## 3 Results
### Microphysical Processes of Snow Formation  
In addition to graupel's important contribution to the formation of precipitation, snow also plays an important role. Table 3 lists the various microphysical processes that can form snow, and their associated conversion rates (10\({}^{-3}\) g kg\({}^{-1}\) s\({}^{-1}\)). During the initial precipitation stage, there are three major processes involved in the growth of snow. They are: the collection of rain-snow to form snow; accretion of super-cooled cloud droplets by snow (both called riming); and deposition of snow crystals. Of these, riming with super-cooled raindrops is of greatest significance. Comparing the conversion rate in two cases, there is more snow riming growth in P-case (0.047) than in C-case (0.036). This is related to the increased amount of cloud water in the mixed-phase region (approximately 4-7 km above the surface). It is noted that, during the maximum precipitation period, snow growth by deposition has become the largest contributor to snow formation. Riming with super-cooled rain and cloud droplets are second and third respectively. This is perhaps related to two things: increased evaporation of cloud droplets; and increased water vapor transported to higher altitudes by enhanced frontal updrafts in the high humidity environment of the Mei-Yu front. Comparing the two cases, the maximum rate of snow production in all of the above processes is greater in P-case than in C-case. Hence, there is much more snowfall under polluted conditions. This is mainly because the P-case has more supercooled cloud water and rain water to foster the formation of snow. By analyzing the physical processes behind the formation of the main precipitable hydrometeors under different pollution conditions, we can see that raindrops mainly come from the melting behavior of graupel and snow during the maximum precipitation period. The primary growth mechanisms of \begin{table}
\begin{tabular}{l c c c c c} \hline \hline \multirow{2}{*}{**Process**} & \multirow{2}{*}{**Description**} & \multicolumn{2}{c}{**15 h**} & \multicolumn{2}{c}{**21 h**} \\ \cline{3-6}  & & **C-Case** & **P-Case** & **C-Case** & **P-Case** \\ \hline
**PSACWG** & Accretion of cloud droplets by graupel to form graupel & 0.39 & 0.66 & 0.8 & 2 \\ PRACG & Collection of raindrops by graupel to form graupel & 0.043 & 0.033 & 0.6 & 1.1 \\ PRDG & Deposition of graupel & 0.089 & 0.14 & 0.27 & 0.55 \\ PSACR & Collection of snow by super-cooled raindrop to form graupel & 0.025 & 0.04 & 0.14 & 0.22 \\ PGSACW & Collection of cloud droplets by snow to form graupel & 0.12 & 0.14 & 0.12 & 0.33 \\ PIACR & Ice-rain Collection to graupel & 0.09 & 0.04 & 0.24 & 0.5 \\ PGRACS & Collection of raindrops by snow to form graupel & 0.0003 & 0.0028 & 0.024 & 0.035 \\ MNUCCR & Freezing of raindrop to form graupel & 0.013 & 0.0008 & 0.01 & 0.0033 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Same as Table 1, but for graupel. \begin{table}
\begin{tabular}{l c c c c c} \hline \hline \multirow{2}{*}{**Process**} & \multirow{2}{*}{**Description**} & \multicolumn{2}{c}{**15 h**} & \multicolumn{2}{c}{**21 h**} \\ \cline{3-6}  & & **C-Case** & **P-Case** & **C-Case** & **P-Case** \\ \hline
**PRACS** & Collection of rain-snow to form snow & 0.0545 & 0.0715 & 0.24 & 0.35 \\ PSACWS & Accretion of super-cooled cloud droplets by snow & 0.036 & 0.047 & 0.16 & 0.24 \\ PRDS & Deposition of snow & 0.016 & 0.0083 & 0.45 & 0.65 \\ PRAI & Auto-conversion of ice to form snow & 0.0043 & 0.0048 & 0.014 & 0.024 \\ PRCI & Collection of ice crystals by snow & 0.011 & 0.001 & 0.013 & 0.024 \\ PRACIS & Accretion of ice by rain to form snow & 0.0001 & 1.3E-5 & 5E-05 & 2E-05 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Same as Table 1, but for snow. groupel and snow are riming and deposition. In high-polluted environment, cold-cloud processes are more pronounced, producing more ice particles, which then melt and produce more raindrops.