*3.1. Subsection Horizontal Distributions of Stratiform and Convective Precipitation in the NCCV* 3.1.1. Precipitation Characteristics

In order to understand the internal precipitation structure of the NCCV, the horizontal distribution of near-surface rain rate (the total rain rate of the precipitation samples in the grid divided by the number of precipitation samples), precipitation frequency (the number of precipitation samples in the grid divided by the number of total observational samples), and storm-top height of total, stratiform, and convective precipitation in the NCCV coordinate system are given, respectively (Figure 3). There are obvious regional differences in the distribution of near-surface rain rate for two types of precipitation. The rain rate is high in the south of the NCCV, especially in the southeast region, while it is the smallest in the northeast (Figure 3a–c), which may be related to the more abundant water vapor transport in the south [1]. The rain rate of stratiform in the southeast quadrant (Figure 3b) is generally higher than 2 mm h<sup>−</sup>1, while it is generally lower than 2 mm h−<sup>1</sup> in other regions. Compared to stratiform precipitation, the convective rain rate (Figure 3c) is generally larger in the NCCV, and the rain rate in the south of the NCCV is higher than 3.5 mm h<sup>−</sup>1, while the rain rate in other regions is also higher than 2 mm h−1. The horizontal distribution of the total rain rate (Figure 3a) is similar to that of stratiform, which is related to the high proportion of stratiform precipitation in the NCCV. The rain rate of stratiform in each region is smaller than that of convective precipitation, while the precipitation frequency is significantly higher, indicating that the precipitation in the NCCV is more composed of stratiform precipitation with weaker precipitation intensity.

There is an obvious comma-shaped rain band for the precipitation frequency distribution of the NCCV, which is consistent with the research results of Chen et al. [1] (Figure 3d–f). For total (Figure 3d) and stratiform (Figure 3e) precipitation, the regions with precipitation frequency higher than 4% are mainly located over the small area near the center in the northeast of the NCCV and a large area in the southeast of the NCCV. This is in good agreement with previous research results that the east side of the NCCV is the main area of precipitation [6,7], while in most of the other areas of the NCCV, precipitation frequency is generally lower than 1.5%. With the increase in the distance from the center of the NCCV, the region with high precipitation frequency shifts from the northeast to the southeast quadrant. Compared to stratiform precipitation, the frequency of convective precipitation is lower in all regions. The precipitation frequency in the comma-shaped rain band is mainly in the range of 0.5–1.5%, while it is basically lower than 0.5% in other regions (Figure 3f). As the distance to the center of the NCCV increases, the high convective precipitation frequency occurs, and the frequency of convective precipitation increases

in the southeast quadrant, which is probably related to the higher instability and more moisture in this region (figure omitted).

**Figure 3.** The distribution of (**a**–**c**) near-surface rain rate (shading, mm h−1), (**d**–**f**) precipitation frequency (shading, %), and (**g**–**i**) storm-top height (shading, km) for total, stratiform, and convective precipitation at each 50 km × 50 km grid in the NCCV coordinate system, derived from GPM DPR for 2014–2019. (The black dots represent the NCCV center; 6432 NCCVs are included in the dynamic composite analysis).

The horizontal distribution of storm-top height in the NCCV also has an obvious asymmetric structure (Figure 3g–i). For total (Figure 3g) and stratiform (Figure 3h) precipitation, the storm-top height is highest (4.5–6.5 km) in the southwest quadrant and lowest in the northeast quadrant. Although the precipitation cloud develops deeply in the southwest regions, the rain rate is small. Conversely, the storm-top height in the northern half of the NCCV is basically lower than 4.5 km, indicating shallower precipitation clouds, and the rain rate is also small. The storm-top height in the southeast region of the NCCV is generally lower than 5 km, but the rain rate is the largest in the NCCV, which may be related to the more abundant water vapor in this area [1]. The convective precipitation cloud in the west of the NCCV develops higher (Figure 3i), and the average storm-top height is higher than 5 km, even up to 8 km in some areas which also have a higher rain rate. The storm-top height in the southeast quadrant of the NCCV is basically lower than 4.5 km, but the rain rate is the highest in the NCCV. One possible explanation is that these areas are mostly located over the ocean, where shallow convection with low storm-top height prevails, according to a previous study [22]. The sufficient water vapor over the ocean may provide favorable moisture conditions for the formation of shallow convection.

