A Modeling Study of the Climate Effects of Sulfate and Carbonaceous Aerosols over China

In this paper, the RIEMS 2.0 model is used to simulate the distribution of sulfate, black carbon, and organic carbon aerosols over China （16.2°-44.1°N, 93.4°-132.4°E） in 1998. The climate effects of these three anthropogenic aerosols are also simulated. The results are summarized as follows： （1） The regional average column burdens of sulfate, BC, OC, and SOC were 5.9, 0.24, 2.4, and 0.49 mg m-2, with maxima of 33.9, 1.48, 7.3, and 1.1 mg m-2, respectively. The column burden and surface concentration of secondary organic carbon accounted for about 20% and 7%, respectively, of the total organic carbon in eastern China. （2） The radiative forcings of sulfate, organic carbon, and black carbon at the top of the atmosphere were -1.24, -0.6, and 0.16 W m 2 respectively, with extremes of -5.25, -2.6, and 0.91 W m-2. （3） The surface air temperature changes caused by sulfate, organic carbon, and black carbon were -0.07, -0.04, and 0.01 K, respectively. The air temperature increase caused by black carbon at 850 hPa was higher than that at the surface. The net effect of the three kinds of anthropogenic aerosols together decreased the annual average temperature by -0.075 K; the maximum value was -0.3 K. （4） Black carbon can reduce the precipitation in arid and semi-arid areas of northern China and increase the precipitation in wet and semi-wet areas of southern China. The average precipitation increase caused by black carbon in China was 0.003 mm d^-1. The net effect of the three kinds of anthropogenic aerosols was to decrease the precipitation over China by 0.008 mmd ^-1.

THE ANALYSIS OF MECHANISM OF IMPACT OF AEROSOLS ON EAST ASIAN SUMMER MONSOON INDEX AND ONSET

RegCM4.3, a high-resolution regional climate model, which includes five kinds of aerosols(dust, sea salt,sulfate, black carbon and organic carbon), is employed to simulate the East Asian summer monsoon(EASM) from 1995 to 2010 and the simulation data are used to study the possible impact of natural and anthropogenic aerosols on EASM.The results show that the regional climate model can well simulate the EASM and the spatial and temporal distribution of aerosols. The EASM index is reduced by about 5% by the natural and anthropogenic aerosols and the monsoon onset time is also delayed by about a pentad except for Southeast China. The aerosols heat the middle atmosphere through absorbing solar radiation and the air column expands in Southeast China and its offshore areas. As a result, the geopotential height decreases and a cyclonic circulation anomaly is generated in the lower atmosphere. Northerly wind located in the west of cyclonic circulation weakens the low-level southerly wind in the EASM region. Negative surface radiative forcing due to aerosols causes downward motion and an indirect meridional circulation is formed with the low-level northerly wind and high-level southerly wind anomaly in the north of 25° N in the monsoon area, which weakens the vertical circulation of EASM. The summer precipitation of the monsoon region is significantly reduced,especially in North and Southwest China where the value of moisture flux divergence increases.

A Study of Shallow Cumulus Cloud Droplet Dispersion by Large Eddy Simulations

Cloud droplet dispersion is an important parameter in estimating aerosol indirect effect on climate in general circulation models（GCMs）.This study investigates droplet dispersion in shallow cumulus clouds under different aerosol conditions using three-dimensional large eddy simulations（LES）.It is found that cloud droplet mean radius,standard deviation,and relative dispersion generally decrease as aerosol mixing ratio increases from 25 mg-1（clean case） to 100 mg-1（moderate case）,and to 2000 mg-1（polluted case）.Under all the three simulated aerosol conditions,cloud droplet mean radius and standard deviation increase with height.However,droplet relative dispersion increases with height only in the polluted case,and does not vary with height in the clean and moderate cases.The mechanisms for cloud droplet dispersion are also investigated.An additional simulation without considering droplet collision-coalescence and sedimentation under the aerosol mixing ratio of 25 mg-1 shows smaller values of droplet mean radius,standard deviation,and relative dispersion as compared to the base clean case.This indicates that droplet collision-coalescence plays an important role in broadening droplet spectra.Results also suggest that the impact of homogeneous mixing on cumulus cloud droplet spectra is significant under all the three simulated aerosol conditions.In weak mixing（strong updraft） regions where clouds are closer to be adiabatic,cloud droplets tend to have larger mean radius,smaller standard deviation,and hence smaller relative dispersion than those in stronger mixing（downdraft or weak updraft） regions.The parameterized cloud optical depth in terms of cloud liquid water content,droplet number concentration,and relative dispersion is only slightly smaller than the result calculated from detailed droplet spectra,indicating that current parameterization of cloud optical depth as used in many GCMs is plausible for low clouds.