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1. Introduction
The natural hazard of hail is generally produced by deep convective storms with an appropriate updraft strength, sufficient supercooled liquid water content, conducive temperature conditions, and an optimal lifetime (Nelson 1983; Ziegler et al. 1983; Hohl et al. 2002; Grant and van den Heever 2014; Punge and Kunz 2016; Dennis and Kumjian 2017; Li et al. 2017). Hail often develops during a short time period (as little as a few minutes and up to a few hours), directly causing huge economic and human losses on a daily scale in China (Cao 2008; Guan et al. 2015; Li et al. 2017). The substantial impacts of hail on agriculture, ecosystems, and human society can last over time scales from months to seasons. In 2015, for example, 57 people were killed directly by hailstorms, 0.67 million houses were damaged, 2.92 million ha of crops were affected, and CNY 32.27 billion of direct economic loss occurred because of severe storms, including hailstorms, tornadoes, and damaging winds (China Meteorological Administration 2016). Therefore, understanding the accurate climatology of hail is the first step toward improving hail disaster prevention strategies.
The climatology of hail has been studied using various types of data over different spatial and temporal scales worldwide (Vinet 2001; Schuster et al. 2005; Tuovinen et al. 2009; Cintineo et al. 2012; Kim and Ni 2014; Burcea et al. 2016; Kahraman et al. 2016; Punge and Kunz 2016; Martins et al. 2017). For example, in Europe, hail risk has been determined using data from 1987 to 1996 from a hail pad network and insurance data (Vinet 2001). Hail climatology has also been explored using the data from newspapers, storm spotters, and eyewitness reports during a relatively long period (1930-2006) in Finland from the perspective of severe hail days and hail size distribution (Tuovinen et al. 2009). Both hail days and hail size have been studied by using data from 105 weather stations in Romania during 1961-2014 (Burcea et al. 2016). A comprehensive review of hail climatology in Europe has been presented by Punge and Kunz (2016). In North America, hail days were recently studied in the United States using multiradar multisensor data for only 4 years, during 2007-10 (Cintineo et al. 2012), and hail size was also studied from...