The indirect effects of aerosols as simulated by the SP-CAM multiscale modeling framework using current versus pre-industrial global aerosol distributions

The indirect effects of aerosols on the radiative forcing are one of the major sources of uncertainties of the climate change as simulated by the global climate models (GCMs). Traditional GCMs have grid spacing that is too coarse to resolve individual clouds; hence virtually all the interactions of clouds with the radiation and resolved large-scale dynamics need to be represented using cloud parameterizations. Accordingly, the interactions between the clouds and aerosols have also been parameterized. In a new kind of GCM, a Multiscale Modeling Framework (MMF), the conventional cloud parameterizations in each grid column are replaced with a small-domain coarse-resolution cloud-resolving model (CRM), often called a "superparameterization". The CRMs develop clouds in response to the large-scale dynamics and radiative tendencies. A CRM in one of the first such MMFs, the SP-CAM, has recently been upgraded to run a two-moment bulk cloud microphysics scheme. The scheme's prognostic variables include mass water content and number concentration for all liquid and solid water species such as cloud liquid and ice water, rain, snow and graupel. The effect of aerosols on simulated clouds is modeled through the specified globally and monthly varying fields of cloud-condensation nuclei (CCN) and ice nuclei (IN). The CCN and IN fields are derived from several species of aerosols as prescribed in the NCAR Community Atmosphere Model (CAM), which is the host GCM for the SP-CAM MMF. The CCN count is derived from the sulfate aerosols, sea salt and organic aerosol fields, while IN count is derived primarily from dust. The results of the global climate simulations using current and pre-industrial global aerosol distributions will be contrasted.