2.3 and 2.4 Mean Precipitation Depths During the Hydrological Half-Years (Non-Corrected)
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2.3 and 2.4 Mean Precipitation Depths During the Hydrological Half-Years (non-corrected) The mean precipitation depths for the hydrological half-years provide particularly important Map Structures information for water resources studies and water management plans. Whereas the water stocks are filled up during the hydrological winter half-year, in Germany they are usually used up again during the summer half-year. In Central Europe the hydrological year is defined as Summer Half-Year (Map 2.3) the period from 1st November of the previous year until 31st October of the current year. In order to be able to draw a direct comparison Maps 2.3 and 2.4 show the precipitation conditions for the hydrological summer (May until between precipitation during the hydrological half- October) and for the hydrological winter (November until April) during the reference period years and the mean annual precipitation totals shown 1961–1990 in a gridded structure. on Map 2.2, the class amplitude of the precipitation In May, the first month of the hydrological summer, changes in the area-wide weather are nor- totals is divided into 50 mm or 100 mm stages here mally largely due to the growing temperature differences between the mainland and the sea. too. Compared to the depiction of the mean annual Cool sea air masses move onto the mainland, which has already warmed up significantly, and precipitation depth, Map 2.3 shows a more balanced result in the formation of convective clouds and showers (cloud images on Map 2.3). These picture. The differences in the precipitation totals are especially strong in the lowlands because the ground heats up most there. During the sum- between the mountains and the lowlands are smaller. mer months, this effect continues to grow and does not reverse until September when the air A precipitation depth of less than 250 mm only oc- temperatures start to fall again. Compared to convective precipitation, precipitation caused by curs in the eastern Harz foothills. By evaluating the advection plays a minor role in the lowlands during the summer months. grid boxes, one can determine that the figures range However, the mountain ranges do experience distinct accumulation effects even during the from 235 mm in the Harz foothills to 1,843 mm in warm season. But since the condensation level rises so much during the summer months that the high areas of the German Alps. But the foothills the highest areas in the low-mountain regions can no longer reach it, the moist air masses from of the Alps also experience above-average summer westerly and northerly directions collide with the Alps region in particular, without having lost precipitation depths in relation to their orographic their moisture by discharging rain on the preceding low-mountain regions. This is why the altitude. This is due to the major accumulation effect accumulation effect of the Alps is particularly significant during the summer months and of the Alps in the summer, as already mentioned. extends far into the foothills of the Alps. Figure 1 shows the north/south intersection of the The friction effect of the coast causes greater pre- grid boxes for Maps 2.3 and 2.4 at longitude 10° 10' east and provides evidence of this pheno- cipitation depths on the North Sea coast than on the menon. Baltic coast. Mountain ranges such as the Harz Mountains, Sauerland, Thüringer Wald (Thuringian Forest) or the Erzgebirge (Ore Mountains) also stand 1500 3000 summer precipitation total out from the lower surrounding areas due to their 1400 winter precipitation total 2750 greater precipitation depths. The impact of increas- 1300 orography Alps 2500 ing continentality combined with decreasing precipi- 1200 2250 tation from the west to the east in relation to the alti- 1100 1000 2000 tude is less significant in the summer months than in 900 Harz the winter because convective precipitation occurs 1750 800 more frequently. By averaging all of the grid boxes, Danube 1500 700 Rhön one arrives at a summer precipitation depth of 1250 600 Elbe Main around 425 mm for Germany. elevation in m 500 1000 precipitation total in mm 400 Winter Half-Year (Map 2.4) 750 300 500 Based on an evaluation of the grid boxes for the 200 hydrological winter half-year, the precipitation totals 250 100 shown on Map 2.4 range from 135 mm to 1,414 mm, 0 0 54 53 52 51 50 49 48 making the totals in the lowlands considerably lower than during summer. The average precipitation total latitude in the hydrological winter in Germany is 360 mm, Fig. 1 North-south section of grid boxes for mean half-year precipitation totals during the around 65 mm lower than the summer level. The reference period 1961–1990 at longitude 10° 10' east coastal effect has a strong impact with