*3.3. Vertical Structure Characteristics of Stratiform and Convective Precipitation in NCCV* 3.3.1. Precipitation Characteristics

The vertical distribution structure of the radar echo of precipitation can effectively reflect the vertical distributions of solid, liquid, and solid–liquid mixed particles in the NCCV precipitation cloud. Contoured frequency by altitude diagram (CFAD) is helpful to clearly show the vertical structure characteristics of the NCCV precipitation. Figure 9 shows the CFAD distribution of radar reflectivity of total, stratiform, and convective precipitation within 2000 km in NCCV coordinate systems from 2014 to 2019. Above 4 km, the radar reflectivity factors for all types of precipitation increase rapidly with the decrease in height, and the hydrometeors such as supercooled water and ice crystals in this layer continue to freeze and grow through the Bergeron process. For total precipitation (Figure 9a) and stratiform precipitation (Figure 9b), radar echoes are mainly distributed between 20 dBZ and 29 dBZ below 4 km, and as the height decreases, the radar reflectivity factor can reach a larger value, showing a wider radar reflectivity factor spectrum. The frequency of 40 dBZ can reach 5%, reflecting the collision-growth process of particles. For convective precipitation below 4 km, the increase in radar reflectivity factor is more obvious with the decrease in height, reflecting the more obvious collision-growth process. Compared to stratiform clouds, the radar reflectivity factor spectrum of convective precipitation below 4 km is wider, which is distributed from 17 dBZ to 50 dBZ. There is a shallow precipitation characteristic area, the echo is concentrated below 3 km, and the echo is between 17 dBZ and 28 dBZ. Another echo center is below 3 km, and the echo is in the range of 32–37 dBZ, indicating deep convection. These two echo centers corresponded well to the two frequency centers of near-surface DSDs (Figure 8). Particularly, there is a clear BB feature for stratiform precipitation, showing a sudden increase in echo impacted by the melted ice particles at the height of approximately 3~4 km in altitude.

**Figure 9.** The CFADs (shading, %) of the Ku-band reflectivity for (**a**) total, (**b**) stratiform, and (**c**) convective precipitation within 2000 km distance of the NCCV center, derived from GPM DPR for 2014–2019.

In order to further explore the vertical variation characteristics and differences of precipitation in different quadrants within the NCCV, Figure 10 shows the CFADs of radar reflectivity for total, stratiform, and convective precipitation in each quadrant within 2000 km in the NCCV coordinate system from 2014 to 2019. For stratiform precipitation (Figure 10a–d), the echo top in the southwest quadrant is the highest and the echo is the strongest, followed by the southeast quadrant of the NCCV. The echo top of stratiform precipitation in the southwest quadrant of the NCCV is about 12 km, and the echo is mainly concentrated below 5 km, distributed between 20 dBZ and 30 dBZ. The echo top of the southeast quadrant is about 11 km, and the echo is concentrated below 4 km, distributed in 20–30 dBZ. The storm-top heights of the northwest and northeast quadrants are 10 and 9 km, respectively, and the echoes are concentrated below 4 km, distributed from 20 dBZ to 28 dBZ. Significant BB characteristic areas can be seen in each quadrant. The BB is higher in the southwest and southeast quadrants, indicating that the melting layer is also higher. Below 5 km, as the height decreases, the radar reflectivity factor value in the southeast quadrant can reach a greater value, the spectral width is wider, and the particle collision-growth process is the most obvious. This may be due to the relatively stronger upward movement in the southeast quadrant, and the raindrops fall against the airflow, which is conducive to rapid collision growth.

**Figure 10.** The CFADs (shading, %) of the Ku-band reflectivity in the (**a**,**e**) northeast, (**b**,**f**) northwest, (**c**,**g**) southwest, and (**d**,**h**) southeast regions within 2000 km distance of the NCCV center for stratiform and convective precipitation, derived from GPM DPR for 2014–2019.

For convective precipitation (Figure 10e–h), the echo height and intensity in each quadrant are greater than those of stratiform precipitation, and the echo spectrum width is much wider. As the height decreases, the echo spectrum width increases more significantly, indicating that the collision growth of particles is more obvious. The CFAD in each quadrant is significantly different. In the southeast and northeast quadrants, the width of the echo spectrum increases more significantly as the altitude decreases, indicating that the collision growth of particles is more obvious. The shallow convection characteristic area can be seen in each quadrant, and the echo is concentrated below 3 km, distributed in 18–27 dBZ. This characteristic is particularly significant in the southeast quadrant of the NCCV. Probably because this area is mostly located in the ocean, shallow convection occurs easily on the sea surface, resulting in a high proportion of shallow convection and an obvious characteristic area. In the other three quadrants, a deep convective feature area can be seen, and the echo is generally concentrated in 32–39 dBZ, which is particularly significant in the northwest quadrant of the NCCV. In the northwest and southwest quadrants of convective precipitation (Figure 10f,g), the storm-top height is higher, and the radar echo above 4 km altitude is stronger, which increases rapidly with the decrease in height, reflecting the microphysical processes such as ice-crystal and rime growth. Below 4 km in altitude, the

radar echo also increases rapidly with the decrease in height, which reflects the obvious collision-growth process in this area, which is consistent with the discovery that the nearsurface particles have larger diameters and smaller concentrations (Figure 5).

