Climate change signal of future climate projections for Aachen, Germany, in terms of temperature and precipitation

A multi-model ensemble of regional climate projections is used to estimate the climate change signal in terms of temperature and precipitation for the city of Aachen, Germany, until the end of the current century. Since the combination of heat and high moisture content in the ambient air is assumed to adversely affect human health, the equivalent temperature is employed as an indicator for heat stress and applied to identify heat waves. As prolonged periods of high temperatures may also cause summer drought, summer precipitation and dry periods are analyzed as well. The study refers to regional climate projections (A1B emission scenario) of the statistical climate models STARII and WETTREG and the dynamical climate models REMO and COSMO-CLM. The model outputs are compared a) with each other in order to assess discrepancies between them and b) with the 30-year baseline period of 1971-2000 in order to estimate future changes in heat load and precipitation patterns for the summer months of June, July and August. Based on median realizations of the model runs, further warming of approximately 1-2 K is expected by the middle of the 21st century and 3-4 K by 2100. These changes will probably be accompanied by an increase in variance. Maximum temperature shows the greatest enhancement (+ 1.9K 2031-2060 and 4.3K 2071-2100, ensemble mean) causing almost a doubling of (extreme) hot days and a trebling of tropical nights as soon as the middle of the century. According to this temperature development heat waves are likely to last longer and occur more often in future whereas their intensity is not expected to change significantly. An increasing number of heat waves is concurrent with a decreasing amount of rainfall in the summer months until 2100. All four climate projections indicate a decrease in precipitation in summer until 2100 with -17% on average. The highest decrease is shown by COSMO-CLM with -30% rainfall in June for the period of 2071-2100. Dry periods are expected to occur 3 times more often at the end of the current century and to last longer by 1 day (COSMO-CLM) to 3 days (WETTREG) compared to the period of 1971-2000.