CR 70:57-76 (2016)  -  DOI: https://doi.org/10.3354/cr01415

ABSTRACT: 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.

KEY WORDS: Cloud droplet number concentration · Climate model · Aerosol activation