Parameterization of cloud droplet formation in large scale models: including effects of entrainment

A parameterization of aerosol activation into cloud droplets is presented for usage in large scale models. This parameterization is based on cloud parcel and Köhler theory, but, unlike previous attempts in the literature, relaxes the adiabaticity constraint and explicitly represents the effect of entrainment on droplet number and size distribution. The parameterization includes a comprehensive description of droplet growth kinetics based on the concept of "population splitting", which allows explicit treatment of the effects of surfactants, organic films and partially soluble species on cloud droplet formation. We also put forward the concept of "critical entrainment rate", a value beyond which droplet activation is not favored, and influences the response of clouds to entrainment. The parameterization was evaluated against the solution of a detailed entraining numerical parcel model over a large set of conditions and exhibited excellent performance. This work offers for the first time a robust, first principles approach for linking aerosol chemical effects, entrainment, and aerosol activation within a global climate modeling framework.