To better understand the contribution of stratiform and convective precipitation within the NCCV and the distribution of NCCV precipitation, the horizontal distribution of precipitation amount contribution (the total rain rate of stratiform/convective precipitation samples divided by the total rain rate of precipitation samples), and precipitation frequency contribution (the number of stratiform/convective precipitation samples in the grid divided by the number of precipitation samples) for stratiform and convective precipitation within 2000 km of the NCCV center are given (Figure 4a–d). Overall, the precipitation in NCCVs is dominated by stratiform precipitation, and the precipitation frequency (mostly more than 70%) and amount contribution (mostly more than 60%) of stratiform precipitation are greater than that of convection precipitation. The difference between convective and stratiform precipitation amount (frequency) contributions is the largest in the northeast quadrant of the NCCV. The latter is significantly higher than the former. For example, in the northeast of the NCCV, the contribution of stratiform precipitation amount (frequency) can reach 80% (90%), while convective precipitation is less than 20% (10%). It is worth noting that in the southwest of the NCCV, although the contribution of convective precipitation frequency is smaller than that of stratiform precipitation (basically less than 30%), the contribution of precipitation amount is higher than 60% locally, exceeding stratiform precipitation. For stratiform and convective precipitation, the distribution of precipitation frequency contribution of the NCCV (Figure 4c,d) is very similar to that of precipitation amount contribution (Figure 4a,b). Generally, the regions with high (low) precipitation frequency contribution also exhibit a high (low) precipitation amount contribution. However, since the rain rate of stratiform is generally lower than that of convective, the contribution of the rain amount is smaller than the frequency for stratiform precipitation. On the contrary, the contribution of the convective precipitation amount increases compared to the frequency contribution.

**Figure 4.** The distribution of (**a**,**b**) precipitation contribution (shading, %), and (**c**,**d**) precipitation frequency contribution (shading, %) for total, stratiform, and convective precipitation at each 50 km × 50 km grid in the NCCV coordinate system, derived from GPM DPR for 2014–2019. (The black dots represent the NCCV center; 6432 NCCVs are included in the dynamic composite analysis).

#### 3.1.2. Characteristics of Near-Surface Microphysics

Rain rate is controlled by both raindrop concentration and raindrop size [28]. The microphysical structure and process inside the NCCV also play a very important role in precipitation [14]. The characteristics of the DSD can reflect the microphysical processes of precipitation. The DSD parameters provided by GPM 2ADPR products are used, including *Dm* and dB*Nw*, where *Dm* represents the size of the particles and *Nw* represents the particle concentration. In order to reduce the influence of DSD retrieval uncertainties caused by ground clutters, the *Dm* and dB*Nw* at 2.5 km height are chosen to represent the near-surface DSDs. In order to facilitate the display, dB*Nw* is used to represent the particle concentration parameter (dB*Nw =* 10*log*<sup>10</sup> (*Nw*)).

The near-surface particle diameter (Figure 5a–c), concentration (Figure 5d–f), and radar reflectivity (Figure 5g–i) are very similar horizontal distributions for stratiform and convective precipitation. The *Dm* and reflectivity factor in the southeast quadrant of the NCCV are small and the dB*Nw* is high, while the dB*Nw* and reflectivity factor in the northwest quadrant are low and the *Dm* is large. For stratiform precipitation (Figure 5b), the *Dm* in the northwest quadrant is generally higher than 1.3 mm and the dB*Nw* is lower than 33, while the *Dm* in the southeast is mostly higher than 1.25 mm and the dB*Nw* is about 34. For convective precipitation, the *Dm* in the northwest is generally higher than 1.45 mm and the dB*Nw* is lower than 32. The *Dm* in the southeast is generally below 1.25 mm and the dB*Nw* is generally higher than 36. For convective and stratiform precipitation, the radar reflectivity is very similar to the *Dm* distribution, which is mainly because the near-surface echo intensity is more affected by particle size. For convective and stratiform precipitation, the near-surface rain rate is close to the horizontal distribution of dB*Nw*. For example, the high rain rate of different types of precipitation in the southeast quadrant of the NCCV (Figure 3a–c) corresponds to the high concentration of particles (smaller size), while the weak precipitation in the northwest quadrant corresponds to the low concentration of hydrometeors (larger size). This shows that the rain rate in the NCCV is closely related to the raindrop concentration. The heavy precipitation in the southeast is mainly contributed to by higher raindrop concentration.

Compared to stratiform precipitation, convective precipitation generally has stronger near-surface echoes, smaller particle concentration, and larger particle size in the western and northwestern regions of the NCCV, while the convective precipitation in the eastern and southeastern quadrants generally has weaker near-surface radar echo, higher particle concentration, and smaller particle size. This shows that there are differences in the microphysical processes of precipitation in different types of precipitation clouds and different quadrants. The following section will further study the variation of precipitation characteristics in different regions in the NCCV.
