Deutscher Wetterdienst, Climate and Environment Division

On the role of the heavy precipitation event of 7-9 August 2010 for the subsequent flood situation of the rivers Neiße, Spree and Elbe in the Czech-Polish-German Euroregion

Andreas Becker, translated from (Bissolli et al., 2010)

1. Introduction and Motivation During the weekend of 7/8 August 2010 partly heavy precipitation occurred in the Czech-Polish- German Euroregion comprising the mountain ranges of Isergebirge, Northern Bohemia and South­ ern Saxony. The substantial amounts of rain water have quickly led to a wide-spread trespassing of the high water alert levels and flooding in the upper parts of the rivers Neiße, Spree and Elbe, caus­ ing catastrophic damages in the affected regions. The day before (6 August) it had already rained in Austria with comparative rates, and similar associated damages along Austrian rivers. This paper shall focus on the climate aspects with regard to cause, evolution and hydrological impact of the early August precipitation event. In doing so each rain gauge and radar based observation are put into the context of their related station specific climatology order to assess their contribution to the soon after developing flooding situation. The assessment revealed that for this event the distinction according to river catchment areas had been of particular relevance for the following reasons: a) The precipitation event featured a particularly high spatial-temporal variability and com­ plexity b) There have been additional catchment area specific events that substantially factored in towards higher severity of the according flooding situation. 2. Synoptic Development

2.1. Development Description The synoptic development showed some features of the later stages of a so-called ‘Vb- Development’ however the centre of the relevant low pressure system ‘Viola’ did not follow the clas­ sical Vb track according to Van Bebber until its arrival in eastern central Europe. In the following, at 7 August 2010, there was a baroclinic zone reaching from the Baltic sea, across Poland, Czechia until Austria, where sub-tropical air with dew points over 200 was advected from south to north at the eastern flank of the associated low pressure trough (Figrue 1), whereas Western Europe was gov­ erned by much drier and cooler air. With the development of a low across Polen in the influence area of a quasi-stationary upper low pressure system centred across the Northern Adriatic Sea and later across Hungary, there was a strong and fast advection of sub-tropical air in the upper troposphere (300 mb) in conjunction with large scale ascending similar to a warm conveor belt according to Browning (19xx). This as­ cendence of air masses with high moisture content (values of precipitable water ranging from 30-40 mm, locally exceeding even 45 mm) gave rise of a large scale precipitation pattern (Figure 2), that persistet for hours at almost the same location. Built in convection and mountain induced forced lift­ ing og air in the mid range mountains of Austria, Czechia and Saxony enhanced the precipitation rates significantly. There has been potentially an additional enhancing effect in the valley of the Elbe river South of Dresden nearby the Elbsandsteingebirge (mountain ranges close to the German-Czech border) given the later establishment of confluent low level northwestern winds in the rear of the low pres­ sure system, but its contribution cannot be quantified conclusively.

Anyway, at this stage the situation was similar to a fully developed classical Vb classified one, how­ ever its genesis was completely different to Van Bebbers description, as the low pressure system did not result from a Genua cyclogenesis but was born from a cold front belonging to a low pressure ______- 1 ­

Deutscher Wetterdienst, Climate and Environment Division

system across the British Islands reaching out eastward into Germany, where it slowed down, be­ came almost stationary and deepened in pressure due to the aforementioned frontogenetical proc­ esses. We believe that from a climatologically perspective the distinction of the genesis of this heavy precipitation event against a Vb development is important, so it should be rather attributed to a zonal situation.

The potentially enhancing contribution of the concurrent high pressure system on the eastern side, causing the well known heat wave across European Russia, and blocking a further zonal propaga­ tion of the low pressure system with it precipitation pattern would need further investigation as the central European low itself might also have sustained the heat high pressure system by means of upper level warm air advection along the front side of the trough contributing to the stabilisation of the high.

Figrue 1: Surface Level Pressure (Black contours), Temperature at 850 hPa level (colour shades) and surface level dew point (Numbers) at 07.08.2010, 12 UTC (Source: DWD, NinJo).

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Figure 2: IR-Channel (MSG, greyscale), PC (Precipitable Clouds, colour scale) and geopotential hight analysis for 500 hPa level (green contours) at 07.08.2010, 12 UTC (Source: SatRepOnline).

