## 3 Results
### Effects on Cloud Microphysical Processes  
#### 3.2.1 Cloud Microphysical Quantities  
At t = 14 h, driven by the Mei-Yu front, cloud water begins to form at a height of 2.5-3 km. The content of cloud water in the two cases is the same, while the number of cloud droplets is higher in P-case, resulting in smaller drop size. At t = 14 h 30 min, surface precipitation begins to appear in C-case. Figure 4 shows the average vertical distribution profile of hydrometeors in the two cases, where \(Q_{c}\), \(Q_{r}\), \(Q_{i}\), \(Q_{s}\), and \(Q_{g}\) represent the mixing ratios of cloud water, rain water, ice crystals, snow, and graupel, respectively. It is notable that, at this time, the two cases contain only cloud water and rain water. \(Q_{c}\) of P-case is greater than that of C-case, but \(Q_{r}\) is less obvious (Figure 4a,b). This indicates that cloud drop-size of P-case is smaller, and less efficient at forming stage of raindrops. Figure 2c shows surface precipitation starts at t = 14 h 40 min in P-case, 10 min later than that of C-case. Figure 3: Evolution of precipitation distribution (t = 15, 18, 21 h) in the model domain (Only a 1000 km \(\times\) 300 km of the domain is presented). The left panels represent C-case, and the right panels represent P-case. In the early stage of precipitation (t = 15 h), cloud top extends to a height of 8 km (0 \({}^{\circ}\)C layer located at 3.75 km height) and ice phase process starts. Even though there is smaller \(Q_{r}\) in P-case than in C-case, P-case has larger \(Q_{s}\) and \(Q_{g}\) (see Figure 5a,b). Overall, P-case has less warm-cloud hydrometeors and more cold-cloud hydrometeors than C-case (Figure 5c). Figure 4: Vertical profiles of domain-averaged mixing ratios (g kg\({}^{-1}\)) of hydrometeors for (**a**) C-case and (**b**) P-case at t = 14 h 30 min. The symbols \(Q_{c}\), \(Q_{r}\), \(Q_{l}\), \(Q_{s}\), and \(Q_{g}\) represent the mixing ratios of cloud water, rain water, ice crystals, snow, and graupel, respectively. Figure 5: Vertical profiles of domain-averaged mixing ratios (g kg\({}^{-1}\)) of hydrometeors at t = 15 h (**a**) C-case, (**b**) P-case, (**c**) Vertical Profile of Total Hydrometeors in two cases. The symbols \(Q_{c}\), \(Q_{r}\), \(Q_{l}\), \(Q_{s}\), and \(Q_{g}\) represent the mixing ratios of cloud water, rain water, ice crystals, snow, and graupel, respectively. More cloud droplets with smaller drop-size in P-case suppress the conversion of cloud droplets to raindrops, which inhibits precipitation in the early stage, and, meanwhile, leads to more cloud water detained and foster more snow and graupel formation at upper levels. These two aspects are the main differences in cloud physical characteristics of two cases at the onset of precipitation. In the developing stage of precipitation (t = 18 h), rain water content exceeds cloud water below a height of 3 km. Snow and graupel content increase dramatically in both cases above a height of 3 km (Figure 6a,b). This shows that cold rain processes are greatly invigorated in the developing stage. Comparing the two cases, the content of rain water is quite similar, but P-case has more hydrometeors of all other species, and larger content of total hydrometeor. The biggest difference is at a height of 6 km (Figure 6c). Note that, in addition to increased ice-phase hydrometeors, the content of cloud water in P-case is much greater now than that at t = 15 h, implying that the condensation process (warm-cloud process) has also been invigorated. In the maximum precipitation period (t = 21 h), ice-phase hydrometeor content of both cases grows dramatically. The two most prominent types of hydrometeors are still snow and graupel (Figure 7). Figure 8 shows the average zonal vertical distribution of hydrometeors content at t = 21 h. Comparing the two cases, the difference in hydrometeor content is greatest in the mixed-phase layer (0 \({}^{\circ}\)C to -20 \({}^{\circ}\)C). The difference is mainly in the graupel, snow, and cloud water content.