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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). _______________________________________________________________________________ - 2 ­ Deutscher Wetterdienst, Climate and Environment Division 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 ­ Deutscher Wetterdienst, Climate and Environment Division 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.

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