2.2. Excecutive Summary of Synoptic Situation As decribed before, there have been a number of partly related stimulating factors for the heavy precipitation encountered in the region of interest (ROI) • An upper level low pressure system as consequence of a cut-off process, • a persistent (quasi-stationty) situation lasting for several days • the inclusion of sub-tropical, high-humidity warm air into the low pressure system circulation • two air masses meeting along a pronounced border zone acting frontogenetic across the ROI • a blocking high across European Russia, • strong lifting in the area of the upper level low pressure system and its short-waved troughs at its margins, partly accompanied by advection of positive vorticity, • a Vb-style track of the surface level low pressure centre after ist arrival in central Europe (however with a quite different storm track and genesis before), • convective cells embedded into the precipitation area, • lifting induced by orography at the Central European mid range mountains

It worth noting, that these factors have been crucial as well for similar flooding situations of the past 15 years across the ROI.

So from climatological perspective the orographically structured region affected, bears a certain predestination for heavy precipitation events, and each of the above listed factors is not rare in the ROI. However, the heavy precipitation of 7/8 August in the ROI encountered all of them acting to­ gether, which is a quite rare event and has lead to precipitation amounts that had locally quite long return periods of up to 100 years.

A note on the genesis of the situaion ______- 3 ­

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The genesis of the weather situation is noteworthy. The cold front of the low pressure system ‘Viola’ which gave rise to the heavy precipitation pattern later on had been formed already across the At­ lantic and reached central Europe already in an almost occluded stage at 4 August 2010. The same time, a cut-off low was splitted from its mother trough reaching from the North Sea towards south­ east. The associated lifting areas led to an enhancement of the frontal precipitation and to the de­ velopments of sub-lows along the front, giving rise to additional precipitation in the Odra basin (Fig. 6) and in the Austrian Alps with daily totals of wide spread more than 50mm and locally more than 120mm for the latter. There were also floodings in upper Austrian catchment areas of the river Danube.

However, the main frontier between the warmer Russian and the cooler Central European air masses, existed already for a number of days (amd lead to the well known Russian heat wave). Therefor, the former occluded cold front transformed to a warm front in conjunction with the cyc­ logenesis and moved on eastward, while the upper level low pressure system kept active in the af­ fected ROI. During the night from 8 to 9 August the next frontal system associated with the low pressure system ‘Wilhelmina’ follwed from the Atlantic to arrive Central Europe, however with lesser precipitation, as this low was much weaker in both, surface and upper levels. Several days later (15 August), another low pressure system („Yvette) arrived with heavy precipitation partly enhanced during thunderstorms, leading again to floodings along the river Spree in Germany. The entire pe­ riod since end of July saw a number of trough arrivals from northwest to southeast with partly con­ vective precipitation events. Therefore, the event was not an isolated one in the seasonal record of summer 2010, but was embedded in a several weeks lasting period of repetetive precipitation events. 3. Totals, temporal evolution and spatial distibution of the precipitation events

3.1. Precipitation totals and their temporal evolution: At the station Aue in western Saxony the major precipitation amount was observed during the 24 hours until 07.08.2010 06 UTC (Figure 3). At several stations at the northerly foothills of the ‘Erzge­ birge’ (i.e. mid mountain ranges along the Czech-German border west of the Elbe river), daily totals of 40 – 60 mm have been measured with highs of 71 and 75 mm in Stützengrün and Chemnitz. Typical totals for the period 6-9 August 2010 have been approximately 90 mm.

In eastern Saxony (Bertsdorf-Hörnitz, Rosenthal-Bielatal) and in Czechia (Liberec) the daily totals collected at 08.08.2010 yielded the highest amounts with more than 100 mm at the station ‘Berts­ dorf-Hörnitz’ (Figure 3). In the entire period of 6-9 August 2010 measurements in ‚Bertsdorf-Hörnitz’ totalled to 163 mm, in Liberec even to 187 mm.

