A Modeling Study Of Anthropogenic Aerosols’ Impacts On East Asian Climate

In this dissertation, the anthropogenic aerosols’impact on East Asian climate is investigated using the NCAR Community Atmospheric Model version5(CAM5), a state-of-the-art climate model considering aerosol’s direct, semi-direct and indirect effects. Five experiments were performed with prescribed monthly mean climatological SST and sea ice. Two experiments are perfomed with emissions of the year2000(PD experiment) and the year1850(PI experiment). The difference between PD and PI experiments represents the effect of all anthropogenic aerosols. Three additional experiments were performed in which sulfur (SO2and sulfate), BC, and POM emissions used data from the year1850, respectively, while keeping emissions of other aerosol species for the year2000. Thus, the effects of all anthropogenic aerosols, and anthropogenic black carbon (BC), sulfate, and primary organic matter (POM) are decomposed from the difference between these simulations and PD runs. The impacts of anthropogenic aerosols on summer monsoon, winter monsoon and spring precipitation are analyzed. The relative contribution of different aerosol species is discussed. We also investigate the seasonal variation of anthropogenic aerosols’ imapct and its possible reasons.The main conclusions are as follows:(1) The anthropogenic aerosols appear to weaken the East Asian summer monsoon, reduce the precipitation in North China and increase the precipitation in South China and in the adjacent ocean areas.The anthropogenic sulfate and POM reduce the solar flux reaching the surface directly by scattering the solar radiation, and indirectly by increasing the cloud droplet number concentration and cloud liquid water path over East China. The surface air temperature over land is reduced, and the precipitation in North China is suppressed. The cooling at surface will reduce the sensible heat transfer, stabilize the boundary layer, and reduce the precipitation at North China. Less condensation heating release at North China cools the up troposphere. This will reduce the thermal contrast between land and ocean and the monsoon circulation is thus weakened. The weaker low level south wind will transport less water vapor to North China and reinforce the process. Unlike anthropogenic sulfate and POM, anthropogenic BC does not have a significant effect on the air temperature at the surface, because of the reduction of the cloud liquid water path and the weakening of shortwave cloud forcing by its semi-direct effect.(2) The anthropogenic black carbon tends to reduce the snow albedo and warm the surface of Tibet Plateau during winter. The warming over Tibetan Plateau tends to weaken the East Asian winter monsoon and prevent cold air from transportation from Siberia to South China. All aerosols generally weaken the winter monsoon like BC.The anthropogenic aerosols increase most significantly at South China in winter. The cloud number concentration is increased and cloud droplet effective radius is reduced due to sulfate and POM. Thus, the large scale precipitation is suppressed in South China due to the second indirect effect. The black carbon decreases the snow albedo, increases the surface short wave flux and warms the surface of Tibetan plateau. The warming over Tibetan plateau persists from November through April of the next year. The winter monsoon circulation is weakened and the cold air in Siberia is prevented from moving to South China due to the warming. Sulfate and POM generally induce a reversed change of the monsoon circulation. All aerosols lead to the weakening of the winter monsoon like BC. The second indirect of sulfate and POM is also important in South China.(3) The anthropogenic aerosols tend to reduce the spring precipitation in South China through generating an anticyclone there. The southwest wind is strengthened, transporting more water vapor to North China and increasing the precipitation there.In spring, the clear sky short wave flux is reduced by9-18W m-2in South China due to the reflection of sulfate and POM and absorption of BC. The short wave cloud forcing change is-25W m-2at30°N. The surface air temperature is reduced in South China and the precipitation is suppressed. The liquid water path is increased by-90g m’in South China and it will reduce the large scale precipitation as well as the surface cooling. The precipitation is increased in North China which cannot be explained by either direct or indirect effect of aerosols. The significant reduction of precipitation in South China generates an anticyclone. This response enhances the southwest wind in South China, bringing more water vapor to North China, and increases the precipitation there.(4) The radiative effect of the anthropogenic aerosols is the most significant in spring and summer, with the maximum center in South China during spring and in North China during summer. The indirect effect of the anthropogenic aerosols is the most significant in spring. The aerosol optical depth (AOD) and short flux change shows a northward movement from South China to China from February to August and a southward movement from October to December. Similar patterns in radiative flux, cloud and precipitation change are also found. It is a direct proof of the anthropogenic aerosols’ impact on East China climate.The maximum of AOD and clear sky short wave flux change in East China is at25°N in spring and40°N in summer. The maximum of AOD and radiative flux change (representing the direct effect of aerosol) moves from25°N to38°N from February to August. The change of cloud droplet number concentration (CDNC) and cloud droplet effective radius at850hPa is large in winter half year (from October to next April). The cloud liquid water path and short wave cloud forcing also shows a northward movement. But the intensity becomes weak from spring to summer. This signal can be also found in the change of surface air temperature, surface heat flux, and convective precipitation.