Simulation Of Radiative Forcing Of Typical Aerosols And Their Effects On Climate

Atmospheric abundances of aerosols have increased since the preindustrial period due to anthropogenic activities. Aerosols can affect climate in several ways. First, aerosol particles can directly scatter or absorb infrared and solar radiation, thereby disturbing the energy budget of the earth-atmosphere system. Second, aerosol particles acting as cloud condensation or ice nuclei can change cloud microphysical and radiative properties and cloud lifetime, and hence indirectly affect the climate. It is very difficult to assess the impact of different effects of aerosols by measurements, although improvements have been made lately. Thus, we finish an on-line coupled model of aerosol?radiation?climate based on the AGCM developed by the National Climate Center of the China Meteorological Administration (NCC/CMA), called BCC_AGCM2.0.1, unidirectionally driving a physical and chemical model of aerosols (CUACE_Aero) developed by the Center for Atmosphere Watch and Services of Chinese Academy of Meteorological Sciences in this work. The concentrations and optical properties of typical aerosols are simulated, and the radiative forcing of aerosols and their effects on global and regional climate are discussed based on the above coupled model. The major conclusions are as follows:(1) The simulated global annual mean column burdens of sulfate, black carbon (BC), organic carbon (OC), dust and sea salt are 1.74 mg m^-2, 0.14 mg m^-2, 1.31 mg m^-2, 40.8 mg m^-2and 14.7 mg m^-2, respectively, using the source emissions that are derived primarily from AEROCOM data. The simulated optical depth, single scattering albedo, and asymmetry parameter of total aerosols (including sulfate, BC, OC, dust, and sea salt) are basically consistent with AERONET observations, except for an obvious underestimated aerosol optical depth (AOD) over South America. The mean relative errors of the simulated single scattering albedo and asymmetry parameter against observations are 4% and 5%, respectively.(2) The simulated global annual means of direct radiative forcing (DRF) due to total aerosols and three species of aerosol mainly produced by human activities (sulfate, BC, and OC) at the top of the atmosphere (TOA) are -2.03 W m^-2 and -0.23 W m^-2 respectively, under all sky conditions. The global annual mean DRFs of sulfate, BC, OC, dust and sea salt at the TOA are -0.19 W m^-2, +0.1 W m^-2, -0.15 W m^-2, -0.9 W m^-2 and -0.83 W m^-2, respectively. The annual mean changes of the net shortwave radiative flux at the TOA and surface due to the direct and semi-direct effects of total aerosols are approximately -3.1 W m^-2 and -3.9 W m^-2, respectively. Consequently, the global annual means of the surface temperature and precipitation rate decrease by 1.6oC and 0.14 mm day-1, respectively.The summer seasonal average DRFs due to sulfate, BC, and OC in East Asia at the TOA and surface are -1.4 W m^-2 and ?3.3 Wm^-2, respectively, leading to decreases of 0.58oC and 0.14 mm d-1 in the summer means of surface temperature and precipitation rate in this area, respectively. The differences of land-sea surface temperature and surface pressure are reduced and the local circulation is changed in East Asian monsoon region due to the direct and semi-direct effects of these aerosols, thus leading to the weakening of East Asian summer monsoon, and moreover decreasing of the summer monsoon precipitation in south and east China.(3) The study yields a global annual mean of ?1.57 W m^-2 for the first indirect radiative forcing of aerosols at the TOA. The second indirect effect of aerosols leads to global annual mean changes in net shortwave flux of ?0.58 W m^-2 at the TOA. The total aerosol indirect effect (AIE) reduces the global annual means of net shortwave flux at the TOA, surface temperature and precipitation rate by 2.27 W m^-2, 0.12oC and 0.03 mm day-1, respectively. In summer, the northeasterly or northerly flows in most areas of east and south China and over the nearby oceans are enhanced due to the total AIE, which weakens the transport of warm and moist air carried by the East Asian summer monsoon, and decreases the summer monsoon precipitation in east and south China.(4) The results show that the global annual mean surface radiative forcing due to BC in snow/ice is +0.042 W m^-2, with maximum forcing found over the Tibetan Plateau and regional mean forcing exceeding +2.8 W m^-2. The global annual mean surface temperature is increased 0.071oC due to BC in snow/ice. Positive surface radiative forcing is clearly shown in winter and spring which increases the surface temperature of snow/ice in the Northern Hemisphere. Snow-melt rates are also increased greatly, leading to earlier snowmelt and peak runoff timings. With the rise of surface temperature in the Arctic, more water vapor could be released into the atmosphere, allowing for easier cloud formation, which could lead to higher thermal emittance in the Arctic. However, the total cloud radiative forcing could be decreased due to the increasing of cloud cover, which will offset some of the cloud positive feedback mechanism.