In order to further explore the precipitation structures in the NCCV at different directions and heights, the azimuthal average profiles of radar reflectivity for total, stratiform, and convective precipitation within 2000 km in the NCCV coordinate system from 2014 to 2019 are presented in Figure 11. For total precipitation (Figure 11a) and stratiform precipitation (Figure 11b) below 10 km, the reflectivity factor increases with the decrease in height. This phenomenon is the most significant in the southeast quadrant, indicating collision growth of particles, corresponding well to the high rain rate in the quadrant. For convective precipitation (Figure 11c), the echo intensity of each position in the NCCV is greater than that of the stratiform. The reflectivity factor in the southwest and northwest quadrants of the NCCV increase with the decrease in height, indicating that the particles continue to collide and grow during the falling process. The height of the convective echo top in some areas within the southwest of the NCCV is higher than 14 km, indicating that there is penetrating convection in the area [34]. In the southeast and northeast, the echo is smaller than that in the west side of the NCCV, which is different from the results of the individual case [17] that the east of the NCCV corresponds to a strong echo. In the southeast quadrant of the NCCV, the storm heights in some areas can be as high as 14 km, but the frequency of occurrence of these deep clouds over this region is relatively low. The reflectivity of the upper layer is the smallest, part of which is lower than 21 dBZ, and the reflectivity near the height of 4 km is up to 28 dBZ. The reflectivity factor increases rapidly with the decrease in height, indicating that the particle collision growth is more obvious. However, within the height of 2–3 km, there is a relatively weak echo area, which may be due to the breakup of particles in the lower layer, showing that the particle diameter decreases significantly and the concentration increases.

**Figure 11.** The azimuthal distribution of Ku-band reflectivity (shading, dBZ) for (**a**) total, (**b**) stratiform, and (**c**) convective precipitation within 2000 km distance of the NCCV center, derived from GPM DPR for 2014–2019. (The sample size in the gray area is less than 0.1% of the maximum sample size).

#### 3.3.2. Microphysical Structures

The intensity of precipitation echo is affected by both particle concentration and particle size. Figure 12 shows the CFAD of *Dm* and dB*Nw* of total, stratiform, and convective precipitation within 2000 km in the NCCV coordinate system from 2014 to 2019. The dB*Nw* of convective precipitation is larger (smaller) than that of stratiform below (above) 5 km in altitude. The *Dm* of convective precipitation is larger than stratiform precipitation above 5 km in altitude. The stronger ascending motion within convective clouds may bring hydrometers to higher altitude and increases the chances of collision, which

eventually leads to large-sized hydrometeors and low concentrations. The *Dm* of total precipitation (Figure 12a) is concentrated between 0.6 and 2.8 mm at each height layer, and the dB*Nw* is almost concentrated between 20 and 48 (Figure 12d); the *Dm* of stratiform precipitation (Figure 12b) is mainly concentrated between 0.8 and 2.4 mm, and the dB*Nw* is almost concentrated between 20 and 46 (Figure 12e). The *Dm* of convective precipitation (Figure 12c) is mainly concentrated between 0.6 and 3 mm, and the distribution of dB*Nw* is mainly concentrated between 18 and 52. As the height decreases, the particle diameter and concentration of convective and stratiform precipitation increase, and the spectral width increases, which corresponds to the increase in reflectivity. In convective precipitation, there are two high-frequency regions for particle concentration and size. Particles with a *Dm* of 0.8 to 1 mm and a dB*Nw* of 38 to 40 correspond to shallow convection and mainly high concentrations of small particles. Another high-frequency characteristic region corresponds to deep convection. The *Dm* is within 1.2 to 1.6 mm, and the dB*Nw* is 30 to 36.

**Figure 12.** The contoured frequency (shading, %) by altitude diagram of *Dm* and dB*Nw* for (**a**,**d**) total, (**b**,**e**) stratiform, and (**c**,**f**) convective precipitation within 2000 km distance of the NCCV center, derived from GPM DPR for 2014–2019.

#### **4. Discussion**

It is important to know the limitations of the present study. To obtain the NCCV center, ERA5 data at a temporal resolution of 1 h and a spatial resolution of 0.25 degree are used in this study. Despite the high spatial–temporal resolution, it is still coarser than that of the DPR observations. The uncertainties during the processes in the identification of the NCCV center may introduce uncertainties to the results. However, the relatively large number of samples (6432 NCCVs) included in the synthetic analysis may partially cancel out the uncertainties. These things considered, since the GPM cannot continuously observe

the precipitation systems, the analysis of the particle growth and precipitation process formation can only be derived by deduction, based on the dynamic composite maps.

Here, we try to provide some explanations on the distributions of NCCV precipitation frequency and intensity. First, the frequency of convective systems is relatively higher in the southern part of the NCCV. This may be because the NCCV provides a favorable circulation background to guide the cold air southward. If there are heating conditions at the lower level, an unstable stratification with high-level dry, cold air and low-level warm and humid air will be formed, which is conducive to the triggering of the convective system [5,35,36]. The warm and cold air usually intersect in the southern half of the NCCV, which is also the rear part of the warm and humid tongue [8]. At the same time, the coupling of high- and low-level jets under the background of NCCV causes a large-scale upward movement on the southeast side of the system, which invigorates deep convection and heavy rain [37]. In addition, the average storm-top height for all types of precipitation is highest in the southwest quadrant, while average rain rate in the NCCV is largest in the south of the NCCV. The mechanism of the obvious phase differences of storm-top height and rain rate is unknown, and a study of this will be carried out in our subsequent research.
