## 5 Physical Mechanism Discussion  
From the analyses of Section 3, it can be noticed that the variation of aerosol concentrations results in the differences of microphysical processes and latent heat release in Mei-Yu frontal system. The differences successively induce dynamic and water vapor variations, which in turn influence microphysical processes. These physical processes create a positive feedback loop, leading to the  
Figure 16: Vertical cross sections of zonal mean Vertical Water Vapor Flux (\(|w\cdot q|/g\), shading, Units: \(g\cdot cm^{-1}\cdot hPa^{-1}\cdot s^{-1}\)) and cross-front wind perturbation vectors \((v^{\prime},w)\) at t = 21 h for (**a**) typical continental C-case and (**b**) polluted continental P-case. Only a 500 km \(\times\) 15 km of the domain is presented. Figure 17: Vertical cross sections of zonal mean horizontal Water Vapor Flux (\(V\cdot q/g\), shading, Units: \(g\cdot cm^{-1}\cdot hPa^{-1}\cdot s^{-1}\)) and cross-front wind perturbation vectors \((v^{\prime},w)\) at t = 21 h for (**a**) C-case and (**b**) P-case. (**c**) is the difference of horizontal Water Vapor Flux between the two cases. Only a 500 km \(\times\) 15 km of the domain is presented. increased precipitation under polluted conditions in Mei-Yu front. In general, four physical mechanisms are proposed to explain the enhancement of precipitation in Mei-Yu front. The first is the cloud microphysical mechanism, including microphysical processes and the latent heat release. The other three mechanisms are three dynamic effects resulted from the microphysical effects. They are the DDE, FDE, and VPE, respectively. Previous studies indicated that riming increased and deposition decreased [77] or deposition enhanced and riming suppressed to produce ice phase particles under polluted conditions [37]. However, our finding shows that not only riming but also deposition enhance to produce snow and graupel in Mei-Yu frontal precipitation systems. There are two possible explanations for the enhanced deposition. First, WBF process is enhanced, because more cloud droplets are transported above the 0 \({}^{\circ}\)C isotherm where they may evaporate and deposited onto ice-phase particles. Second, the increased cold-cloud processes enhance latent heat release, leading to stronger updrafts and transportation of water vapor to high levels to strengthen deposition. On the other hand, the increased deposition releases more latent heat and increases updrafts that enhance riming process. The mutual promotion of riming and deposition increases the growth of snow and graupel in the strong updraft region and then increases cold-rain process. During the developing and maximum precipitation stages of Mei-Yu front system, the enhanced melting and evaporation can strengthen dynamic processes at the low-level frontal boundary, and drive and strengthen warm-cloud processes (condensation and collision-coalescence). Likewise, strengthened warm-cloud processes release more condensation heating, strengthening updrafts, which in turn promotes cold-cloud processes. Thus, with the cold-cloud \"towing\" effect, there is mutual promotion of cold and warm-cloud processes. This could be an even more complicated and peculiar cloud microphysical mechanism of aerosol effects on Mei-Yu front precipitation. Regarding latent heat release, previous researchers emphasized that increased high-level freezing heating was the major difference under polluted conditions [25,77]. Our research shows that in polluted Mei-Yu frontal cloud systems, high-level heating due to both freezing and deposition processes increases, but deposition heating is much greater than freezing heating. In addition, due to the increased high-level cold-cloud heating, there is increased updraft that not only increases collision-coalescence processes, but also greatly increased vertical transportation of water vapor because of the high humidity environment of Mei-Yu fronts. This leads to deposition and condensation processes all increasing as well (Figure 10).