| Thirty-nine aerosol vertical distribution and liquid water cloud aircraft observations from the field studies (the National (CHINA) Key Program 'Precipitation Enhancement Technique and Demonstration') have been analyzed to investigate distribution of aerosol in boundary layer of Beijing, the variation of the effective radius of cloud droplets in layer clouds, the first and second indirect aerosol effects and compared the relationship between cloud droplet effective radius and cloud albedo for clean and polluted clouds by using the data set from RACE field experiments, which is supposed to represent coditions in clean area. The results indicate that the number concentration of below-cloud aerosol particles (>0.10μm diameter) was correlated with cloud droplet number concentration. Particle number was anticorrelated with droplet size. For the same liquid water path the polluted clouds have more and smaller cloud droplets and thus a higher cloud albedo and less drizzle size drops. The effective radius is positively correlated with cloud albedo for polluted clouds caused by the absence of drizzle size drops. Conversely effective radius is negatively correlated with cloud albedo for clean clouds. These results verify that higher particle concentrations do directly affect the microphysics of stratiform clouds.A warm cloud microphysical parameterization scheme was incorporated into a regional model to study the sensitivity of cloud-radiative properties and precipitation to aerosols by simulating a case of March 17 2000 from ARM field campaign in United States of America.. Assuming a tri-modal lognormal aerosol size distribution, the aerosol numbers are explicitly calculated from prognostic aerosol masses, considering advection, diffusion, and cloud and rain drop activation/deactivation processes. Clean continental, average continental and urban aerosols, each with different modal parameters, were used to serve as condensation nuclei of cloud and rain drops, whose activations depend on super-saturation and aerosol composition. Consistent with other studies, simulation indicate that more aerosols result in more cloud water, and more but smaller cloud drops, yielding increases in cloud albedo and decreases in surface precipitation. The presence of giant nuclei increases both the cloud drop effective radius and the precipitation, while the use of volumetric cloud drop radius tends to calculate larger cloud solar radiative forcing than the use of effective cloud drop radius.Simulations were also conducted over Beijing during summer of 2005 when aircraft and ground measurements, in particular the aerosol and cloud size distributions, were available. In those simulations, a tri-lognormal aerosol size distribution is used with the accumulation mode fitted to the aircraft measurements. Experiments were conducted for 4cases, including six periods aircraft measurements. Result show that the model is capable of catching the circulation, and the temporal and spatial distribution of observed precipitation. In addition, the model also can simulate the observed effective cloud drop radius, although the model over estimates the cloud water. Sensitivity experiments using the clean continental and urban aerosol models were conducted to compare the effects of different aerosols on the cloud radiative properties. It is found that more aerosols result in more but smaller cloud drops and more cloud water, leading to an increase in albedo. The effects on rains are complex: smaller cloud drops has less efficient coagulation for raindrop to form (autoconversion), leading to fewer raindrop numbers and less rainwater; while more cloud water enhances the accretion for raindrop growth. Nevertheless, for the whole cloud system, the impact of increasing aerosols inhibits precipitation. |
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