Topics: Major floods Precipitation patterns Observational problem Flood assessment Trends? Example: Century flood of Rhine and Mosel in Dec 1993 and in Jan 1995 again. This photo was published on New York Times cover page. Meteorological and climatological aspects of river flood assessment in Central Europe Bruno Rudolf (et al.), Global Precipitation Climatology Center, Deutscher Wetterdienst, Offenbach am Main

Selected major floods in Central Europe during the past ten years: 1993 Sep 23/24 heavy flash floods in the southern Alps (Brigs!) and nothern Italy 1993 December Rhine and Mosel river flood, Germany 1995 January Rhine and Mosel river flood, Germany 1997 July Odra river flood, Poland and Germany 1999 May Danube river, Lake Constance, Austria, Germany 2000 Sep-Nov Persistent river floods in wide-spread regions of western and southern Europe 2001 July Vistula river flood, Poland 2002 August Danube river flood, Germany and Austria Elbe river flood, Czech Republic and Germany 2002 Sep 08/09 heavy flash floods, Southern France (Rhone) 2003 January floods of many tributaries of Rhine and Elbe

Century flood of Rhine and Mosel in December 1993 GPCC analysis of monthly precipitation based on data received via GTS The situation is similar to the one of January 1995, when a 2nd century flood of Rhine and Mosel occured. Spatial extent ~ 1000 km relevant period ~ 1 month rainfall and snow melt Precipitation Dec Precipitation December normal Anomaly (mm) Dec % of normal Dec. 1993

Wide-spread floodings in Europe, Autumn 2000 Map: Precipitation for the period Sep-Nov 2002 (dark green > 600mm) Diagrams: Daily precipitation as observed at selected stations (scale 0 – 125 mm/d)

Floods of Danube and Lake Constance in May 1999 Red bars: Daily precipitation at Zugspitze black lines: Snow depth at Zugspitze and Wendelstein Snow melt ( m msl) and a short-time extreme rainfall event

Odra Flood in July 1997 and Vistula flood in July 2001 Monthly precipitation totals in mm spatial extent ~ 300 km relevant period < 1 week Vb Wetterlage + orographical effects => intense rainfall spatial extent ~ 100 km relevant period < 1 week

The Elbe Flood, August 2002 Meteorological situation on 11 August, 500 HPa and surface

The Elbe Flood, August 2002 Storm track of the cyclone (location of occlusion

The Floods of Danube and Elbe, August 2002 Precipitation totals in mm for the two episodes August 2002 and August 2002 (6-6 UTC)

The Floods of Danube and Elbe, August 2002 Time series of monthly precipitation (mm) for August Highest observed values for Zinnwald-Georgenfeld (Erzgebirge): Monthly precipitation total = 470 mm, anomaly = 456 % Maximum daily precipitation = 312 mm, occurence = 1 per 150 years maximum 24h precipitation = 357 mm Maximum hourly precipitation = 30.5 mm

The Elbe Flood, August 2002 Hourly precipitation at Dresden and Zinnwald on 12 to 13 August 2002

The Elbe Flood, August meteorological causes: Meteorological factors leading to the extreme conditions: Convergence, large-scale strong and quasi-stationary lifting Very high atmospheric liquid water content Embedded convective cells Additional orographically forced lifting Saturated deep soil and already high river levels Fractions of the different precipitation mechanisms: Stratiform precipitation: 2 parts Convective precipitation: 1 part Orographic amplification: duplication of the quantities

Precipitation accumulated for the period 10 Aug 06 UTC - 13 Aug 06 UTC GME forecast LM forecast Observation The Elbe Flood, August 2002

Extreme Flash Flood in the lower Rhone area, 8 and 9 Sept The largest reported daily precipitation is 650 mm observed on 8 Sept. at Anduze. Other precipitation reports of Meteo France:

Heavy Flash Floods in the Rhone delta, 8 and 9 Sep A first precipitation analysis of the GPCC for the period 7 Sep. 18 UTC - 10 Sep. 6 UTC was based on synoptical reports received from the GTS. The highest 54h total from GTS: mm The extreme values observed at national stations have not been available on GTS.

Maximum river levels of Elbe in Dresden since 1640 Most of the Elbe floods occured in winter or spring. Typical summer floods happened in 1890 and A statistical evaluation of the river levels is problematic: Extreme floods of a specific river are rare. The runoff is influenced by human inventions. The causes of extreme floods are different.

Types of floods related to meteorological conditions (in particular for Central Europe) Flash floods (mostly summer) Heavy thunderstorms, squall lines River floodings (summer, autumn) Regionally large rain quantities, synoptic regime (Vb), orography River floodings (winter, spring) Largescale rainfall and/or snow melt River floodings (winter) Frost: ice barriers blocking the river flow Continent wide floodings Persistent large scale cyclonic and humid synoptic regime, convergent stream flow

Conclusions NWP models mostly well simulate the meso-scale atmospheric structures. However, they do not forecast extreme precipitation quantities and the location of the affected region with sufficient accuracy. Thus, the models may fail in forecasting individual flash floods, but they can base conditional warnings on the probability of rainstorms for a certain region. Forecast of large river floods requires not only prediction of precipitation and snow melt but also of the river level, the progressing of the flood wave, based on monitoring of the river level, analysis of the observed precipitation and soil moisture conditions. Floods represent individual extrem events stamped by various interacting processes. Any interpretation of the frequency of flood events is problematic due to the different meteorological causes and human influence (river bed regulation, discharge management). Classification of precipitation episodes with respect of the areal coverage and duration is a suitable basis for statistical assessment of the frequency of relevant episodes and extreme events. => A suffient data base is required.

Additional material: Bruno Rudolf und Jörg Rapp (2003): Das Jahrhunderthochwasser der Elbe: Synoptische Wetterentwicklung und klimatologische Aspekte. In Klimastatusbericht 2002 des DWD, Offenbach, erscheint Juni Bruno Rudolf and Jörg Rapp (2003): The Century Flood of the River Elbe in August 2002: Synoptic Weather Development and Climatological Aspects. To be published by DWD in July Bruno Rudolf und Clemens Simmer (2002): Niederschlag, Starkregen und Hochwasser. In Wiss. Mitteilungen aus dem Inst. für Meteorologie der Univ. Leipzig, Sonderheft zum Jahr der Geowissenschaften, S