2.2 Mean Annual Precipitation Depth (non-corrected)

annual precipitation total The spatial distribution of the mean annual precipitation totals in millimetres provides a pre- 2600 2500 liminary overview of the regionally differentiated precipitation patterns in Germany. Map 2.2 2400 mean annual precipitation total is therefore a base map for hydrological considerations concerning the hydrologic cycle, as re- orography 2250 2200 Alps quired for, for example, preservation of water resources, water storage and use of rainwater, 2000 precipitation/discharge ratios, ground water recharge or water management framework plans. 2000 The accuracy and quality of the precipitation measurements, the network density, the statisti- 1800 1750 Harz cal presentation of the precipitation time series and the regionalisation method used determine 1600 1500 the quality of the statements that can be derived from the spatial distribution of the mean Schwäbische Alb 1400 annual precipitation depths (cf. introduction 2.2 to 2.6 “Maps Showing Mean Precipitation 1250 Depths”). 1200 Rhön

Danube elevation in m Elbe 1000 1000 All maps and statistics published so far for regional or national precipitation patterns result- Main precipitation total in mm ing from various measurement and reference periods (K ELLER 1979, S CHIRMER 1979, 800 750 MÜLLER -W ESTERMEIER 1990, 1996, etc.) are based on measured, non-corrected precipitation 600 totals. In order to be able to compare the various periods with one another and the measured 500 with the corrected precipitation depths, the mean annual and half-year precipitation totals are 400 250 depicted both with and without correction of the systematic measurement error. The correc- 200 tion involves a location-dependent supplement in order to compensate for the reduced values 0 0 registered by the measuring device and mostly created by the wind and the snow proportion 54 53 52 51 50 49 48 of the precipitation. latitude Depending on the area of application, the time levels and the degree of detail or generalisa- tion, the resulting precipitation databases with and without corrections will continue to be used Fig. 2 North/south intersection of the grid boxes for mean annual precipitation totals during the reference period 1961–1990 at longitude 10° 10' east side by side in the future. totals below 500 mm/a. The lowest values can be seen in the south-easterly foothills of the Map Structures Harz Mountains (Atzendorf 399 mm/a). As would be expected, the greatest mean annual precipitation totals occur in the high areas of Map 2.2 shows the mean annual precipitation depth (non-corrected) in the form of a gridded the Alps (e.g. Balderschwang 2,450 mm/a). With its meridional orientation, the Schwarzwald 2 structure with a resolution of 1 km with class amplitudes of 50 mm or 100 and 200 mm. The also experiences mean annual precipitation totals of more than 2,000 mm/a in its high areas. mean annual precipitation totals for Germany for the period 1961–1990 vary from around However, the north of the Schwarzwald, with relatively low foothills to the west, receives 400 mm on the leeward side of the Harz Mountains to 3,200 mm in the Alps, although values more precipitation than the south of the Schwarzwald despite lower altitudes. In the south, the between 500 mm (in the east) and around 800 mm (in the north-west) are typical for most of rain-shadow effect of the Vogesen (Vosges Mountains) results in less precipitation when there Germany. The precipitation pattern is significantly dependent on the influences of the west- are westerly airstreams. In view of the major accumulation effect of the Schwarzwald and the erly drifts and on the orography. Alps, it is not necessarily the station altitudes that determine whether precipitation is above- Due to the extensive lifting processes, the precipitation fields, which are accompanied by the average because even stations such as Aschau-Stein (680 m above sea level) and Ruhpolding- low-pressure areas coming in from the west, can extend to a longitude of a few thousand and Seehaus (753 m above sea level) have mean annual precipitation totals of more than 2,000 mm a latitude of several hundred kilometres. They move with the fronts or the vertical airstream per year. and their structure alters depending on the development stage of the associated low-pressure At the west/east intersection of the mean annual precipitation total at latitude 51° 50' north area. This usually involves warm fronts assuming the form of cyclonic fronts, whereby the (Fig. 1), the windward/rain shadow effect can be clearly identified looking at the mean advancing warm air moves obliquely onto the cold air, which is slowly withdrawing. This lift weather data in the Harz region. Moist air masses from the west lead to an increase in the pre- creates a stable updraft cloud that begins far away from the warm front with stratiform cloud cipitation total on the western side of the elevation, whilst on the eastern side, on the leeward at the highest level. This condenses into mid-height altostratus cloud and finally grows into a side of the Harz Mountains and in the Thüringer Becken, there is a large drop in the mean vertical nimbostratus cloud of a sizeable vertical dimension, from which the precipitation falls annual precipitation totals. in the form of snow in winter and sustained rain in the transitional seasons and the summer (cloud images on Map 2.4). The north/south intersection of the mean annual precipitation total at longitude 10° 10' east Cold fronts involve a cooling-down process, which usually covers the entire troposphere. In (Fig. 2) provides clear evidence of the accumulation effect in the foothills of the Alps. The the case of active cold fronts, the cold air at the top rushes away, producing a destabilising influence of the accumulation effect on the precipitation pattern extends right down into the effect with cumulus clouds that can result in showers and thunderstorms, particularly in sum- Donauebene (Danube flats). But the Harz Mountains and the foothills of the Thuringian For- mer (cloud images on Map 2.3). In the case of passive cold fronts, the wind subsides as the est, the Rhön Mountains and the Schwäbische Alb (Swabian Mountains) also have outstand- height increases, and the cold air is driven under the warm air obliquely. It is forced to rise and ing secondary maximum precipitation totals. In addition, the high areas of the Sauerland and a stable stratified cloud develops, from which extensive precipitation falls, particularly in the Bavarian Forest experience an average of more than 1,400 mm of precipitation per year. winter. These three-dimensional processes of cloud and precipitation formation can be But it is precisely in the mountainous regions that the high level of unreliability linked to the observed, for example, on meteorological charts and satellite images. measurement of solid precipitation is made apparent: there are precipitation deficits caused by In the north of Germany up to the northern edge of the low-mountain regions, where the pre- strong winds and an increased proportion of snow in the overall precipitation. This makes it cipitation total is not significantly dependent on altitude, decreasing mean annual precipitation particularly clear that it is necessary to correct the precipitation measurements. totals are the result of the influence of increasing continentality from the west to the east. But there is also less precipitation in relation to altitude in the eastern low-mountain regions, such as the Thüringer Wald (Thuringian Forest), Bayerischer Wald (Bavarian Forest) and the Practical Information Erzgebirge (Ore Mountains), than there is in the western low-mountain regions, such as the The mean temporal fluctuation range of the annual precipitation totals during the 30-year ref- Schwarzwald (Black Forest) and the Sauerland, due to the increasing continental influence. erence period is, with regional differences, around ± 20 %. However, some annual values can Moving from the north to the south, the decreasing cyclonic nature of the weather conditions differ considerably from the mean values of the 30-year period observed and this is reflected causes the precipitation to subside and low-pressure areas occur less frequently. In Germany in Table 1, which shows data from selected stations. this effect is added in the south by a higher level of precipitation caused by increasing orographical altitude. Table 1 Mean and extreme annual precipitation totals during the period 1961-1990 for In coastal areas the impacts of the precipitation-forming processes as described in the intro- selected stations duction to Maps 2.2 to 2.6 “Maps Showing Mean Precipitation Depths” can be clearly identi- Mean value Deviation from Deviation from Station Highest annual value Year Lowest annual value Year 1961–1990 in mm the mean value in mm the mean value fied. With 700 to 800 mm per year, the North Sea coast receives more annual precipitation in mm in % in % based on the quarterly mean, due to the westerly, inland precipitation-promoting sea air Arkona 521 658 126 1965 331 64 1971 masses, than the Baltic Sea coast with its offshore winds, where mean precipitation of 500 to Brocken 1814 2338 129 1981 1080 60 1963 Cottbus 563 864 153 1974 335 60 1976 600 mm/a is recorded (e.g. St. Peter-Ording on the North Sea coast with 810 and Marien- Erfurt 500 671 134 1987 296 59 1982 leuchte on the Baltic Sea coast with 560 mm/a for the period 1961–1990). Feldberg/Schwarzwald 1909 2494 131 1965 1345 70 1971 Freiburg im Breisgau 955 1222 128 1965 682 71 1971 Hamburg 770 988 128 1980 542 70 1971

