Fei Luo, Adam Monahan, Knut von Salzen

Aerosol radiative forcing can be difficult to quantify both accurately and precisely in global climate models. Long climate model integrations are often required and levels of statistical uncertainty can be substantial for some of the diagnostic methods and diagnosed forcings. Instantaneous estimates of sulfate aerosol direct radiative forcings in the present-day climate are compared to forcings that were determined using different diagnostic methods based on nudged climate model simulations with specified sea surface temperatures and sea ice. Our results show that the accuracy and precision of aerosol radiative forcing estimates vary considerably among different methods. Nudging of model trajectories to constrain natural meteorological variability in the simulations leads to highly accurate and precise estimates of sulfate radiative forcings for a wide range of nudging parameters. The radiative forcing of black carbon aerosols, for which instantaneous estimates of the direct radiative forcing are not available, was also assessed using these methods. We show that the nudging process also significantly increases the precision of these radiative forcing estimates.