1) and relative humidity at 925 hPa (•10-2 Pa s-1) for the flash-flood event in class 6 according to HYDRATE classification.
    • Common factors that yield to cause extreme rainfall events in Romania
      • The presence of southerly strong moisture flow at low levels associated with surface boundary aligned roughly parallel to the mean flow . Such an alignment increases the chances of cell training along the boundary;
      • Deep convective cells are organized such that they move repeatedly over a given area, a process commonly called "echo training".
      • The development of the rotational structure that yields to grout of duration of rainfall events.
      • Radar depiction of flash-flood producing mesoscale convective systems has shown a linear organization in many cases.
      • Size, organization and motion characteristics of the mesoscale convective systems are the principal factors that determine heavy rainfall.
      • The convection can interact with its environment to develop new convection in preferred locations relative to the existing cells.
      • In addition to mesoscale convective systems, high precipitation supercells have significant flash flood potential.
    • Factors generating flash flood and/or worsening flooding conditions
      • From the hydrological point of view, factors that have a major influence on the occurrence of FF (apart from the intensity and duration of the rainfall) are:
        • the topography; the soil conditions; the coverage of the terrain.
      • Topography is a key factor for the genesis and evolution of the quasi-stationary convective systems (or in simplified terms “slow moving rainfall cells that release large amounts of water over a relatively small area”) that are often the cause of flash floods.
      • Disadvantageous topographical conditions such as high-exposure (steeply sloping) highland terrains, narrow valleys or ravines hasten the runoff and increase the likelihood of flash flood occurrence.
      • Saturated soil or shallow watertight geological layers increase surface runoff.
      Wood deposits at the confluence (Uleasa river - Izvorului river, 2006)
    • Factors generating flash flood and /or worsening flooding conditions (cont.)
      • Terrain coverage can have a similar effect. Urbanization processes and affiliated constructions are able to make runoff 2 to 6 times greater in comparison to terrains with natural coverage (fields, meadows, forests).
      • Even if geographical features are considered to change little over time, it should be remembered that changes in basin - area management (e.g. deforestation, sealed constructions, agro technical operations) can have a vital impact on the rainfall-runoff transformation, or on the stability of slopes.
      • Consequence : water accumulation and flooding u p stream and downstream the small bridges, huge deposits of sediments and wood offals from forests exploitation at rivers confluences .
      Solid deposits (Trotus river, 2004)
    • Conclusions
      • The Romanian flash floods are due to:
        • Massive and sudden rainstorms combined or not with
        • A rapid snowmelt in the mountains;
        • Failure of natural or man-made water defenses
      • Flash-flood situations represent a spectrum of synoptic and mesoscale possibilities. The main synoptic regimes associated with the flash flood events in Romania, according to the "HYDRATE” weather classifications:
        • #1. Zonal West
        • #2. Mixed
        • #5. Meridional Northeast and East
        • #6. Meridional Southeast and South
      • Heavy precipitation events in the Romanian region typically occur downstream of a significant cyclone aloft, often exhibiting “cut-off” cyclone characteristics .
      • Mediterranean and Black Sea proximity provide a source of moisture for Romanian territory and the S shape of the Carpathian Mountains provides local circulations that help the forcing.
    • This work has been carried out in the framework of the HYDRATE project which is a currently ongoing EC funded project (FP6-2006-STREP-037024) ACKNOWLEDGEMENT
    • Thank you for your kind attention !