Participate to model intercomparisons and -evaluations regarding aerosol indirect effects; and to re-use the data from past experiments in future assessments. Achievement: Contributed to the first indirect effect intercomparison (Penner et al., 2006), and led the second phase indirect effect intercomparison study (Quaas et al., 2009). Penner, J., et al., Model intercomparison of indirect aerosol effects, Atmos. Chem. Phys., 6, 3391–3405, 2006; Quaas, J., et al., Aerosol indirect effects - general circulation model intercomparison and evaluation with satellite data, Atmos. Chem. Phys., 9, 8697–8717, 2009.