## 3 Results  
### Vertical Structure and Temporal Evolution of the Orographic Clouds as Observed by Radars  
#### 3.1.1 Characteristics of Precipitation Observed by MRR  
Figure 4 shows the time-height diagrams of the reflectivity and Doppler velocity measured by the MRR. As seen from the figure, the precipitation process started at 8:50 UTC and ended at 12:20 UTC, and the MRR reflectivity near the surface ranged from approximately 10 dBZ to 30 dBZ, corresponding to a rain rate of 0.1-1 mm h\({}^{-1}\). The entire observed process can be partitioned to three segments, as indicated by the red boxes in Figure 4a. In Segment 1, precipitation quickly intensified after occurring and remained relatively strong until 9:15 UTC, then gradually weakened. In Segment 2, the low-level reflectivity was mostly around 20 dBZ, with two very short periods exceeding 25 dBZ. In Segment 3, the observational site received light and discontinuous precipitation. In all the three segments, a bright band could clearly be seen in the melting layer at about 3.7 km. The reflectivity below the melting layer was higher than that above it, partly because of the melted falling ice particles and partly because of the efficient warm rain process, as evident from cloud radar observation, which will be discussed later. The Doppler velocity was larger than 6 m s\({}^{-1}\) in the region with Z \(>\) 20 dBZ below the melting layer, indicating rain drops with an equivalent diameter of about 2 mm. Taking into account the possible updraft in the clouds, the actual terminal velocity could be larger than the measured Doppler velocity. Above the melting layer, the measured Doppler velocity was mostly 1-2 m s\({}^{-1}\), which is a typical terminal velocity of snowflakes [30]. The clear separation of the Doppler velocity above and below the melting layer indicated that the large particles in the mixed-phase region were mostly ice. However, in some regions, such as that at a 4-4.6 km height at 8:55 UTC, the Doppler velocity exceeded 4 m s\({}^{-1}\) and the reflectivity exceeded 25 dBZ, suggesting the existence of large, supercooled drops or graupels.  
The drop size distributions, concentrations, and VMDs below the melting layer in the three precipitation segments are shown in Figure 5, respectively. It can be seen that the drop size distributions broadened as the rain fell toward the ground in all the segments (Figure 5a-c), indicating that the accumulation of drops was occurring, and the maximum drop diameter increased from 4 mm to approximately 4.6 mm as the rain fell from the melting layer to the surface.  
Figure 4: Time–height diagram of (**a**) reflectivity and (**b**) Doppler velocity measured by the MRR.  
The drop VMDs and concentrations had different height dependencies in the three segments. In Segment 1, the VMD generally increased (Figure 5d) and the size distribution broadened (Figure 5a) as the height decreased, indicating the continuous accumulation of rain drops. However, the evaporation and accumulation of rain drops led to a decrease in the concentration as the height decreased (Figure 5d). In Segment 2, VMD first decreased, then increased as height decreased, while the concentration varied in an opposite way (Figure 5e). This was because the evaporation of rain drops was faster than the rate of accumulation as the drops fell, and the breaking up of drops might happen; however, when approaching the surface, many small drops had fully evaporated, resulting in a broad size distribution and fewer small drops (Figure 5b); thus, the VMD was large near the surface. In Segment 3, the concentration and VMD varied similarly compared to that in Segment 2, except that, above 3.2 km, the VMD increased and the concentration decreased with the decrease in height, mainly due to the accumulation of drops.