Aerosol processing in mixed‐phase clouds in ECHAM5‐HAM. Model description and comparison to observations

Abstract

The global aerosol‐climate model ECHAM5‐HAM has been extended by an explicit treatment of cloud‐borne particles. Two additional modes for in‐droplet and in‐crystal particles are introduced, which are coupled to the number of cloud droplet and ice crystal concentrations simulated by the ECHAM5 double‐moment cloud microphysics scheme. Transfer, production, and removal of cloud‐borne aerosol number and mass by cloud droplet activation, collision scavenging, aqueous‐phase sulfate production, freezing, melting, evaporation, sublimation, and precipitation formation are taken into account. The model performance is demonstrated and validated with observations of the evolution of total and interstitial aerosol concentrations and size distributions during three different mixed‐phase cloud events at the alpine high‐altitude research station Jungfraujoch (Switzerland). Although the single‐column simulations cannot be compared one‐to‐one with the observations, the governing processes in the evolution of the cloud and aerosol parameters are captured qualitatively well. High scavenged fractions are found during the presence of liquid water, while the release of particles during the Bergeron‐Findeisen process results in low scavenged fractions after cloud glaciation. The observed coexistence of liquid and ice, which might be related to cloud heterogeneity at subgrid scales, can only be simulated in the model when assuming nonequilibrium conditions.