*3.1. Dataset after QC*

In total, 25,499 (934) 1-minute raindrop spectra passed (failed) the QC. The validated spectra account for a total rainfall of 1013.78 mm. According to the histogram in Figure 2, DSD samples failed to pass the QC mainly appear when rain rates (*R*stn) measured by collocated rain gauges at 1-min-interval were lower than 15 mm h−1. Falling beyond the threshold of the empirical *V*–*D* relationship is the major factor leading to droplet removal from the dataset, and accounts for 3.2% of total rainfall. It was also noted that most of the removed DSD samples were characterized by abnormally rain rates (*R*) compared with *R*stn, most of which occurred when *R*stn < 10 mm h−<sup>1</sup> or *R*stn > 100 mm h−<sup>1</sup> (red points in the scatter plot of Figure 2). The Pearson correlation coefficient (PCC) between the pairs of (*R*, *R*stn) was higher after QC (0.96 vs. 0.91). The linear fitting curve based on the dataset with *R*stn > 0 mm h−<sup>1</sup> after QC (blue line; denoted "QC + *R*stn > 0") is close to the diagonal line.

**Figure 2.** Histogram (top) of the number of 1-min raindrop spectra coinciding with rain gauge measurements (*R*stn); and scatterplot (bottom) of rain rate calculated by PARSIVEL2 disdrometer measurements vs *R*stn observations from rain gauge at BJ during the experiment period. The solid black line in the scatterplot is the 1:1 line. Data before (NonQC) and after (QC) quality control are indicated by red and blue dots, respectively.

As shown in Figure 3, the distribution of raindrops is almost entirely within the threshold of ±60% based on Equation (1). The filtered particles are mainly below 3 mm in diameter. They generally have low fall speeds but with relatively large size, likely due to the influences of strong winds or splashes from instrument surface during heavy rainfall [20]. The accumulated disdrometer data after QC are almost symmetric along the empirical *V*–*D* relationship of Atlas et al. [40] and the highest number concentrations of raindrops are nearly superimposed.

**Figure 3.** Scattergram of raindrop size distribution (DSD) at different diameter size and fall velocity classes after QC for the entire experiment period. The solid curve indicates the empirical *V*–*D* relationship described by Atlas et al. [40] which considers the air density effect; dashed curves indicate the ±60% ranges of the empirical *V*–*D* relationship.

A summary of rainfall observations after QC during the experiment period is listed in Table 1. The precipitation mainly occurred from June to August, which contributed up to 81.5% of the total rainfall amount. The mean and maximum rain rates, *R* and *R*max, were much higher during these three months than other months. The number of DSD samples, *N*mins, collected between June−August and in October, was much higher, contributing 78.3% of total samples. Although *N*mins in October was higher than June, *R*, *R*max, and the rainfall amount were much lower in October, especially *R*max (12.17 mm h−<sup>1</sup> vs. 84.92 mm h−1). The most (least) contribution of rainfall amount, as well as *R*max, came from July (September), while the least *R* and *N*mins came from April and September, respectively. Compared with 2017, the precipitation intensity in 2018 was heavier with higher *R* and *R*max but lower *N*mins and total rainfall amount. All these imply that the selected rainfall events consist of a wide variety of rainfall types.

**Table 1.** Summary of rainfall during the experiment period.


Note: *N*mins is the number of 1-min DSD samples. *R* and *R*max are the mean and max rain rate, respectively.
