Aerosol specification in single-column Community Atmosphere Model version 5

摘要

Single-column model (SCM) capability is an important tool for generalcirculation model development. In this study, the SCM mode of version 5 ofthe Community Atmosphere Model (CAM5) is shown to handle aerosolinitialization and advection improperly, resulting in aerosol, cloud-droplet,and ice crystal concentrations which are typically much lower than observedor simulated by CAM5 in global mode. This deficiency has a major impact onstratiform cloud simulations but has little impact on convective case studiesbecause aerosol is currently not used by CAM5 convective schemes andconvective cases are typically longer in duration (so initialization is lessimportant). By imposing fixed aerosol or cloud-droplet and crystal numberconcentrations, the aerosol issues described above can be avoided.Sensitivity studies using these idealizations suggest that the Meyers etal. (1992) ice nucleation scheme prevents mixed-phase cloud from existing byproducing too many ice crystals. Microphysics is shown to strongly depletecloud water in stratiform cases, indicating problems with sequentialsplitting in CAM5 and the need for careful interpretation of output fromsequentially split climate models. Droplet concentration in the general circulation model (GCM) versionof CAM5 is also shown to be far too low (~ 25 cm−3) at thesouthern Great Plains (SGP) Atmospheric Radiation Measurement (ARM) site.