The polish stations in the Euroregion, for example Jelenia Gora and the 1600 m elevated mountain station Sniezka registred 97 and 68 mm, respectively for the 48h period 6-8 August 2010. Also here the highest daily totals occured at 8 August 2010 (60 mm for Jelenia Gora, 57 mm for Sniezka).

______- 4 ­ ______tribu­ of areas catchment Moreover river. Spree the and Neiße Lusetian the of headwaters the ing includ­ 6), Figure und 5 (Figure bohemia northern in located totals precipitation highest of area The Saxony. Eastern in stations of a atchoice rates precipitation Hourly 4: Figure amounts tation in terms oftheirperiods return in chapter 4.3. the precipi­ the ofassessment climatological a itself.will be of The rate hourly and the rate peak the hourof day the both, of terms in figures exceptional featured that ‘Bertsdorf-Hörnitz’ station the true the not that for is reveals This stations. all 4) almost at (Figure 2010 Saxony August 7 Eastern of afternoon in the during stations occured rates of highest choice a at rates hourly the at Looking Czechia Saxony and in stations of choice a at day) given the of UTC 6 at (ending totals precipitation h 24 3: Figure Division and Environment Climate Deutscher Wetterdienst,

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ölt 149 Kbcüz r ate O8) oln/pe O8) ürenrdr O8) Bertsdorf-Hörnitz(O598) (O580) Dürrhennersdorf (O484) Sohland/Spree (O384) Bautzen Kr. Kubschütz, (10499) Görlitz

60.00 70.00 80.00 09.08.2010 08.08.2010 07.08.2010 06.08.2010 06.08., 18 18

Liberec Rosenthal-Bielatal Bertsdorf-Hörnitz Aue

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07.08., 06 06 Termin [MESZ] O484 O484

O580 07.08., 12 12

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08.08., 00 00

08.08., 08 08

Deutscher Wetterdienst, Climate and Environment Division

tary rivers to the Elbe River in Northern Bohemia and Saxony (Figure 7) including the Kirnitzsch, and the Kamenice River along the Czech-German border and the Polish Ploucnice have been affected. With regard to Germany, only the Zittauer Gebirge (Zittau low mountain ranges) in Eastern Saxony including the station Bertsdorf-Hörnitz has been affected by the heaviest precipitations (Table 1).

Therefore, it was the coincidence of locally very heavy daily or short-term precipitation events and the spatial range of the precipitation pattern matching catchment areas of the headwaters of affected rivers that has lead to the high waters and flash floods.

At 7 August there have been also substantial amounts of precipitations falling in the area southeast­ of Dresden called ‘Sächsische Schweiz’. Another centre of heavy precipitation was located to the north in Northwestern Poland only affecting the lower flows of the Neiße and Odra rivers, without any substantial impact on the high water situation in the Euroregion CZ-D-PL. All regions of heavy precipitation encountered 3-days totals of more than 50 mm with local peaks exceeding 150 mm.

Figure 5: Preciitation distribution regarding 48h totals (ending at 8 August 6 UTC) in the Euroregion CZ-D-PL including the headwaters of Neiße and Spree as derived from the RADOLAN network of pre­ cipitation radar systems operated at Deutscher Wetterdienst. For the German territories (mainly Ea­ stern Saxony) IDs for river catchment areas are inserted into the map. IDs starting with 66 belong to the Neiße & Odra River catchment area, the initial digits ‘58’ denote the Spree and the Elbe River catchment area and ‘53’ the catchment area of further tributary flows of the Elbe River located to the west.

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Deutscher Wetterdienst, Climate and Environment Division

Figure 6: 2-day totals (ending at 9 August, 0 UTC) interpolated on a 1°x1° evaluation grid as derived from SYNOP messages collected for that period across central europe. (Source: Global Precipitation Climatology Centre, GPCC operated by DWD).

Figure 7: Daily totals of SYNOP stations in the Euroregion CZ-D-PL (indicated by bars) for 5 samping stop days of 6-10 August 2010, 6 UTC. Moreover the following catchment areas from west to east are highlighted: Elbe-Eger, Elbe-Polzen until Schwarze Elster, Schwarze Elster, Elbe - Moldau, Polzen, Spree, Lusetian Neiße, Iser and Bober.

