Simulation of aerosol effects on climate system by aerosol climate model

In this study effects of anthropogenic aerosols on the climate system are analyzed from simulations by a general circulation model, MIROC, coupled with an on-line aerosol transport-radiation model, SPRINTARS. The MIROC is a coupled atmosphere-ocean general circulation model developed by Center for Climate System Research, University of Tokyo (CCSR), National Institute for Environmental Studies (NIES), and Frontier Research Center for Global Change (FRCGC). The SPRINTARS is one of the components in the MIROC to simulate aerosol distributions and interaction with the climate system. It treats the aerosol direct, semi-direct, and indirect effects due to main tropospheric aerosols, i.e., black carbon (BC), organic carbon (OC), sulfate, soil dust, and sea salt. The aerosol transport processes include emission, advection, diffusion, sulfur chemistry, wet deposition, dry deposition, and gravitational settling. The emission of natural aerosols, such as soil dust, sea salt, and DMS that is a precursor gas of sulfate, are calculated on-line with some parameters (wind velocity, vegetation, etc.). Historical data of anthropogenic emissions for BC, OC, and SO₂ are edited by the NIES research group based on several emission and statistical database. The radiation scheme in MIROC is extended for the aerosol direct effect related to scattering and absorption by aerosol particles. The cloud droplet number concentration is diagnosed by a parameterization considering Köhler theory, which includes not only aerosol particle number concentration but also the updraft velocity, size distributions and chemical properties of each aerosol species, and saturation condition of the water vapor. Then the diagnosed cloud droplet number concentration changes the cloud droplet effective radius and precipitation rate. The global mean radiative forcings of the direct and indirect effects at the tropopause by anthropogenic aerosols are calculated to be -0.1 and -0.9 W m^-2, respectively. The simulation suggests that the radiative forcing at the surface as well as the tropopause can be appropriate index to explain changes in the surface air temperature in the 20th century. A rapid increase in the negative radiative forcing at the surface by the aerosol effects can explain the cooling of the surface air temperature in the mid-20th century. The model is coupled with an ocean model to analyze the climate feedback mechanism caused by the aerosol effects. The ensemble equilibrium experiments are integrated for 50 years and analyzed for the last 30 years. The simulated results show that aerosols decrease the cloud droplet effective radius and increase the cloud water path through the cloud microphysical effect, similar to the recent analysis of satellite data. However, the aerosol-induced changes in surface insolation, evaporation, hydrological cycle, and regional meteorology principally due to the aerosol direct and first indirect effect decrease the cloud water path in a lot of regions. The simulation also suggests that the observed trend of precipitation in the 20th century can be partly explained by the aerosol effect.