**4. Discussion**

In this section, we discuss and analyses the reasons for the difference of precipitation particles of heavy precipitation over different terrain, including plains, mountains, and high mountains. For heavy CP, the occurrence probability of larger precipitation particles (D m ≥ 2.6 mm) increases with elevation (Figure 6d–f). We sugges<sup>t</sup> that strong updraft is more likely to form due to mountainous topographic lifting, and the strong updrafts tend to hold up the falling raindrops, slowing them down, and can carry some small raindrops to high altitudes to collide and merge with the falling raindrops, which leads to higher collision efficiency. In addition, the strong updraft can also carry sufficient water vapor into the cloud, which is also conducive to the formation of large raindrops. Yan et al. [45] proposed that the strengthening of ascending movement will also cause the snow and graupel particles above FzH to grow rapidly, making it easier to generate large rain droplets. Strong turbulence facilitates raindrops colliding and merging into large raindrops, meanwhile, it is facilitating raindrops breakup into little raindrops. This phenomenon is well evidenced by the wider D m horizontal distribution over mountains and high mountains than those over plains (Figure 6d–f). Another explanation is due to the seeder–feeder mechanism. In Figure 15, the relationship between near-surface D m and nearsurface dBN w is shown for the heavy CP over the three areas. For heavy CP, when dBN w is > 40 mm<sup>−</sup>1/m3, it generally corresponds to D m < 2 mm, and this relationship is best in high mountains. In Figure 8f, it can be clearly observed that the occurrence probability of region with dBN w > 40 mm<sup>−</sup>1/m3 below FzH of high mountains is much higher than those of plains. Wilson and Barros [46] showed that the seeder-feeder mechanism leads to an accelerated growth of small and moderate size raindrops (D m < 2 mm). This process could explain the enhancement of coalescence and the increase of the concentration of small drops [27]. Yan et al. [45] mentioned that for heavy rain, the cloud ice particles with large number concentrations (>600 <sup>L</sup>−1) seldom occur, and they are more inclined to gather at moderate concentrations (100–250 <sup>L</sup>−1) above 9 km over the Tibetan Plateau (roughly corresponds to the high mountains in this study) compared with the northern India and south of the Tibetan Plateau (NIST) and tropical ocean (TO). The cloud ice particles with smaller number concentrations usually correspond to the larger sizes of cloud ice particles. In this study, for heavy precipitation, precipitation particles over high mountains have the characteristics of lower number concentration and larger scale above 10 km. We speculate that there is some connection between the cloud ice particles and precipitation particles. However, this assumption needs further exploration and validation in future studies.

**Figure 15.** Scatter distribution of near-surface D m-dBN w of heavy CP over plains (**a**), mountains (**b**), and high mountains (**c**). The color indicates the scatter density.

For heavy SP, the occurrence probability of precipitation particles Dm below FzH within 1.3–1.6 mm over high mountains are significantly higher than those over plains (Figure 6a,c). The total water vapor near FzH (altitude at 4.5–6.5 km) over high mountains is more sufficient than those over plains (Figure 16). In addition, the underlying surface of the high mountains is closer to FzH, and the updraft is more likely to transport water vapor to FzH, which makes snow and graupel particles above FzH grow rapidly and form larger raindrops more easily. Owing to sufficient water vapor, the occurrence probability of Dm in the range of 1.1–1.3 mm over high mountains is much lower than those over plains, and the Dm over high mountains tends to concentrate in the range of 1.3–1.6 mm (Figure 6a,c).

**Figure 16.** Probability distribution functions (PDFs) of total water vapor content at 4.5–6.5 km of heavy SP of different topographic. The green, black and red solid lines represent plains, mountains and high mountains, respectively.
