*3.2. Azimuthal Distributions of Stratiform and Convective Precipitation in NCCV* 3.2.1. Precipitation Characteristics

The above studies show that there is a significant asymmetric structure in the horizontal distribution characteristics of stratiform and convective precipitation in the NCCV. In order to understand the distribution characteristics and differences of precipitation in different directions within the NCCV, the azimuth distribution of precipitation frequency, near-surface rain rate, precipitation frequency contribution, precipitation amount contribution, and storm-top height within 2000 km of the NCCV center are given in Figure 6.

**Figure 5.** The distribution of near-surface (**a**–**c**) droplet mass-weighted mean diameter *Dm* (shading, mm), (**d**–**f**) particle concentration parameter dB*Nw* (shading, no unit), and (**g**–**i**) average reflectivity (shading, dBZ) 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 peak value of precipitation frequency is in the southeast quadrant of the NCCV for different rain types (Figure 6a). In each direction of the NCCV, the frequency of stratiform precipitation is greater than that of convective precipitation. This indicates that the NCCV precipitation is dominated by stratiform precipitation, which is consistent with the horizontal distribution (Figure 3e,f). The convective precipitation frequency changes slightly in the northern half of the NCCV, while it gradually increases from west to southeast, and then gradually decreases to the east. The peak of the precipitation frequency is about 1% in the southeast of the NCCV. The frequency of stratiform precipitation varies greatly in different directions of the NCCV. From the east side of the NCCV to the southwest, the frequency of stratiform precipitation gradually decreases and reaches the minimum in the southwest of the NCCV, about 1.3%. Then to the southeast, the frequency of precipitation increases rapidly and reaches the peak in the southeast of the NCCV, about 4%. The azimuthal distribution of the total precipitation frequency is basically consistent with that of stratiform precipitation but is larger than that of stratiform precipitation in each direction.

**Figure 6.** The azimuthal distributions of (**a**) precipitation frequency, (**b**) near-surface rain rate, (**c**) precipitation frequency contribution, (**d**) precipitation contribution, and (**e**) storm-top height for total (black solid lines), stratiform (blue solid lines), and convective (red solid lines) precipitation in the NCCV coordinate system, derived from GPM DPR for 2014–2019. (A total of 6432 NCCVs are included in the dynamic composite analysis; samples should be within 2000 km distance of the NCCV center).

The peak of average rain rate in the NCCV is in the south of the NCCV (Figure 6b) for convective and stratiform precipitation, which is more westward than the peak of precipitation frequency. In all directions in the NCCV, the convective rain rate is higher than that of stratiform precipitation. The convective rain rate varies greatly in different directions of the NCCV. It gradually increases from the east side to the south side in the NCCV and reaches the peak in the southwest (about 6 mm h−1), and then gradually decreases to the east side. The stratiform rain rate changes little in the northern region of the NCCV and gradually increases from the west to the south. The peak is in the south of the NCCV, about 2.67 mm h<sup>−</sup> 1. From south to east, the stratiform rain rate gradually decreases. Due to the high proportion of stratiform precipitation, the azimuth distribution characteristics of the total rain rate are basically the same as the stratiform rain rate, which is not described here.

In all directions within the NCCV, the precipitation frequency and amount contribution for stratiform precipitation is greater than those of convective precipitation; they are generally between 80% and 90%, and 60% and 90%, respectively. The precipitation amount and frequency contribution for stratiform precipitation are the lowest in the southwest quadrant and the highest in the northeast quadrant of the NCCV (Figure 6c,d). On the contrary, the contribution of convective precipitation is the highest in the southwest region and the lowest in the northeast region. The precipitation frequency and amount contribution for stratiform precipitation increase slightly from the east to the south of the NCCV, reaching the peak in the northeast quadrant, about 92% and 87%, respectively. After that, they gradually decrease and reach the valley in the southwest, at about 80% and 62%, respectively. On the contrary, the peak value of convective precipitation contribution is in the southwest of the NCCV center; the contribution of precipitation frequency and the amount is about 19.8% and 37%, respectively. The valley value is in the northeast, where the frequency contribution is about 7.7% and the precipitation amount contribution is about 13%.

The peak of average storm-top height for all types of precipitation in the NCCV is in the southwest quadrant of the NCCV, while the peak of convection is more westward. The valley of the storm-top height is in the east (Figure 6e). The storm-top height of convective precipitation in the southeast of the NCCV is smaller than that of stratiform, while it is opposite in other quadrants. The storm-top height of convective precipitation varies greatly in different directions. From the east side to the southwest side, it gradually increases, reaching a peak in the southwest, about 6.24 km, and then to the east side, the storm-top height gradually decreases, reaching the valley value in the east. The storm-top height peak value of stratiform precipitation is about 5.4 km in the southwest, and the valley value is about 3.83 km in the east of the NCCV. The azimuth distribution of storm-top height for total precipitation is basically consistent with that for stratiform precipitation, which is related to the large proportion of stratiform precipitation in the NCCV.

