High spatial and temporal organization of changes in precipitation over Germany for 1951–2006

Temporal changes in daily precipitation observed at more than 2300 stations in Germany during the second half of the 20th century are analysed. Compared to other studies, this analysis is based on a very high spatial density of observation locations and complete areal coverage of Germany. Changes in four precipitation characteristics are investigated: (1) total amount of seasonal and monthly precipitation, (2) mean and 95% quantile (q95) of daily precipitation, (3) transition probabilities to quantify wet and dry spells, and (4) precipitation amounts for a 7-day event with return period 100 years. For all parameters, strikingly clear trend patterns in space and time (of the year) emerged. Stations with increasing and decreasing trends are never found in direct neighbourhood, but are well separated from each other. Changes are season and even month specific. These clear spatial and temporal patterns are an expression of the organization of precipitation mechanisms over Germany. These findings add a note of caution in regard to trend analyses: Spatially and temporally aggregated trend studies might not disclose the complete range of changes and might miss important details. Interestingly, the variability of daily precipitation has changed in parallel with the mean behaviour: Those regions and seasons that show an increase in mean show also an increase in standard deviation, leading to a disproportional increase in heavy precipitation. In addition, there is a tendency towards higher persistence, in particular, longer wet spells in winter, spring, and autumn, and longer dry spells in summer. If these trends continue, there will be an increasing potential for floods in winter and spring, and increasing problems for water availability in summer in regions that show signs of water stress today.