*2.1. Observations*

The PARSIVEL2 disdrometer is located at Beijing station in the North China Plain surrounded by the Yan mountains to the west and north, and the gulf-like Bohai Sea to the southeast (Figure 1). The mean annual precipitation was 575 mm during the most recent decade (2009–2018). In this study, 14 months continuous DSD measurements in 2017–2018, ranging from 1 April to 31 October in each year, were used, which made up 96.7% of the total rainfall (1085.8 mm out of 1122.7 mm) during this period. In order to focus on rainfall analysis, winter precipitation (mainly snowfall from November to March the next year), as well as the solid precipitation (such as hail), was removed according to the ground weather reports.

**Figure 1.** Topographic (m) information around the PARSIVEL<sup>2</sup> disdrometer site at Beijing station (BJ, the red circle). The districts of Beijing are highlighted in black curves.

### *2.2. Quality Control (QC)*

Particle diameter and fall speed, each divided into 32 nonuniform classes, were measured by the PARSIVEL2 disdrometer with a 1-min sampling interval. The mean values of particle diameter (0.062–24.5 mm) and fall speed (0.05–20.8 m s−1) are described by the manual [37]. The first two size bins are not included in the analysis, because of the low signal-to-noise ratios. As a result, the smallest detectable mean diameter is 0.312 mm. The effective sampling area of PARSIVEL2 droplet size measurements is affected by the so-called border effects, and the method of Jaffrain and Berne [38] is utilized to account for these effects. In particular, defining *Di* (mm) as the central volume-equivalent diameter for the *i*th size bin, the effective sampling area can be calculated as 180 mm × (30 mm − 0.5*Di*).

The empirical terminal velocity–diameter (*V*–*D*) relationship of Gunn and Kinzer [39] with air-density correction factor (ρ0/ρ*a*) 0.4 [40,41] was used to assess raindrop observations and is repeated as follows:

$$V\_t(D\_i) = [9.65 - 10.3 \exp(-0.6D\_i)] \left(\frac{\rho\_0}{\rho\_a}\right)^{0.4} \,\mathrm{}\,\tag{1}$$

where *Vt*(*Di*) is the mean particle terminal velocity for the *i*th size bin; ρ*<sup>a</sup>* and ρ<sup>0</sup> (1.20 kg m<sup>−</sup>3) are the air density at the observation altitude and at sea level, respectively. Following the method described in Atlas et al. [40] and Foote and Toit [41], the mean value (1.008) of the correction factor was selected for simplicity.

Some droplet observations may deviate from the *V*–*D* relationship shown in Equation (1). A commonly used method to eliminate those abnormal particles is to set a threshold regarding Equation (1). A value of ±60% was selected as the threshold [20] in this study, which means droplets with velocities of *V*obs(*Di*) were discarded when they met the condition *<sup>V</sup>*obs(*Di*) <sup>−</sup> *Vt*(*Di*) <sup>&</sup>gt; 0.6*Vt*(*Di*). In addition, the 1-min DSD spectrum with a total number of raindrops *CT* < 10 or a rain rate lower than 0.01 mm h−<sup>1</sup> was considered to have no rain. Rain drops larger than 8 mm in diameter were also removed. Then, continuous spectra with rain-free periods of no longer than 1 h were defined as a rain event, and rain events lasting less than 5 min were eliminated to reduce the statistical errors. The dataset after quality control is further described in Section 3.1.
