4.3.1. The Polarimetric Radar Signatures

To further understand the differences between the two rain patterns on 29 and 30 August, the polarimetric radar fields over the two extreme stations are compared. Figure 11 shows the time series of polarimetric variables over Doumen and Gaotan stations observed by GZRD and MZRD 0.5-degree sweeps, respectively. The size of radar pixel over Doumen (Gaotan) station is around 0.25 × 1.49 km (0.25 × 2.23 km), and the horizontal/vertical distances are 93 km/0.81 km (137 km/1.21 km) between the radar pixel and the station. Generally speaking, Figure 11 shows that the timing changes of radar variables are consistent with the rainfall changes at both stations (Figure 6). During the extreme rainfall periods of Doumen station and Gaotan station, the mean values of *ZH* over both stations are similar while the mean value of *Zdr* over Doumen station is higher than that over Gaotan station, indicating that bigger drops are more likely to occur at Doumen station than Gaotan station, which is similar to the DSD analysis of Zhuhai and Huidong.

**Figure 11.** Time series of different polarimetric radar variables over (**a**) Doumen station observed by GZRD, (**b**) Gaotan station observed by MZRD from 00:00 UTC 29 August to 12:00 UTC 31 August.

To eliminate the random error of one radar pixel, two extreme regions were defined as circular areas with a radius of 5 km centered on Doumen station and Gaotan station, respectively, hereafter referred to Doumen rain region and Gaotan rain region. The polarimetric variables from 0.5-degree sweep over these two regions from 00:00 UTC 29 August to 12:00 UTC 31 August were analyzed. Figure 12 shows the joint frequency distributions of *Zdr* vs. *ZH*, *Kdp* vs. *ZH*, as well as log10 (*Kdp*/*Zh*) vs. *Zdr* observed by radars over two regions as well as the simulated radar moments based on DSD data at Zhuhai station and Huidong station. The frequency at each pair of data bin (e.g., the bin width

is 1 dBZ for *ZH* and 0.1 dB for *Zdr*, 0.1 degree km−<sup>1</sup> for *Kdp*, and 0.1 for log10(*Kdp*/*Zh*)) refers to the total number of radar pixels with values of *Zdr* vs. *ZH* (*Kdp* vs. *ZH* or log10 (*Kdp*/*Zh*) vs. *Zdr*) falling into the bins, and the frequency distribution is further normalized by dividing by the maximum frequency among all the pairs of data bins.

**Figure 12.** Joint frequency distributions of polarimetric radar at (**a**,**c**,**e**) Doumen and (**b**,**d**,**f**) Gaotan areas from 00:00 UTC 29 August to 00:00 UTC 31 August: (**a**,**b**) *Zdr* vs. *ZH*, (**c**,**d**) *Kdp* vs. *ZH*, (**e**,**f**) log10 (*Kdp*/*Zh*) vs. *Zdr*. The black dots are the simulated radar moments based on DSD data at (**a**,**c**,**e**) Zhuhai station, (**b**,**d**,**f**) Huidong station.

Frequencies for the observed *ZH*, *Zdr* at Doumen rain region peaked at approximately 28 dBZ, and 0.3 dB, respectively. While at Gaotan rain region they peaked at high values, around 35 dBZ for *ZH* and 0.8 dB for *Zdr*. The *ZH* maximum values with the 10% normalized frequency of two regions

are similar around 52 dBZ, which are consistent with the extreme rainfall periods of two stations. Meanwhile, the *Zdr* maximum values with the 10% normalized frequency at Gaotan rain region is smaller (2.1 dB vs. 2.5 dB). Meanwhile, for the same *ZH*, the range and mean *Zdr* at Doumen rain region is larger than that at Gaotan station, which indicates that larger drops more likely occurred at Doumen compared with Gaotan. For both regions, the slope between *ZH* and *Kdp* shows a sharp transition over sampling grids with *ZH* exceeding 40 dBZ, resulting in a rapid increase of *Kdp* along with the increase of *ZH*. Although the peak frequencies for *Kdp* at two rain regions are similar around 0.1 degree km<sup>−</sup>1, *Kdp* increases faster as the *ZH* increases at Doumen rain region which indicates that there are more relative larger drops at Doumen rain region. Then, considering the higher rain rate and smaller raindrops at Gaotan station, the number concentration must be larger. The extreme lower value of log10 (*Kdp*/*Zh*) could confirm this, which means the concentration of small drops at Gaotan is higher than that at Doumen. It can be concluded from the polarimetric radar signatures that compared with Doumen rain region, the raindrops at Gaotan rain region have higher concentration but smaller drops, which is consistent with the DSD characteristic at Huidong station.
