Investigating Ice Nucleation with an Aerosol Enabled Multi-scale Modeling Framework

Ice nucleation processes involving aerosols are key to the formation and properties of cold clouds, and thereby impact latent and radiative heating processes in the global energy budget and hydrological circulation. However, compared to droplet formation in warm clouds, ice nucleation is more difficult to represent in atmospheric models, due to complicated nucleation mechanisms responsible for ice formation under various circumstances. It is even more challenging to simulate ice nucleation in global models, because accurate cloud treatments and aerosol schemes are required. In this study, a new multi-scale modeling framework (MMF) is employed to investigate ice formation mechanisms in the development of mixed phase and cirrus clouds. In the MMF approach, cloud processes are explicitly calculated by cloud-resolving models (CRMs) embedded in each grid column of the NCAR Community Atmosphere Model (CAM5), and aerosol processes are driven by the CRM results. A new ice nucleation scheme based on recent research has been implemented. The ice nucleation scheme takes into account aerosol properties, including composites and concentrations. Ice formation in cirrus clouds features competition between homogenous and heterogeneous freezing, and in mixed phase clouds, deposition nucleation is related to number concentrations of dust and black carbon aerosols. The ice nucleation results are compared with observations in an Arctic mixed phase cloud region to investigate aerosol effects on the persistence of these clouds. In the ITCZ and storm track regions, explicit parameterization of clouds and aerosols helps improve our understanding of how convective transport of aerosols influences ice cloud development.