Urban effects on summertime air temperature in Germany under climate change

The impact of global 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 summers with a regional climate model (RCM) in conjunction with a multilayer urban parametrization when driven by three global circulation models (GCMs). The GCMs are selected because they project different warming rates for Germany. Specifically, each GCM provides boundary conditions for the RCM for two past and two future summers that resemble average conditions for Germany with respect to 2-m air temperature statistics during a historical (1976-2005) and a future (2031-2060) period. Thus, one emphasis of our study is on the effects of the different driving GCMs on urban-rural air temperature differences (Delta Tu-r). The simulations show climate change signals (CCSs) in summer-mean Delta Tu-r of the city ensemble of up to 0.15 K. Across all driving GCMs, the largest changes are projected for the city of Berlin. Characteristics of the diurnal courses of Delta Tu-r, the magnitude of the CCS and its physical reasons are GCM specific. Specifically, CCSs are caused either by positive or negative changes in the Bowen ratio of the rural boundary with little change in the urban surface fluxes, or vice versa. The study emphasizes the importance of the driving GCM in RCM simulations when investigating urban effects on air temperature under global climate change.