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Deutscher Wetterdienst, Climate and Environment Division

4. Climatological Assessment of the Precipitation Totals

4.1. Local Comparisons to the Climate Normals As shown in Figure 8, the totals at station Liberic, CZ did exceed the climate normals (1961-1990) partly by far during August, but also in May and July. For the latter the main amount occurred in the second half of July with a maxiumum of 61 mm at 18.07.2010. Altogether July at Liberec has seen 180% of the climate normal, but this was clearly outperformed by the following month, that started already quite wet with 35 mm at 3-4 August 2010, just before the very heavy precipitation started at 6 August. For the entire month of August the precipitation in Liberec totalled to 460% of the climate normal. Also Western Poland has seen in August 2010 along the border to Germany 300% of a cli­ mate normal with 1971-2000 as reference period (Figure 11). 450

400

350

300 Referenzperiode 1961-1990

250 Monatssummen 2010

200

150 Niederschlag [mm]Niederschlag

100

50

0 Jan Feb Mär Apr Mai Jun Jul Aug

Figure 8: Monthly precipitation totals at Liberec – Czechia in terms of climate normals for 1961-1990 reference period (yellow bars) and totals of year 2011 until August (blue bars).

4.2. Area averaged (gridded) precipitation amounts and their rela­ tive contribution to annual totals Analysing the precipitation in central Europe of 6-9 August according to its percentage of the long term (1951-2000) mean of August in Europe (Figure 9), the same patterns as for the absolute pre­ cipitation distribution (Figure 6) appear. Interestingly, just theses 3 days of precipitation totalled al­ ready to 80-100% of the long term mean in August, which is a ten times longer period. This demon­ strates how far the amounts have exceeded the expectation values (normals). On the other side, it should be noticed that during summertime the region of interest encounters regularily rather convec­ tive precipitation events, leading to just the opposite of a uniform temporal distribution of precipita­ tion rates. Considering the spatial distribution of the long term mean precipitation, the Euroregion CZ-D-PL does not feature higher climate normals compared to other nearby low mountain range regions. This is true for August but also for the whole summer season, June, July, August as de­ picted in Figure 10.

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Deutscher Wetterdienst, Climate and Environment Division

Figure 9: Area averaged relative precipitation percentage of August long-term (1951-2000) mean, across central Europe for the 72 hours until 09 August 2010, 06 UTC. Source: GPCC, DWD

Figure 10: Precipitation normals in mm/month averaged to a 0.5°x0.5° grid for all summer months (June-August) of 1951-2000. Source: GPCC operated by DWD.

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Deutscher Wetterdienst, Climate and Environment Division

Figure 11: August 2010 precipitation totals in terms of percentage of long term (1971-2000) mean (Source: IMGW Polen).

4.3. Return Periods of the Heavy Precipitation Events

4.3.1. Eastern Saxony Method of Work Data of regional precipitation monitoring as taken from selected DWD stations in Eastern Saxony and the DWD radar network system are compared to the regionalised extreme precipitation clima­ tology of the Deutscher Wetterdienst called KOSTRA-DWD-2000-Starkniederschlagshöhen1 (KOSTRA-DWD-2000-Extreme precipitation heights). From west to east the DWD stations Rosen­ thal, Lichtenhain, Sohland/Spree, Bertsdorf-Hörnitz and Görlitz have been examined.

1 The location specific KOSTRA-DWD-2000 extreme precipitation heights hN(D;T) in dependence of the per­ sitence level D (length of event including short interrupts) and the return period T are available on a grid of 8.5x8.5 km2 resolution for the entire astronomic year and for the half-year seasons Summer (May to Septem­ ber) and Winter (October to April). ______- 10 ­

Deutscher Wetterdienst, Climate and Environment Division

Results As detailed in Table 1, there have been substantial precipitation amounts at all stations during the periods of investigation covering a temporal range from 6-8 August, 6 UTC. However, their clima­ tological significance has to be strongly differentiated. In doing so, the 24h precipitation of 44 mm at a station located in the city of Görlitz being strongly affected by high waters and floodings of the River Neiße, was not exceptionell from a climatological perspective, as even the 6h total of 33mm in Görlitz has a return period of 5 years. The 24h totals at Rosenthal and Lichtenhain have been al­ ready substantially stronger with 50 and 70 litres, but the corresponding return periods of 10 and 20 years are still moderate, and would on their own, so without consideration of other factors, give rise for a flooding situation.