1400 2000 Karlsruhe 770 1022 133 1965 462 60 1971 Kassel 698 1086 155 1981 496 71 1976 Harz mean annual precipitation total Köln 803 1078 134 1966 510 63 1976 orography 1750 München 967 1201 124 1965 796 82 1976 Weser- Teutoburger Zugspitze 2003 2724 136 1981 1476 74 1963 1200 bergland Wald 1500

Weser 1250 These differences between the precipitation totals registered for individual years and the mean 1000 value clearly demonstrate that a 30-year averaging period is not necessarily representative of Ems 1000 the mean precipitation totals at any one station. It is perfectly possible for one extremely high or low annual value to have a decisive influence on the mean value. The time series of the 800 750 elevation in m Görlitz station, which spans back more than 100 years, is an example of the large fluctuation Spreewald precipitation total in mm range of annual precipitation totals (Fig. 3). In order to be able to compare mean annual pre- 500 Saale cipitation totals internationally despite this fluctuation, a standard 30-year reference period 600 (currently 1961–1990), as recommended by the WMO, is used internationally. 250

1100 400 0 8 9 10 11 12 13 14 15 jährlicheannual precipitation Niederschlagshöhe total Mittelwertmean value 1858-1996 1858-1996 gleitendesmoving average Mittel über30 years 30 Jahre Mittelwertmean value 1961-1990 1961-1990 longitude 1000 gleitendesmoving average Mittel über10 years 10 Jahre Fig. 1 West/east intersection of the grid boxes for mean annual precipitation totals during the reference period 1961–1990 at latitude 51° 50' north 900

Regional differences in the mean precipitation total of individual mountains in relation to the 800 orography are also caused by mountain ranges facing the westerly to south-westerly vertical airstream which affects the weather and brings precipitation with it. For example, the north- 700 south orientation of the Schwarzwald causes above-average annual precipitation depths due to lifting processes and accumulation effects (Table 1, Feldberg/Schwarzwald). The meridi- 600

onal orientation of the Spessart hill range and the Odenwald Mountains also results in greater precipitation total in mm mean precipitation totals in relation to altitude than in the nearby Taunus Mountains, the orientation of which runs more parallel to the airstream. 500 Low-precipitation areas with less than 600 mm/a can be found in the east of Germany, due to the continental influence there, and on the leeward side of such elevated areas as the Eifel pla- 400 teau, Spessart hill range and Haardt. Mean annual precipitation totals below 500 mm/a are observed on the leeward side of the Hunsrück Mountains and in Oderbruch, where even the 300 minor elevations of the Barnim have rain shadow effects. In the Altmark, on the leeward side 1855 1865 1875 1885 1895 1905 1915 1925 1935 1945 1955 1965 1975 1985 1995 of the Lüneburger Heide and in Ueckermark too, low elevations cause mean precipitation year Fig. 3 Annual precipitation totals at Görlitz station for the period 1858 to 1996