Single-column model (SCM) capability is an important tool for general circulation model development. In this study, the SCM mode of version 5 of the Community Atmosphere Model (CAM5) is shown to handle aerosol initialization and advection improperly, resulting in aerosol, cloud-droplet, and ice crystal concentrations which are typically much lower than observed or simulated by CAM5 in global mode.The goal of this study is to test several fixes for this problem.
Three fixes to the aerosol specification problem in the SCM are evaluated. One solution is simply to fix cloud droplet and ice crystal concentrations at observed values. Another is to use aerosol concentrations specified by the prescribed aerosol functionality recently added to CAM. A third solution is to specify aerosol concentrations at observed values. We tested these fixes for 4 different SCM case studies: DYCOMS RF01 drizzling subtropical marine stratocumulus, MPACE-B mixed-phase Arctic stratocumulus, RICO tropical marine shallow convection, and ARM95 midlatitude continental deep convection.
The corrective measures listed above have a major impact on stratiform cloud simulations but has little impact on convective case studies because aerosol is currently not used by CAM5 convective schemes and convective cases are typically longer in duration (so initialization is less important). All corrective methods are successful at improving non-convective simulations. Over the course of our analysis, we found evidence that the Meyers et al. (1992) produces too many ice crystals, which prevents the existence of mixed-phase cloud. Microphysics is also shown to strongly deplete cloud water in stratiform cases, indicating problems with sequential splitting in CAM5 and the need for careful interpretation of output from sequentially split climate models. Droplet concentration in the general circulation model (GCM) version of CAM5 is also shown to be far too low ( ∼ 25 cm−3) at the southern Great Plains (SGP) Atmospheric Radiation Measurement (ARM) site.