Amount Station D[h] (collection stop) Return Period [mm] 33,0 6 h (7. Aug, 6 UTC) 5 a Görlitz 44,4 24 h (7. Aug, 6 UTC) 1 a >57,0 12 h (7. Aug, 6 UTC) 10 a Rosenthal >109,0 48 h (8. Aug, 6 UTC) 10 a >68,0 12 h (7. Aug, 6 UTC) 20 a Lichtenhain >120,0 48 h (8. Aug, 6 UTC) 20 a 40,4 2 h (7. Aug, 15 UTC) 20 a 53,7 3 h (7. Aug, 15 UTC) 30 a Sohland/ 72,9 6 h (7. Aug, 16 UTC) 100 a Spree 91,4 12 h (7. Aug, 18 UTC) 100 a 101,7 24 h (7. Aug, 18 UTC) 50 a 104,0 48 h (8. Aug, 23 UTC) 20 a 35,4 1 h (7. Aug, 8 UTC) 20 a 57,2 2 h (7. Aug, 9 UTC) 100 a 66,2 3 h (7. Aug, 9 UTC) 100 a Bertsdorf- 85,5 6 h (7. Aug, 12 UTC) > 100 a Hörnitz 130,4 12 h (7. Aug, 18 UTC) > 100 a 145,6 24 h (8. Aug, 6 UTC) > 100 a 159,8 48 h (8. Aug, 6 UTC) > 100 a Tabelle 1: Observer precipitation amounts at varous persistency levels (‚Dauerstufen’), D[h] in East­ ern-Saxony and their corresponding return periods in years (‚Jährlichkeiten) as taken from the DWD­ KOSTRA extreme precipitation climatology

However, the precipitation in the vicinity of Bertsdorf-Hörnitz was truly exceptions, with a 48h hour total ending at 8 August, 6 UTC of more than 150 litres. An extreme event of this scope happens more rarely than every 100 years. Bertsdorf-Hörnitz is located in the Zittauer Gebirge (Zittau low mountain ranges) that dewaters into the Lusatian Neiße. However, this observation would have been representative also for areas north-west of Bertsdorf-Hörnitz, that dewater into the Spree River. This is revealed by the extreme event statistics for the observations at Sohland/Spree, where a 24h total until 8 August, 6 UTC of more than 100 mm was observed. 70 mm of this total rained within 6 hours, an event already less frequent than every 50 years. Analysing the hourly records at Sohland/Spree independent from SYNOP observa­ tion dates in order to reveal a sum with a peak return period, one can find 6 and 12h sums of 72.9 and 91.4 litres, respectively, that correspond to return periods of more than 100 years. This explains to a certain extend the high water levels encountered also at the Spree River in the following days.

4.3.2. Euroregion CZ-D-PL As discussed in the previous section, the Eastern-Saxonian observations within the 3 day of 6-8 August showed return periods of 20 years in general and more than 100 years sporadically. Accord­

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Deutscher Wetterdienst, Climate and Environment Division

ing to the data available from the ECA&D-portal2 daily totals of 50 mm in the Euroregion are associ­ ated to return periods of 50 years. The maximum amounts observed in every of the three countries of the Euroregion have exceeded the 50 mm level, in particular at the station Liberec where a 24h sum at 8 August of 99 mm exceeded the 50 years return period levl of 82 mm.

Therefore it can be summarized, that the precipitation event has been exceptional from climatologi­ cal perspective in many areas subsequently affected by flooding events, but not in all of them. How­ ever, it can only locally at scattered locations be assessed as an extremely exceptional event.

Figure 12: Maximum daily totals corresponding to 50 years return period in August evaluated on base of an 18 years data set covering the period 1991-2009. (Source: ECA&D; http://eca.knmi.nl )

5. Hydrological Impacts and Damages Due to the strong and persisting precipitation the headwaters in the affected catchment areas quickly swelled and transformed to torrential rivers. The high amounts of waters could not be dewa­ tered within the river basins, so there have been consequently, floodings, flash floodings and even dam failures.

