The Numerical Study Of Direct Radiative Forcing Due To Black Carbon And Its Effects On Climate

The global mean distribution and seasonal changes of direct radiative forcing due to black carbon (BC) at the top of atmosphere (TOA) and surface and the effect of cloud on BC direct radiative forcing have been investigated in this work by using the CAM3 (Community Atmosphere Model Version 3) developed by NCAR (National Center For Atmospheric Research). Additionally, we also discuss the influence of global BC on summer precipitation in China and the effects on Asian summer monsoon due to BC in the South Asia. Some significant conclusions are gained as following.There exist obviously seasonal changes in the global mean distribution and intensity of direct radiative forcing due to BC at the TOA and the surface in the influence of seasonal changes of emission of aerosols, surface albedo and solar altitude angle. At the TOA, the distribution of radiative forcing is the most extensive in spring, but the global mean value of radiative forcing is the most intensive in summer; the distribution and global mean intensity of radiative forcing are the least in winter. At the surface, the distribution and global mean intensity of radiative forcing are the most in summer and autumn; both are the least in winter.In cloudy sky, BC induces a positive radiative forcing at the TOA and negative radiative forcing at the surface. The global mean values of the direct radiative forcing at the TOA and the surface due to BC are +0.33 W/m~2 and -0.56 W/m~2, respectively. In clear sky, the corresponding values are +0.21 W/m~2 and -0.71 W/m~2, respectively. The positive forcing at the TOA is enhanced and the negative forcing at the surface is weakened because of the existing of cloud. The precipitation increases obviously in the north of China in summer between 30°N and 45°N, whereas it decreases obviously in the south of Yangtze River except for some cities of Hainan and Guangxi Provinces because of the effect of BC. The results of our simulations have not produced the "north drought/south flooding" precipitation pattern that has frequently occurred in the summer of China during the past 50 years.The South Asia that locates at the south of the Tibetan Plateau is an area of high value of BC emission. The air general circulation and Asian summer monsoon will be greatly affected due to the interaction between the effects of BC and the Tibetan Plateau. The results show that BC aerosol strongly absorbs solar radiation and heated the lower-to-mid troposphere in the South Asia during the late spring. This may lead to an advance of the rainy periods of the Bay of Bengal and its coast and the outbreak of the Indian summer monsoon. In summer, the heated air ascends along the southern slope of the Tibetan Plateau and forms a steady heating layer at upper air that gives birth to a local motion of deep convection. In this way, a meridional circulation that ascends in the north and subsides in the south appears between the continent of the South Asia and the Indian Ocean, which causes the northward motion to enhance and intensifies the Indian summer monsoon. But the BC of the South Asia weakens the East Asian summer monsoon by impacting on the surface pressure, vertical motion, precipitation, wind field of 850 hPa and so on, and induces western Pacific Subtropical High northward shift and westward extension, and causes the Mei-yu rain belt of China to shift northeastward.