## 5 Physical Mechanism Discussion
Furthermore, deposition and condensation processes cause much greater heating than freezing. Because of this, the key mechanism behind increased heating is increased deposition and the condensation processes with the cold-cloud \"towing\". On the other hand, previous studies pointed that low-level evaporative cooling had a key impact on low-level convergence [30]. Our studies indicated that evaporative cooling is the main driver of low-level convergence, and melting cooling also plays a role (Figures 11 and 12). Regarding dynamic effects, the similarities between the previous results of convective systems [25,30,34,77,78] and our findings of Mei-Yu front are that increased heating strengthens updrafts, and increased cooling strengthens downdrafts, which in turn strengthens low-level convergence. However, there are also some differences and new insights in our findings. We refer to the stronger updrafts (due to the latent heat-dynamic effect) and convergence (due to the cool pool effect) pointed out by [PERSON] et al. [78] as DDE, which has a very significant effect on the precipitation enhancement in convective clouds and mixed clouds. However, in Mei-Yu system, in addition to DDE, FDE is also an important mechanism for the precipitation enhancement under polluted conditions. Under the role of the frontal vertical wind shear, FDE forces ageostrophic circulation and forms slantwise convective circulation patterns (updrafts and downdrafts are separated) which contributes to strengthening and maintaining updrafts, causing a more \"well-organized\" Mei-Yu frontal system. And this further strengthens and prolongs precipitation. Regarding the VPE, [PERSON] et al. [25] mentioned that increased squall line precipitation maybe related to the gathering of water vapor from a larger region into a smaller region, but they did not analyze further. This paper presents a \"water vapor pump\" effect in Mei-Yu front caused by aerosols in positive feedback loops. It is particularly pointed out that Mei-Yu front is monsoon precipitation system. With the large-scale sufficient water vapor conditions, deep frontal clouds, and long-time precipitation, the long-lasting water vapor convergence and vertical transportation can tow and drive large-scale domain water vapor to Mei-Yu front. The enhanced water vapor transportation effect, in turn, strengthens condensation, deposition and even collision-coalescence processes, leading to more latent heat release, which further enhance the \"microphysical-dynamic feedback loop\". Note that this effect has not been reported in the previous studies yet. The four physical mechanisms resulted from high aerosol concentrations in Mei-Yu frontal systems have different influences in different stage of precipitation. The first mechanism, the microphysical mechanism, affects the entire precipitation process. Especially, the microphysical effects on precipitation dominate the early stages of precipitation. With the precipitation development, the other three dynamic effects gradually become active. Among them, DDE is the most direct. FDE are driven by latent heat release and DDE in Mei-Yu frontal environments. The increasing and maintaining low-level frontogenesis can strengthen the intensity of the original front and play an important role in strengthening and prolonging the precipitation. That is to say, FDE only in the frontal precipitation system is more significant. Furthermore, VPE becomes obvious only in Mei-Yu front associated with monsoon systems. It may not be obvious in short-lived isolated convection systems. The aerosol impacts on Mei-Yu frontal clouds and precipitation are summarized in a schematic diagram (Figure 18). The figure describes the main physical processes through which high aerosol concentration affects Mei-Yu frontal clouds and precipitation via four mechanisms.