Urban Effects on Summertime Air Temperature in Germany under Climate Change

The impact of climate change on urban relative to rural summertime air temperatures is investigated for nine major cities in Germany, under the Representative Concentration Pathway trajectory 8.5. The analysis is based on simulations of representative historical and projected future summers with the regional climate mode COSMO-CLM in conjunction with the multilayer urban parametrization DCEP when driven by three global circulation models (GCM). Each GCM provides boundary conditions for two historical and two future summers that resemble average conditions for Germany with respect to 2-m air temperature statistics for the periods 1976–2005 and 2031–2060, respectively. The simulations show climate change signals (CCS) in summer mean urban-rural air temperature differences of the city ensemble of up to 0.15K. Across all driving GCMs, the largest changes are projected for the city of Berlin. Characteristics of the diurnal courses of urban-rural air temperature differences, the magnitude of the CCS and its physical reasons are GCM specific. Specifically, CCS are caused either by positive or negative changes in the Bowen ratio of the rural boundary with little changes in the urban surface fluxes, or vice versa. The study emphasizes the importance of the driving GCM in COSMO-CLM simulations when investigating urban effects on air temperature under global climate change.