Exploring Process Controls on Global Cloud Droplet Number Concentrations (Invited)

Changes in cloud droplet number concentration (N_d) over the industrial period control aerosol indirect radiative forcing of climate. Large biases persist in state-of-the-art GCM simulations of present-day N_d, and likely also affect changes in N_d over the industrial period. In many pristine regions, key uncertainties are associated with the number concentrations and activation of Aitken-mode aerosols, which mainly result from new particle formation. In this talk I will describe global and regional simulations aimed at disentangling and improving simulated aerosol and cloud processes that control N_d in different situations, focused on the marine atmosphere. I will explain how we are using satellite, surface and aircraft observations to evaluate simulations of N_d in the Met Office Unified Model. I will invite discussion of the extent to which fixing biases in an over-parameterized climate model can constrain pre-industrial to present-day changes.