Improving aerosol activation in the double-moment Unified Model with CLARIFY measurements

Abstract. Representing the number and mass of cloud and aerosol particles independently in a climate, weather prediction or air quality model is important in order to simulate aerosol direct and indirect effects on radiation balance. Here we introduce the first configuration of the UK Met Office Unified Model in which both cloud and aerosol particles have double-moment representations with prognostic number and mass. The GLOMAP aerosol microphysics scheme, already used in the HadGEM3 climate configuration, is coupled to the CASIM cloud microphysics scheme. We demonstrate the performance of the new configuration in cloud-resolving simulations of a case study defined from the CLARIFY aircraft campaign in 2017 near Ascension Island in the tropical south Atlantic. We improve the physical basis of the activation scheme by representing the effect of existing cloud droplets on the activation of new aerosol, and we also attempt to account for the effect of unresolved vertical velocities. The first of these improvements should be applicable to the representation of aerosol activation in other microphysics schemes. While these changes lead only to a modest improvement in model performance, they reinforce our confidence in the ability of the model to simulate aerosol-cloud microphysical interactions. Capturing these interactions accurately is critical to simulating aerosol effects on climate.