Decadal-scale trend of surface solar radiation simulated by a global aerosol transport-climate model

Long-term records of surface radiation measurements indicate a decrease in the solar radiation between the 1950s and 1980s (“global dimming”), then its recovery afterward (“global brightening”) at many locations all over the globe (Wild 2009; Ohmura 2009). It is suggested that these trends of the global dimming and brightening are strongly related with a change in aerosol loading in the atmosphere which affect the climate change through the direct, semi-direct, and indirect effects. In this study, trends of the surface solar radiation are analyzed based on the hindcast simulation from the year 1980 to 2009 according to the AeroCom (Aerosol Model Intercomparison Project) Phase II experiment using the global aerosol transport-climate model, SPRINTARS.

SPRINTARS is coupled with MIROC which is a general circulation model (GCM) developed by Center for Climate System Research (CCSR)/University of Tokyo, National Institute for Environmental Studies (NIES), and Research Institute for Global Change (RIGC)/Japan Agency for Marine-Earth Science and Technology (JAMSTEC) (Takemura et al. 2000, 2002, 2005, 2009). The horizontal and vertical resolutions are T106 (approximately 1.1˚ by 1.1˚) and 56 layers, respectively. SPRINTARS includes the transport, radiation, cloud, and precipitation processes of all main tropospheric aerosols (black and organic carbons, sulfate, soil dust, and sea salt). The model treats not only the aerosol mass mixing ratios but also the cloud droplet and ice crystal number concentrations as prognostic variables, and the nucleation processes of cloud droplets and ice crystals depend on the number concentrations of each aerosol species. Changes in the cloud droplet and ice crystal number concentrations affect the cloud radiation and precipitation processes in the model. The emissions of soil dust, sea salt, and dimethylsulfide (DMS) from oceanic phytoplankton and land vegetation are calculated within the model. The other historical emissions, that is consumption of fossil fuel and biofuel, biomass burning, aircraft emissions, and volcanic eruptions are prescribed from database provided by the Global Aerosol Model Intercomparison Project (AeroCom).

There are general agreements between the simulation and long-term records of the surface solar radiation in many regions. An analysis of the simulated results shows that year-to-year variations and decadal trends of it are strongly related to the cloud fraction and aerosol direct radiative forcing, respectively. There are, however, significant differences between the simulation and observations in some regions, suggesting a problem in traditional bottom-up emission inventories.

Acknowledgments. The simulation in this study was performed on the NIES supercomputer system (NEC SX-8R). This study is partly supported by the Global Environment Research Fund (RF-0901) by the Ministry of the Environment of Japan, Grant-in-Aid for Young Scientist (21681001) by the Ministry of Education, Culture, Sports, Science, and Technology of Japan, and Mitsui & Co., Ltd. Environment Fund (R08-D035).