5.1. Spree At the water gauge Bautzen the corresponding catchment area sizes just 276 km². During the early Sunday hours at 8 August2 010, the fast rising waters of the Spree River caused a dam failure on the premises of the train wagon producer Bombardier in Bautzen, Saxony. The entire premises have been flooded at a water level of 1.5 m. The same night the high water protection mechanism of the dam in Bautzen has been activated in order to protect the dam. In doing so the maximum flow rate of the Spree river upflow the dam lake of 165 m3/s could be reduced to 65 m3/s downflow the dam.

2 ECA&D: European Climate Assessment & Data, http://eca.knmi.nl ______- 12 ­

Deutscher Wetterdienst, Climate and Environment Division

As there had been maintenance work at the outlet of the dam, prior to the arrival of the water the barrage was at a rather low water level, hence able to take a substantial amount of water masses originating from precipitation events dewatering. The outlet flow had been increased in several steps from 10 to 70 m3/s. The dam’s water content increased by approximately 15 Million m3. The flood peak in Spremberg reached a level of 384 cm, which was high enough to issue hiqh water alert level 3. Due to the increased outlet flow of the Spremberg dam, the Spree water level in Cottbus reached 225 cm, which was still below the threshold to issue alert level 1 there. 5.2. Neiße The largest damages and the biggest impact within the Lusetian Neisse catchment area occurred in the Polish town Bogatynia, where a little brook all for a sudden transformed to a torrential river de­ stroying major parts of the 18 000 inhabitants town. In contrast to the Spree where the peak flood level had been sucessufully controlled downflow the Spremberg dam, a dam failure of the Witka barrage had boosted the peak flood in a catastrophic and unpredictable manner. In total 5 Million m3 of water was additionally discharged into the Lusatian Neiße. This total corresponds to an 80-days average discharge at the water gauge Görlitz. Soon after the Witka dam failure the rising Neiße wa­ ter levels flooded substantial parts of Görlitz. The flood peak was an alltime high exceeding with 720 cm the old record of 1912 by 40 cm.

Another dam failure at the Berzdorf Lake led to a fast filling of the co-located former, so far partly flooded, open lignite mining area yielding a rise by 45 cm of the 9 km2 sized lake. Despite the quick dewatering, flooding of the city area of Görlitz could not be prevented. Downflow Bad Muskau and Guben/Gubin had been affected. In Bad Muskau parts of the Fürst-Pückler gardens, being member of the UNESCO world heritage list, had been flooded, as well as nearby streets in Guben/Gubin at German/Polish riversides. In Groß Bademeusel the flood alert level 4 has been issued, in Guben the flood levels trespassed the alert level 3 thresholds. 5.3. Rivers Chemnitz and Pleiße In western Saxony, in the vicinity of Chemnitz, and around Zwickau there have been severe flood­ ings along the Rivers Pleiße and Chemnitz and their tributary rivers. In Neukirchen three inhabitants where drowned. 5.4. Tributories of the Elbe River In the Czech part of the Elbe River catchment area, the vicinities of Liberec, Decin and Usti na La­ bem, have been affected worst. This was the centre of gravity of the precipitation activity of the low pressure system ‘Viola’, turning small brooks into torrential rivers. In Frydlant people had to be evacuated with helicopters as they couldn’t get out of the danger zone in time. The Czech rescue staff has been supported by German colleagues. The water gauges of Ploucnice (Figure 13) in Czeska Lipa and Benesov triggered flood alert level 3. In the area of the Elbsandsteingebirge (mountain ranges south-east of Dresden) there had been flooding on both sides of the Czech- German border. The valleys of the rivers Kamenice and Kirnitz were affected. The peak flood level did raise very quickly, but left the area quickly as well. Therefore the levels went down quickly to reveal a picture of entire devastation.

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Figure 13: Flooding situation in Czechia at 9 August 2010 with flood alerts (red dots) in Northern and Southern Bohemia. (Source, CHMI: http://hydro.chmi.cz) 6. Casualties and other Damages The death toll of the flooding totalled to 11, with three casualties in western Saxony, three in Poland and even 5 in Czechia.

