## 4 The Dynamic Effects
### Third Dynamic Effect: The Water Vapor Pump Effect  
Mei-Yu front is an important part of the East Asian monsoon rainfall system, which is associated with plenty of water vapor conditions. From the above analyses, aerosol has significant influence on the dynamic field of Mei-Yu front. In addition, this rainfall system usually maintains long time, which could have important influences on the transportation of water vapor, even though very few studies have investigated the indirect vapor effect of aerosols.  
Figure 16 shows the vertical cross sections of zonal mean vertical water vapor flux at t = 21 h for both cases. Maximum vertical water vapor flux in P-case is approximately double C-case. Horizontal water vapor flux is also clearly greater in P-case than C-case (Figure 17). Note that around 200 km south of the rain band, P-case horizontal water vapor flux is still 5% higher than C-case (Figure 17). This indicates that both vertical and horizontal transportation of water vapor increase significantly of Mei-Yu frontal system under polluted conditions.  
From the previous sections, in polluted Mei-Yu frontal environments, both the first dynamic effect (DDE) and second dynamic effect (FDE) of aerosols can enhance low-level convergence and updrafts, which can increase low-level water vapor convergence near the front, leading to the enhanced transportation of water vapor above the frontal zone. On the other hand, in the high humidity conditions associated with the southwest monsoon, strong vertical water vapor transportation and consuming, in turn, increase the horizontal water vapor transportation in the larger-scale domain (Figure 17c). Overall, the increased updrafts and vapor transportation further enhance condensation, deposition, and collision-coalescence processes to produce more precipitable particles and to release more latent heat, which in turn can increase updrafts and further strengthen the positive feedback loop between microphysical and dynamic processes. Thus, the enhanced vapor transportation effect is driven by the first and second dynamic effects, which are rooted to latent heat release. We suggest this effect may be referred to as the third aerosol dynamic effect--vapor pump effect (VPE).  
In fact, the VPE has been presented in Section 3.2 and Section 3.3, where it was pointed out that, under polluted conditions, not only freezing and deposition but also condensation is increased (Figure 10). In other words, as cold-cloud processes strengthen, warm-cloud processes also strengthen. During these processes, enhanced water vapor transportation should play an important role.  
This increased water vapor transportation effect is closely related to the Mei-Yu frontal system during monsoon precipitation. Southwest flow brings plenty of water vapor to the Yangtze River Basin, forming precipitation and even long-lasting torrential rainstorms in suitable convergence conditions. Under these environments, the proper convergence near the frontal boundary can drive large-scale domain water vapor to Mei-Yu front. During the developing and mature stage of the Mei-Yu frontal precipitation system, the microphysical-thermal-dynamic coupling effect created by high concentration aerosols causes stronger updrafts, high level divergence and low-level convergence, forming the VPE. This effect enhances water vapor transportation, and further leads to more rainfall production.