precipitation totals differing by around 200 mm between the The moist sea air masses that move inland during the autumn and winter months cause dis- North Sea and Baltic coasts. Accumulation can al- tinct windward and rain shadow phenomena in the low-mountain regions, particularly in ready be seen in the north German lowlands. Minor December and January. This is even true of minor elevations because of the low condensation elevations, such as Ankumer Höhe, Lüneburger level during the cold months. These processes result in a high altitude-dependence and thus Heide or Schorfheide, experience windward and rain in a spatial precipitation pattern that is heavily characterised by orography. In winter, the shadow effects, which involve greater precipitation orographical influences are added to by the increasing continentality moving further away depths on the west side and lesser precipitation from the sea. The Alps also experience precipitation of great depth but as the height increases amounts on the east side of the elevations. These the increase in precipitation is less pronounced than in the low-mountain regions. The west/ phenomena are, however, more pronounced in the east intersection of Maps 2.3 and 2.4 at latitude 51° 10' north (Fig. 2), which includes the low-mountain regions: Hunsrück, Odenwald, Fig. 3 Regional precipitation types mountains and hills of the Sauerland, the Rothaar Mountains and the Hainich as well as the Thuringian Forest and other low-mountain regions based on standardised cause particularly dry areas on their eastern sides annual developments during foothills of the Harz Mountains, illustrates the cloud and precipitation formation on the west the reference period 1961– side in the winter, caused by advection, and the cloud dispersal on the east side (cloud images with longterm mean precipitation totals below 200 1990 on Map 2.4). mm during the hydrological winter. Although the precipitation depths in the Alps are significant, the A Sea Type altitude-related increase in precipitation is lower B Summer Type I 900 200 0 C Summer Type II summer precipitation total Sauerland Rothaargebirge than in the low-mountain regions, such as the D Low-mountain Type winter precipitation total 800 175 0 Schwarzwald (Black Forest), Thuringian Forest or E Winter Type orography Sauerland. Thus, the winter precipitation totals are 700 150 0 less substantial than the summer levels (Fig. 1). However, it should be noted that the high pro- Kellerwald 600 125 0 portion of snow in the winter precipitation in the high areas of the German Alps causes a ma- Hainich Elbe Thüringer Becken jor systematic measurement error (Maps 2.5 and 2.6). 500 100 0 Saale 400 750 elevation in m Practical Information precipitation total in mm 300 500 The isohyetal maps shown and discussed here are a selection of the digital maps available at 200 250 the DWD, which can be adapted for a variety of purposes. As described in the introduction to 100 0 the maps showing the precipitation totals and the text accompanying Map 2.2, “Mean Annual 6 7 8 9 10 11 12 13 14 Precipitation Depths”, the processes that trigger precipitation depend on the weather condi- longitude tions and regional influences. The proportion of the average monthly precipitation depths in the overall precipitation total during the course of the year can be used to classify Germany’s Fig. 2 West-east section of grid boxes for mean half-year precipitation totals during the reference period 1961–1990 at latitude 51° 10' north precipitation into regional types (Fig. 3). But the distribution of the quantified monthly pre- cipitation depths also depends on the season. Figure 4 illustrates this by evaluating all grid boxes with monthly precipitation depths for the months of February and June during the ref- The precipitation pattern in Germany is not only determined by the prevailing weather condi- erence period 1961–1990. This shows that monthly precipitation totals in summer often tions, but is also modified by geographical and orographical influences. For this reason, the exceed the winter totals by more than 40 mm. The reason for this is again the fact that in the course of the precipitation pattern during the year is divided into the following regional types summer months there is an increase in convective precipitation (showers) which can result in (Fig. 3, A-E): substantial local precipitation, even in the lowlands. Sea Type (A) 140000 The delayed cooling and warming of the sea water compared to the mainland temperature FebruarFebruary causes a distortion in the course of the precipitation pattern during the year, resulting in a pre- 120000 JuniJune cipitation minimum in spring and a maximum in autumn; 100000 Summer Type (B + C) 80000 This type is characterised by a maximum in the summer months because the heating of the ground, caused by radiation, mostly results in convective precipitation.