Advanced two-moment bulk microphysics for global models. Part II: Global model solutions and aerosol-cloud interactions

Abstract

A modified microphysics scheme is implemented in the Community AtmosphereModel, version 5 (CAM5). The new scheme features prognostic precipitation. The coupling between the microphysics and the rest of the model is modified to make it more flexible. Single-column tests show the new microphysics can simulate a constrained drizzling stratocumulus case. Substepping the cloud condensation (macrophysics) within a time step improves single-column results. Simulations of mixed-phase cases are strongly sensitive to ice nucleation. The new microphysics alters process rates in both single-column and global simulations, even at low (200 km) horizontal resolution. Thus, prognostic precipitation can be important, even in low-resolution simulations where advection of precipitation is not important. Accretion dominates as liquid water path increases in agreement with cloud-resolving model simulations and estimates from observations. The new microphysics with prognostic precipitation increases the ratio of accretion over autoconversion. The change in process rates appears to significantly reduce aerosol-cloud interactions and indirect radiative effects of anthropogenic aerosols by up to 33% (depending on substepping) to below 1Wm-2 of cooling between simulations with preindustrial (1850) and present-day (2000) aerosol emissions.