Most people were just surprised by the flashfloods so badly, that they could not find a save place anymore. Others drowned during the attempt to rescue their belongings.

In many places evacuations were necessary in order to schelter people from the approaching wa­ ters. There had been shutdowns of electric supplies and the fresh water supplies in Görlitz and Bautzen broke down.

The government of the Saxonian Federation has accessed the damages to reach 150 Million Euros. Many producers and factories had to stop their production with according losses in total revenues. Bombardier and the Outdoor equipper ‘Yeti’ had to entirely shut-down their production, and tourists cancelled their bookings

In the Czech county of Liberec many bridges had been damaged, and 7 just washed away.

7. Climatological Assessment of the Flood Situation The floodings of the rivers Elbe, Spree and Lusatian Neiße and their tributaries can be conclusively attributed to the precipitation events of 6-8 August in their catchment areas. However, precipitation­ flood relationships turned out to be rather complex, as the catchment areas have been affected by precipitations events of highly variable strengths, and been dewatered through a variety of brooks and rivers. Moreover a non climate (change) related dam failure factored into the flood situation.

Despite these complexities it can be stated that at least for the bigger rivers Elbe and Lusatian Neiße the heavy precipitation in Northern Czechia have ben crucial. For the Spree River the precipi­ taion in the Zittauer Gebirge (low mountain ranges) and the Lusatian ranges are rather to be attrib­ uted. These areas were located more to the northern edge of the major precipitation pattern.

7.1. Comparison to earlier Floodings In general floodings happen quite often in the central European area of investiagion (Table 2).

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Year Month Catchment Area 1997 July Odra 1999 May Danube 2001 July Vistula 2002 August Elbe/Danube 2005 August Danube 2010 May Odra/Vistula 2010 August Neiße/Spree/Elbe 2010 September Neiße/Elbe-Elster

Tabelle 2: Comparative flooding events attributable to Vb-style of Weathersituations of the previous years

Just seven weeks after this 6-8 August 2010 event, another flooding occurred in Saxony at the end of September 2010, this time in particular in the Neiße catchment area with comparative precipita­ tion totals. A few months before, in May 2010, there had been severe floodings in eastern central Europe (Belz and Wiechmann, 2010), that had been analysed by DWD as well (Bissolli et al. 2010). However its synoptic genesis had been significantly different to the August one, despite the fact that it was also a Vb-style of weather pattern. Moreover the centre of major precipitations was located far more to the East and its spatial range was much bigger. Further comparisons to earlier flooding events can be found in Bissolli et al. (2010). 8. Assessment of the Role of Climate Change The so far most recent (fourth) IPCC assessment report (Salomon et al, 2007) points to the fact of an increasing percentage of extreme precipitation events on the annual totals during the most recent decades. On the other hand, the IPCC AR4 projects rather a decrease of precipitation totals (despite the North). It should be noted, however, that the spread of the climate projections in AR4 is quite high; hence a projection of the future precipitation rates and frequencies of extreme precipitation events is still very uncertain in central Europe. The effects of global warming advocate formation of increased water vapour contents in the atmosphere. On the other hand the expected decrease of long term precipitation totals advocate an according impact on the number and scale of extreme events. Moreover the AR4 projects enhanced anti-cyclonic circulation across the Northeastern Atlan­ tic, and consequently more frequent anti-cyclonic conditions across the Western Europe and more frequent trough (low pressure) situations across Eastern Europe. This would advocate more fre­ quent heavy precipitation events in Eastern Europe, in particular during La Niña seasons. The exact locations of these extreme events, however, cannot be reliabily predicted by the climate models, due to their limited spatial resolution. Climatologic considerations keep the Vb type storm tracks and oro­ graphically structure terrain to be the most favourite areas of extreme precipitation events, also in a future climate.

