## 6 Summary and Conclusions  
In this study, the impact of polluted (P-case) conditions versus typical (C-case) continent conditions on an ideal Mei-Yu front was simulated via WRF coupled with a bulk two-moment microphysics scheme [63]. The primary focus was on exploring physical mechanisms of simulated aerosol effects in the face of complex process interactions and feedbacks between the cloud microphysics and dynamics in Mei-Yu frontal system. We summarized three dynamic effects resulted from the microphysical mechanisms and revealed two microphysical/dynamic positive feedback loops in polluted Mei-Yu frontal environments. The main conclusions are as follows:  
1. Aerosols play an important role in affecting local precipitation around Mei-Yu fronts. Polluted conditions delay the onset of precipitation but strengthen precipitation during the intense precipitation period and increase the amount of total precipitation. This may be one of the reasons for the increased frequency of the intense Mei-Yu precipitation in recent years. Note that anthropogenic pollution has a significant effect on Mei-Yu front precipitation. This may be an important feature influencing climate change in East Asia. 2. Aerosols affect Mei-Yu frontal precipitation by first changing microphysical processes. Under polluted conditions, these more numerous and smaller cloud droplets suppressed auto-conversion and collision-coalescence processes, and hence resulted in late raindrop formation, which initially delays the onset of Mei-Yu precipitation. However, a larger amount of cloud water is transported to above the freezing level, increasing ice-phase particle growth by riming and the WBF processes, and producing large amounts of snow and graupel. The melting of snow and graupel is the main reason why rain increases. Furthermore, during the developing and mature stages, cold rain processes are more vigorous under polluted conditions. With cold-cloud processes \"towing\", warm-cloud progresses are encouraged. Thus, cold-cloud and warm-cloud interactions mutually boost each other. This is perhaps the more complicated and particular microphysical mechanism by which aerosols affects Mei-Yu frontal precipitation. 3. High aerosol concentrations first influence cloud microphysical processes, enhance latent heat release, and then strengthen updrafts, downdrafts, and low-level convergence, forming DDE. In turn, DDE further strengthens microphysical processes and creates the first positive feedback loop. In Mei-Yu frontal environments, the enhanced DDE and the cool pool effect result in stronger frontogenesis, which strengthens frontal dynamic processes. This further increases both vertical and horizontal transportation of water vapor and form VPE, which in turn strengthens microphysical processes and creates the second positive feedback loop. Many physical processes and effects interact with each other, forming and strengthening both microphysical-dynamic feedback loops, leading to the well-organized Mei-Yu front precipitation system. Overall, the combined effect is to increase Mei-Yu front precipitation. The interaction of microphysical processes and dynamic processes, and the positive feedback loops they create, are the main physical mechanisms behind the impacts of aerosol on Mei-Yu frontal precipitation. Notably, a series of ideal frontal experiments are carried out herein by using a domain-averaged initial sounding of a typical Mei-Yu system. The results could represent the typical Mei-Yu frontal systems, but not all of Mei-Yu frontal temperatures and humidity. On the other hand, the cloud physics scheme used in this paper simplifies the cloud-aerosol interactions by setting cloud droplet concentrations to reflect changes in aerosol concentration. In addition, it does not consider the impacts of aerosols on ice nuclei. The above simplifications may affect the accuracy of our conclusions about the impacts of aerosols on the magnitude of precipitation.