## 2 Ice Accretion Tests
### Facilities and Experimental Procedure  
The laboratory had a work section with dimensions of 4.7 m (length), 3.7 m (width), and 2.6 m (height). The temperature \(T\) in the testing zone could be adjusted continuously between \(-20\) and 20 \({}^{\circ}\)C with an accuracy of \(\pm 0.1\)\({}^{\circ}\)C which was controlled and measured by a thermostat. Icing objects were supported on the steel brackets, as shown in Figure 2. Several air-atomizing nozzles were used to spray water to simulate the required rainfall intensity and environmental humidity. The vertical distances between atomizing nozzles and the top surface of icing objects were about 50 cm. Generally, the range of the mean volume diameter (MVD) in freezing rain is from 500 \(\upmu\)m to 3-4 mm [33]. The diameters of the sprinkler heads were 2.0 mm. [PERSON] et al. [34] showed that freezing rain is generally produced at temperatures ranging from 0 \({}^{\circ}\)C to \(-10\)\({}^{\circ}\)C. The laboratory air temperature was set to \(-7\)\({}^{\circ}\)C to accelerate the icing progress. The average humidity was set to around 80% at the position of icing objects based on the results of [PERSON] and [PERSON] [35]. Freezing rain was reproduced in the tests, the precipitation rate in freezing rain is usually 1-2 mm/h, and in extreme cases 10 mm/h [30]. Unfortunately, the precipitation rate was set 45 mm/h in the experiment which was much higher than typical values encountered during freezing rain to hasten the icing process by referring to the recommend values 60 \(\pm\) 20 mm/h in [PERSON] et al. [19]. It was impossible to produce more typical freezing rainfall rates of 1 mm/h without compromising the uniformity of the water supply along the cylinder [16]. Water was chilled to 0-2 \({}^{\circ}\)C before being transferred into the nozzles. The precipitation intensity was controlled by the water pressure, air pressure, nozzle size, and water flow rate supplied to the nozzles, which was measured near the center line of the icing objects with a pluviometer. Precipitation occurred in the top-down direction under calm condition. The temperature of the injected water droplets was higher than the surface temperature of the icing  
Figure 2: Experiment setup. object, and the latent heat of the impinging supercooled water droplets was released during freezing. Therefore, the accreted ice would partly melt and the nozzles were easy to freeze if the water were continuously sprinkled if the water were continuously sprinkled. Hence, the water was not sprinkled continuously, but instead, cycles of a 2-min sprinkle and 5-min pause were repeated. Based on the test results as shown in the follow, the radial ice shapes in the experiment were similar with previous studies [14,26], which indicated that the pulsed spraying method did not affect the radial ice shape. After each spraying, the nozzles would be put into another holding room where temperature was about 1 \({}^{\circ}\)C to prevent the nozzles from freezing. Ice accretion is a complex natural phenomenon which depends on various factors, including the wind velocity, external temperature, droplet MVD, and precipitation rate, and so on [7,14,30,36,37,38]. There are too many uncertainties for the ice accretion, and it is difficult to be refined. The influence of above-mentioned factors has been ignored, and some specific climatic conditions have been studied experimentally in the paper. Although icicles growth process was considerably affected by wind speed by affecting the heat exchange and droplets trajectory, resulting growth rate of diameter and length of icicles increase with the wind speed [39], and the radial ice size was also affected by the wind speed [7,13,14].