The majority of Regional climate models predict for Central Europe a trend to more zonal synoptic scale weather patterns, howeve, with a pronounced Northwest to Southeast gradient as far as the derived trends in precipitation are concerned. In general more frequent zonal patterns should lead to accordingly more frequent marine type of precipitation events. Fricke (2009) has discussed the relationship of heavy precipitation events to the frequency of the weather classification type ‘central European low pressure system’ (Troglage, Mitteleurope). The hypothesis whether the past precipitation events give rise to a further differentiation on this relation­ ship, towards a superposition of the Vb classified situations and those only leading to a ‘Vb alike’ type of situation with a different genesis from zonal patterns remains to be investigated. Further in­ vestigations are also necessary to clarify whether the ‘Vb alike’ are related to a further eastward dis­ placement of East-Atlantic cut-off processes with the cut-off low moving into Europe or whether global warming related increased potential evaporation are successively substituting the traditional ______- 15 ­

Deutscher Wetterdienst, Climate and Environment Division

mechanism of Vb classified lows, to be ‘fueled’ with humid and instable layered air masses across the gulf of Genua.

DWD in-house re-analysis of in-situ precipitation observation records for the period 1951-2000 have not shown any significant trend towards more frequent extreme precipitation events. However, there is a trend to more frequent events of heavy precipitation exceeding daily totals of 30 mm. Even this trend is only significant during wintertime. 9. Conclusions The heavy precipitation in the Euroregion PL-CZ-D from 7-9 August 2010 was a remarkeble event, however, only at a few locations in Saxony it was truly exceptional. The event was formed during a synoptic scale weather situation and a number of further factors, whereby each one on its own was not exceptional, but their coincidence was indeed. There are indications that this constellation of factors was at least supported by the La Niña situation in place during and before the event. More­ over the enhanced likelihood for heavy precipitation in the regions has been partly indicated three months in advance by the seasonal forecasts from ECMWF and MetéoFrance (2010). The future frequency of this kind of precipitation in the Euroregion in connection with global warming is yet to be investigated.

References Belz and Wiechmann, 2010: Frühjahrshochwasser im Osten und Südosten Deutschlands (Spring flood events in the East and South of Germany). Bundesanstalt für Gewässerkunde, Referat M1, 4. Juni 2010. http://www.bafg.de/cln_007/DE/07__Aktuelles/20100604__hw__oder,templateId=raw,property=publi cationFile.pdf/20100604_hw_oder.pdf

Bissolli, P, Friedrich, K., Rapp, J., Ziese, M., 2010: Hochwasser im östlichen Mitteleuropa im Mai 2010 (Floods in Eastern Central Europe in May 2010). http://www.dwd.de/ecsm

Ceski Hydrometeorologicky Ustav – Informacni Servi, http://www.infomet.cz/index.php?id=read&idd=1281288892 http://www.e-pocasi.cz/srazkove-uhrny-pri-povodnich.html

Fricke, W., 2009: Wetterlagen im Klimawandel – Welche Veränderungen sehen wir bereits und wel­ che können wir erwarten? (Large scale weather patterns and climate changes – What changes are visible already, what cn be expected?). Vortrag beim 4. ExtremWetterKongress Bremerhaven, 19.02.2009

Hochwasserwarnungen und –informationen des Landesamtes für Umwelt, Gesundheit und Ver­ braucherschutz Brandenburg: Hochwassermeldezentrum Cottbus http://www.luis.brandenburg.de/w/

Landeshochwasserzentrum des Sächsischen Landesamtes für Umwelt, Landwirtschaft und Geolo­ gie www.hochwasserzentrum.sachsen.de

Météo France (2010): Seasonal Outlook for Summer Season (17(05/2010 – JJA). Bulletin Cli­ matique Global 131, 1-20

Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor and H.L. Miller (eds.), 2007: Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental

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Deutscher Wetterdienst, Climate and Environment Division

Panel on Climate Change, 2007. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA. http://www.ipcc.ch/publications_and_data/ar4/wg1/en/contents.html

Tschechisches Hochwasservorhersagezentrum http://hydro.chmi.cz/hpps/index.php

ZAMG: Unwetterbericht für August 2010, http://www.zamg.ac.at/klima/klima_monat/unwetterbericht/

Contact for Feedback Dr. Andreas Becker, Head Unit KU42 (Precipitation Monitoring) and GPCC Deparment for Hydrometeorology Frankfurter Str. 135, 63067 Offenbach am Main, Germany Phone: +49 69-8062-2900; Fax: +49 69-8062-3987 e-mail: [email protected]

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