4.2.3. The DSD Spectra

The characteristics of DSD of this rain system are investigated to understand the precipitation microphysical processes. To eliminate to the effect caused by rain rate and duration, the DSD measurements are divided into six classes according to the associated rain rate (*R*): C1, 0.1 ≤ *R* < 1; C2, 1 <sup>≤</sup> *<sup>R</sup>* <sup>&</sup>lt; 5; C3, 5 <sup>≤</sup> *<sup>R</sup>* <sup>&</sup>lt; 10; C4, 10 <sup>≤</sup> *<sup>R</sup>* <sup>&</sup>lt; 25; C5, 25 <sup>≤</sup> *<sup>R</sup>* <sup>&</sup>lt; 50 mm h−1. Such classification is based on the rain rate distribution of this system, as well as several previous studies [25,35]. The DSD sample relative frequency, rain rate statistics and mean DSD parameters for each class are summarized in Tables 1 and 2. For lower rain rate classes (C1–C3), the rain rate of Huidong station is lower than that of Zhuhai station. For C5, the rain rates of two stations are close to each other and for the other two high rain rate classes (C4 and C6), the rain rate of Huidong station is higher than that of Zhuhai station, which caused the higher hourly rainfall at from 18:00 to 19:00 UTC 30 August. However, due to the lower relative frequency of high rain rate classes, the overall average rain rate of Huidong station is smaller than that of Zhuhai station.

**Table 1.** Number and DSD retrieved rain rate statistics of each rain rate class for Zhuhai and Huidong station data.




It can be shown from Figure 10 that the total averaged DSD of Zhuhai station has wider spread as well as lower concentration at small and medium-sized drops (*D* < 2 mm) but higher concentration at big and large drops (*D* > 2 mm) compared with that of Huidong station, which corresponds to a larger *Dm*, more *W*, but a lower *Nt* for Zhuhai station compared with those for Huidong station (Table 2). For the same rain class, the DSD of Huidong shows the same characteristics with higher concentration at smaller drops as well as lower concentration at larger drops. Additionally, the diameters where the

raindrop concentrations of two stations become larger as the rain classes increased, from 1.2 to 3.8 mm, due to the higher the concentration of larger drops at higher rain rates for both stations.

**Figure 10.** Mean DSDs observed at Zhuhai and Huidong stations from 00:00 UTC 29 August to 12:00 UTC 31 August: (**a**) all DSD data, (**b**) DSD data classified by different rain rates. The solid and dashed lines in (**a**) and (**b**) represent Zhuhai and Huidong stations, respectively. The dash-dot line in (**b**) shows the DSD intersection of two stations for each rain rate class.

Table 3 summarizes the relative contributions of each size class to *Nt*, *W*, and *R*, which are obtained through dividing the parameters calculated for the raindrops of each size class by the corresponding parameters calculated for all the size classes. For both stations, the small raindrops (*D* < 1 mm) have the highest concentration (51.99% and 57.92%) of the total data set and the second highest relative frequency (Td 38.08% and 44.26%), but relatively low contributions to total water content and rainfall. The contribution of small and medium-sized raindrops (*D*: 1–2 mm) to the total number is the greatest, accounting for 47.48% (45.99%) at Zhuhai (Huidong) station. However, for Zhuhai station, the middle-sized drops (*D*: 2–3 mm) accounts for the most rainfall (34.39%), while the small and medium-sized raindrops (*D*: 1–2 mm) account for the most for Huidong station (40.50%). Moreover, for Zhuhai station, the medium-sized and larger drops (*D* > 2 mm) account more for rainfall and water content compared with Huidong station, as the higher relative frequency. This result is consistent with the conclusion shown in Figures 8 and 9 that larger drops are less likely to occur at Huidong, where complex topography could exacerbate the collision.


**Table 3.** Relative contributions of each size class to the total drop numbers *Td*, total drop concentration *Nt*, liquid water content *W*, rain rate *R*. The bold number highlights the maximum.
