Simulation of dimming and brightening in Europe from 1958 to 2001 using a regional climate model

The present study applies a regional climate model with coupled aerosol microphysics and transport in order to simulate dimming and brightening in Europe from 1958 to 2001. Two simulations are performed, one with transient emissions and another with climatological mean emissions over the same period. Both simulations are driven at the lateral boundaries by the ERA-40 reanalysis and by large-scale aerosol concentrations stemming from a global simulation. We find distinct patterns of dimming and brightening in the aerosol optical depth and thus clear-sky downward surface shortwave radiation (SSR) in all analyzed subregions. The strongest brightening between 1973 and 1998 under clear-sky conditions is found in mid-Europe (+3.4 W m(-2) per decade, in line with observations). However, the simulated all-sky SSR is dominated by the surface shortwave cloud radiative forcing (CRF). The correlation coefficient R between 5 year moving averages of the CRF and all-sky SSR equals 0.87 for all of Europe. Both model simulations show a similar evolution of cloud fraction and thus all-sky SSR due to the constrained circulation induced by the reanalysis at the lateral boundaries. For most subregions, the modeled differences in all-sky SSR due to transient versus climatological emissions are insignificant in comparison with estimates of the model's internal variability.