Implementation of a new empirical relationship between aerosol and cloud droplet concentrations in a climate model

A new empirical relationship between aerosol and cloud droplet number concentrations (N_aer and N_d, respectively) was established based on observed data from different regions around the world to implement in the Hadley Centre Global Environmental Model version 2-Atmosphere-Ocean (HadGEM2-AO). Compared to the original relationship used in HadGEM2-AO, which utilized observed data from only limited regions, this new relationship provides a lower N_d than the original relationship for N_aer < 680 cm^-3, and the opposite for N_aer > 680 cm^-3. The complete historical run with the new relationship (newHIST) shows a number of significant differences from the original historical run (HIST). N_d increased in industrialized regions while it generally decreased over the oceanic regions in newHIST compared to HIST. Interestingly, in newHIST a significant change in total cloud amount occurred mainly in the intertropical convergence zone (ITCZ) due to a change in the wind circulation pattern over the North Pacific. Notably, the surface temperature in newHIST was closer to the observational data than that in HIST - especially over the Northern Hemisphere. Despite the small change in N_d in the new relationship, not only the variables directly associated with cloud microphysics but also the variables representing atmospheric circulation patterns changed significantly. This implies that accurate parameterization of N_d is critical for better climate model prediction.