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

Figure 7a,b shows the azimuthal distribution of near-surface *Dm* and dB*Nw* of the total, stratiform, and convective precipitation in the NCCV. The *Dm* for different types of precipitation shows obvious asymmetry, and the azimuth distribution characteristics are similar (Figure 7a). The peak values are in the west of the NCCV, and the valley values are in the east. Except for the southeast quadrant, the *Dm* of convective precipitation is generally larger than that of stratiform in all directions. The *Dm* of convective precipitation changes the most in different directions within the NCCV (the asymmetry is more obvious). From the east side of the NCCV to the west side, the *Dm* gradually increases and continues to the east side, where the *Dm* gradually decreases. The peak value in the west is about 1.66 mm, and the valley value is in the southeast, about 1.24 mm. For stratiform precipitation, the average azimuthal variation of *Dm* is small, with a peak of about 1.32 mm in the west and a valley of about 1.25 mm in the east. The *Dm* distribution of total precipitation is similar to that of stratiform precipitation.

**Figure 7.** The azimuthal distributions of near-surface (**a**) droplet mass-weighted mean diameter *Dm*, and (**b**) particle concentration parameter dB*Nw* for total (black solid lines), stratiform (blue solid lines), and convective (red solid lines) precipitation in the NCCV coordinate system, derived from GPM DPR for 2014–2019. (A total of 6432 NCCVs are included in the dynamic composite analysis; samples should be within 2000 km distance of the NCCV center).

The azimuth distributions of dB*Nw* for different types of precipitation are also obviously similar (Figure 7b). In the southeast or south of the NCCV, the concentration of precipitation particles is the highest (~36); the lowest is in the northwest (~31) for both convective and stratiform precipitation. The variation of convective precipitation in different directions is larger than that of stratiform clouds. From the east side of the NCCV to the northwest side, the particle concentration gradually decreases, and then to the southeast side, the particle concentration gradually increases. The peak value in the southeast is about 35.9, and the valley value in the northwest is about 31. For stratiform precipitation, the peak value is about 33.8 in the south, and the valley value is about 32.3 in the northwest. The particle concentration distribution of total precipitation is basically consistent with that of stratiform. In general, the concentration of convective precipitation systems will have lower concentration and larger hydrometeors, while in Figure 7b, except in the southeast and northeast quadrants of the NCCV, the concentration of droplets in convective precipitation is relatively smaller, which proves that the microphysical distribution varies with weather systems [21,23,24,33]. However, for a specific type of precipitation, the peak and valley values of particle concentration are opposite to that of *Dm*, which is related to the characteristics of the droplet-size distribution (high-concentration small particles or low-concentration large particles). This is consistent with the research results of Qi et al. [15] that particle concentration and diameter showed a negative correlation. In addition, compared to *Dm*, near-surface rain rates for the different types in the NCCV (Figure 6b) are closer to the azimuth distribution of the dB*Nw*. Especially for stratiform precipitation, the peak of heavy rain rate in the south corresponds well to the peak of high concentrations of hydrometeors in the south.

The above results reveal the different characteristics of near-surface DSDs of the NCCV precipitation in different directions. In order to further analyze the near-surface microphysics at different positions in the NCCV and study the corresponding relationship between particle concentration and particle size, Figure 8 shows the two-dimensional frequency distribution of dB*Nw* and *Dm* at 2.5 km in four quadrants for convective and stratiform precipitation in the NCCV coordinate system. The stratiform and convection in each quadrant generally have high concentrations of small particles and low concentrations of large particles. The DSDs of stratiform precipitation are more concentrated, while the DSDs of convective precipitation are much wider, with higher concentrations and larger hydrometeors.

**Figure 8.** The frequency pattern (shading, %) in two-dimensional space of *Dm* and dB*Nw* at 2.5 km 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. (The black (white) solid line represents the frequency higher than 4% (36%)).

For stratiform precipitation (Figure 8a–d), the particle *Dm* is mainly concentrated between 0.8 and 2.4 mm, and the dB*Nw* is mainly between 22 and 46. There are differences in DSDs in different quadrants. The DSDs in the southeast quadrant are the widest, followed by the southwest quadrant, and the average particle concentration is higher and the diameter is larger in the two quadrants, corresponding to a high rain rate. For convective precipitation (Figure 8e–h), the *Dm* is concentrated between 0.6 and 3.0 mm, and the dB*Nw* is between 18 and 52. The difference of DSDs in different quadrants is more significant than that in stratiform clouds. There are two peak centers of DSDs, located in the northeast and northwest quadrants of the NCCV. One peak center is concentrated on *Dm* in the range of 0.8–1 mm, and the dB*Nw* is concentrated in the range of 38–40, which is composed of a high concentration of small particles. Another peak center is concentrated when *Dm* ranges from 1.25 to 1.75 mm, and the dB*Nw* is concentrated from 30 to 34, which indicates a low concentration of large particles probably resulted from deep convection. However, there is only the former DSD peak center in the southwest and southeast quadrants, which may be caused by a higher proportion of shallow convective precipitation in these regions. For the southeast and southwest quadrants with higher convective rain rates, the strong convective precipitation on the southeast side of the NCCV is mainly contributed to by near-surface high-concentration hydrometeor particles. The average diameter of the hydrometeor particles is the smallest, 1.27 mm, while the dB*Nw* is the largest, 35.82. The DSDs are also more concentrated on large dB*Nw* values and small *Dm* values (high particle concentration and small particle diameter, indicated by white solid lines), with few particles with *Dm* higher than 2 mm. The stronger convective rain rate in the southwest quadrant is contributed to by higher particle concentration (dB*Nw* = 34.01) and larger particle size (*Dm* = 1.47 mm).
