## 3 Results
### Effects on Cloud Microphysical Processes
#### 3.2.2 Cloud Microphysical Processes  
The differences in precipitation and hydrometeor content caused by aerosols are closely related to microphysical processes. In order to better understand the microphysical differences, we analyze the various sources (sinks) of precipitable hydrometers in both cases. Tables 1-3 give the sources (sinks) of the main precipitable hydrometeors (rain water, graupel, and snow), as well as their associated conversion rates during the onset of precipitation period and the peak precipitation period. Figure 8: Vertical cross sections of zonal-mean mixing ratios (g kg\({}^{-1}\)) of hydrometeors at t = 21 h (**a**,**b**) cloud water (shadow), ice crystals (black dashed line), snow (red solid line);(**c**,**d**) rain water (shadow) and graupel (red solid line);(**e**,**f**) the total hydrometeors contents (shadow). Only a 500 km \(\times\) 15 km of the domain is presented. The left panels show C-case, the right panels show P-case. Microphysical Processes of Rain Formation  
During the early stage of precipitation (t = 15 h), the formation of rain water mainly comes from collision-coalescence between raindrops and cloud droplets. The second significant sources of rain water at this time include the melting of snow and graupel (see Table 1). Noted that auto-conversion of cloud droplets into raindrops is very important in the initial formation of raindrops, even though it occurs with a very low efficiency. The concentration of cloud droplets in C-case is lower. Accordingly, their size is larger, and they are more efficient at auto-converting into raindrops than in P-case. In fact, the maximum rate of auto-conversion of cloud droplets into raindrops is 25 times in C-case than in P-case (see Table 1). Therefore, raindrops form earlier; then grow larger by collision coalescence and fall faster in C-case, leading to the rainfall to surface earlier (Figure 2c,d). By contrast, the formation of raindrops in P-case is inhibited in the initial stage of the precipitation, due to the initial concentration of cloud droplets is relatively higher and cloud drop-size is smaller. Furthermore, low conversion rate means more retention of cloud water, which will be transported to higher levels by frontal updraft, leading to enhancement of cold-cloud process. During the maximum precipitation period (t = 21 h), conversion rates of both cases have increased dramatically, and the largest conversion rate is the melting of graupel. At this time, except for the auto-conversion of cloud water into rain, all the other rainfall source items (conversion rates) in P-case is greater than in C-case. The ratios of the maximum conversion rate are as follows: the melting rate of graupel melting is about 2 times; melting rate of snow is 1.3 times; and the collection rate of cloud water by rain is 1.8 times. These results show that cold-rain processes enhance greatly under the polluted conditions. It is noted that evaporation of raindrops also strongly increases in the maximum precipitation period. Raindrop evaporation in P-case is twice stronger than C-case. By the above analysis, it can be inferred that cold-rain processes play a key role in precipitation enhancement during the maximum precipitation period, especially the melting of graupel and snow. Therefore, it is important to study the formation of graupel and snow, which helps understand the impacts of aerosol on precipitation formation mechanism. Melting of graupel is the most important source of rain formation. Table 2 lists the various microphysical process of graupel formation. It can be seen in the table that during the early stage of precipitation, the most significant physical processes of graupel formation are: Accretion of cloud droplets by graupel to form graupel, deposition of graupel, and collection of cloud droplets by snow to form graupel. During the maximum precipitation period, the production rate of graupel enhanced greatly due to increased upward movement in cold-cloud, as well as the presence of large amounts of supercooled water droplets.