*4.1. Precipitation Pattern Observed by a Gauge Network*

Figure 4 shows the spatial distribution of the cumulative rainfall in Guangdong during the period from 00:00 UTC 27 August to 00:00 UTC 2 September based on the gauge hourly precipitation data using Inverse Distance Weighted (IDW) interpolation method. This result was only used for visualization so as to get a general sense of the geographic distribution of rainfall. Obviously, this rainstorm affected most regions of Guangdong, with the extreme rainfall occurring at the southeast part and gradually decreased to the northwest of Guangdong. The gauge maximum cumulative rainfall was 1394.6 mm recorded at Gaotan station (23.1883 ◦N, 115.3044 ◦E), which set a new record of cumulative rainfall during a single event in Guangdong. Meanwhile, the maximum daily rainfall of 1056.7mm was also recorded at Gaotan station from 21:00 UTC 29 August to 21:00 UTC 30 August, which broke the historical daily rainfall record of 924.3 mm on 17 August 2013 [7].

**Figure 4.** The spatial distribution of the accumulated rainfall from 27 August 2018 to 1 September 2018 of Guangdong province. The red squares and triangles represent the locations of radars and disdrometers, respectively.

Figure 5 shows the distribution of the daily rainfall that exceeded 100 mm from 28 to 31 August. It is clear that the rainfall is mainly concentrated on 29 and 30 August. There is a clear rain band along the coast on 29 August, with a gradient of accumulated rainfall decreasing from the coast towards the inland region, which can be partially attributed to the land-sea roughness contrasts [38]. On 30 August, the rain band moved a little towards northeast and inland region, with extremely high accumulated value (1041.1 mm) concentrated at Gaotan station, which could be due to the combined effect of terrain

and evolved synoptic condition. To further reveal the two rainfall processes, Figure 6 shows the time series of rainfall collected at two gauges where extreme daily rainfall concentrated on these two days. One station is located at Doumen (22.1967 ◦N, 1153.1131 ◦E) in Zhuhai, representing the rainfall on 29 August, when the daily rainfall reached 458.6 mm. Another station is Gaotan station, representing the most extreme rainfall pattern both for 30 August and the whole event.

**Figure 5.** Daily rainfall accumulation (in mm, a day is defined from 0000 UTC to 0000 UTC of the next day) for (**a**) 28 August, (**b**) 29 August, (**c**) 30 August, (**d**) 31 August, (**e**) a zoomed area for 30 August, (**f**) a zoomed area for 29 August. Each dot represents a gauge station with daily rainfall accumulation exceeding 100 mm.

**Figure 6.** Hourly and accumulated precipitation at (**a**) Doumen station, (**b**) Gaotan station from 00:00 UTC 29 August to 12:00 UTC 31 August.

The differences between the two stations are clear: Doumen station experienced heavy rainfall mainly on 29 August, while Gaotan station experienced a heavier and longer rainfall from late 29 August to early 31 August. Doumen station experienced three major rainfall stages on 29 August: first from 00:00 to 06:00 UTC, second from 06:00 to 14:00 UTC, and third from 14:00 to 23:00 UTC. The third stage was longer and stronger than the first two stages, with a maximum hourly rainfall of 70.2 mm from 1800 to 1900 UTC 29 August. After 0800 UTC 30 August, rainfall at Doumen station almost disappeared. At Gaotan station, rainfall exhibited two major stages: first from 00:00 to 20:00 UTC on 29 August, then from 20:00 UTC 29 August to 08:00 UTC 31 August. The first stage, with only one hourly rainfall exceeding 20 mm is much weaker than the second. During the second stage, the hourly rainfall showed an increasing trend, and reached a maximum of 98.6 mm from 1900 to 2000 UTC 30 August. Moreover, the cumulative rainfall of the second stage (1206.6 mm) accounted for 87% rainfall of the whole event at Gaotan station.

#### *4.2. Raindrop Size Distribution*

#### 4.2.1. DSDs Time Series at Two Observation Stations

As there are no disdrometers deployed at the Doumen and Gaotan stations, two nearby Parsivel2 disdrometers were selected to analyze the raindrop size distribution from 00:00 UTC 29 August to 12:00 UTC 31 August. One disdrometer is located at Zhuhai (22.2750 ◦N, 113.5669 ◦E) near the Doumen station (45 km away); the other is located at Huidong (23.0261 ◦N, 114.6681 ◦E) near the Gaotan station (67 km away) (see Figure 1). The comparisons of hourly rainfall between the disdrometers and the gauges at Zhuhai and Huidong stations (within 20 m for each pair) are shown in Figure 7. Although the disdrometer might slightly underestimate the rainfall compared with gauge data because of the absence of some records and the overlap of drops along the laser beam, overall, they captured the rainfall pattern very well. As such, we ignored the impact on DSD observations in this study. Moreover, the rainfall pattern at Zhuhai station is similar to that of Doumen station, and the rainfall pattern of Huidong station is similar to Gaotan station. The similar patterns indicate that these two disdrometers could, respectively, represent the DSD characteristic of Doumen and Gaotan stations.

**Figure 7.** Hourly rainfall measured by gauges and collocated disdrometers: (**a**) Zhuhai station, (**b**) Huidong station from 00:00 UTC 29 August to 12:00 UTC 31 August.

Figure 8 shows the time series of the drop size distribution (*N*(*D*): m−<sup>3</sup> mm<sup>−</sup>1) in the logarithmic scale from 00:00 UTC 29 August to 12:00 UTC 31 August, derived by the 1-min disdrometer observations at the two stations. In line with the synoptic condition shown in Figure 2, it can be seen that the Zhuhai station experienced stronger rainfall during 29 August, and it reached the maximum hourly rainfall of about 40 mm (Figure 7). The maximum raindrops greater than 7 mm were recorded from 12:00 to 14:00 UTC 29 August. After that, rainfall at Zhuhai station began to weaken with the decreasing number of raindrop concentration and size of raindrops, while that at Huidong station gradually increased. After 23:00 UTC 29 August, this long-lasting storm disappeared at Zhuhai station following with two short storms characterized with lower concentration but bigger raindrops around 04:00 UTC 30 August and around 11:00 UTC 31 August. The rain at Huidong station showed an increasing trend from 13:00 UTC 29 August, and the number of raindrop concentration and size of raindrops also show an increasing trend, and the hourly rainfall reached the highest from 18:00 to 19:00 UTC 30 August. Compared to the extreme rainfall period at Zhuhai station, the number of large drops (*D* > 5 mm) at Huidong station is much less during the strong rainfall period which we hypothesize is due to the collision caused by sea-inland orographic effect [38–40].
