Projekt A 1.1.2:
Long-term Behaviour of Areal Precipitation
in Baden-Wuerttemberg and Bavaria
Summary of the results of the project
(valid as of 1997)
For years now a lively discussion has been under way, both among experts and the general public, on the causes and consequences of the climate change. The hydrologic cycle, as basis for a number of energy and material cycles, is immediately affected by climate change. As a consequence of the worldwide observed warming, an intensification of the hydrologic cycle and the pertaining components of the water budget such as precipitation in general is assumed, as a result of the physical relations. However, due to the atmospheric circulation, differing developments can also occur regionally and seasonally, resulting in deviations from this general trend. According to the IPCC Report (2001) the mean annual precipitation (period 1900 - 1999) has risen in central Europe (30°-55° N) by around 7%, with the trend increasing in the past 25 years. Albeit for Germany the time series of the areal mean of the annual precipitation (1901-2000) shows only a slight, statistically insignificant rise (DWD Climate Status Report 2001, author Müller-Westermeier). Regions in Germany with low precipitation, such as Brandenburg, even showed a downward trend in precipitation.
The KLIWA Project A 1.1.2 (Long-term behaviour of areal precipitation in Baden-Württemberg and Bavaria) is concerned with the question as to how the mean precipitation in the comparatively rainy regions of southern Germany has developed in the past. According to IPCC a change in the mean value also means changes in the extreme values of the precipitation. These have a direct effect on society, especially in the nature of floods or drought. In the evaluation of the highest annual values of precipitation at stations in southern Germany, it turned out that there was, for example, a statistically significant increase in the six months of the hydrological winter (November-April), which occurred with differing regional intensity (see KLIWA Project A 1.1.3 (Long-term behaviour of heavy precipitation in Baden-Württemberg and Bavaria).
Course of Action
In order to be able to estimate such changes and their magnitude, both extensive evaluation of historic climatological and hydrological data and also the intensive study of scenarios are necessary. The paper presented is a contribution to the first of these two subtasks and the work was carried out by the Hydrometeorology Department of the DWD on behalf of the State Office for Environmental Protection in Baden-Württemberg and the Bavarian State Office for Water Management.
The subject of the investigation was in each case a series of diurnal values of areal precipitation for each of the 33 KLIWA investigation areas in Baden-Württemberg and Bavaria for the period 1931 to 1997 (see fig. 1 and tab. 1). The basis for the computation of the area value series was the interpolation of the precipitation observations from 1815 stations on a grid with an edge length of around 7 km (2139 grid point rows) by means of a geostatical method (KLIWA Project A 1.1.1. - Provision of long series of interpolated grid point values of precipitation for Baden-Württemberg and Bavaria). A big advantage of this course of action for the statistical analysis of areal precipitation lies in the minimisation of inhomogeneity at certain stations that influences trends, and an areal statement for certain river areas.
Tab 1: KLIWA areas of investigation with number of grid points.
On the basis of the diurnal values of the areal precipitation, five statistical characteristic quantities, i.e. diurnal mean value, standard deviation, lower quartile, median and excess frequency of 15 mm/d, were determined for each month of the period under investigation (1931 - 1997). The monthly value series resulting therefrom (extent of spot checks - 804 values in each case), were subject to a time series analysis by means of different investigation methods, in order to determine inhomogeneities and the general trend of developments.
When describing the long-term processes, this paper concentrates on the meteorological summer (June to August) and the meteorological winter (December to February), as it is precisely the opposing tendencies of these two yearly sections that show up the occurring changes, while spring and autumn have less meaning as "transition periods". The study also contains statements relating to the calendar year.
Changes relating to the calendar year
The changes with regard to the calendar year can be summarised as follows:
Changes in summer
- In the course of the period under investigation the annual precipitation total only shows a significant increase of 0.1 % to 0.2 % per annum in six areas of Baden-Württemberg and Bavaria. In more than 80 % of the areas there was no evidence of a tendentious development.
- More distinct than the change in the diurnal precipitation mean is the change is the frequency distribution. Both the "dry" days, i.e. days without or with only slight precipitation, as well as the diurnal precipitation values with above-average, however not extreme amounts, are on the increase.
- The frequency of days with very heavy areal precipitation (more than 15 mm/d) is increasing only in a part of the eastern areas (Vils, Rott and Naab river areas, and upper reaches of the Main, and in the south-west (river areas of the Neckar above Fils, and the Enz), while in certain areas in the north-west (Tauber and middle stretches of the Main) there is a decrease.
The results for the meteorological summer show the following changes:
- The majority of the areas under investigation is characterised by a decrease in the total precipitation, represented by the diurnal mean precipitation value (fig. 1). This decrease can mainly be attributed to the simultaneous decrease in the frequency of days with high areal precipitation.
- In comparison there is also an increase in the frequency of "dry "days. This change, however, does not have much effect on the change in the total precipitation (represented by the diurnal mean value).
- Another development was only recorded in southern Bavaria. Here, in contrast to the other areas, there is a slight increase in precipitation, which is caused by an increase in the frequency of days with heavy precipitation. However, this increase cannot be proved everywhere with sufficient significance. The area mainly affected is the Lower Bavarian hill country.
Fig. 1: Distribution of the areal precipitation trend (diurnal mean value) for the meteorological summer (JJA) in the 33 KLIWA areas of investigation in Baden-Württemberg and Bavaria, 1931 to 1997.
Changes in winter
The long-term development of the areal precipitation during the meteorological winter appears in certain respects as counterpart to the process in summer, even if not reflected in every area and for every quantity evaluated.
- In most areas there is an increase in precipitation in winter, which is characterised by both a rise in the mean diurnal precipitation (fig. 2) and a higher number of days with a large amount of precipitation.
- Comparatively speaking the Fore-Alps experience hardly any significant increase in precipitation. A slight increase does occur here, but can be belayed statistically only in a very few cases.
Fig. 2: Distribution of the areal precipitation trend (diurnal mean value) for the meteorological winter (DJF) in the 33 KLIWA areas of investigation in Baden-Württemberg and Bavaria, 1931 to 1997.
All statements on the general tendencies in the whole period of investigation (1931 to 1997) should be augmented by the fact that many changes in areal precipitation took place mainly in the 1960's and 1970's. On the one hand they are shown in a redistribution of the precipitation during the course of a year, and on the other in a change in the frequency distribution of the diurnal values of the areal precipitation.
In the project possible causes for the detected changes were also investigated, i.e. whether there are relations between the time series behaviour of the precipitation and the seasonal frequency of the atmospheric forms of circulation (zonal, meridional, mixed), based on the catalogue of the synoptic situations in central Europe (Gerstengarbe et al. 1999). A comparison of the areal value series with the long-term series of the frequency of certain weather situations in central Europe in winter shows that both developments are connected with each other and that the increase of precipitation in winter is related to a higher frequency of the zonal and mixed forms of circulation.
Finally it can be said that, without seasonal differentiation, precipitation will change very little long-term. However, it will rain less frequently in summer and there will be fewer cases of extreme precipitation, whereas in winter both the amount and the variability of precipitation and thus the frequency of extreme values will increase. Some of these changes had already begun before or around 1960, and others did not happen until towards the end of the 1970's.