Review of advances in hail formation process and hail suppression research

As a kind of short-period severe weather disaster, hails often have severe impacts on agriculture, construction, electricity, transportation and even lives and properties, etc. Therefore, hail suppression is widely concerned worldwide. This paper provides a detailed review of research progress in hail formation mechanism and hail cloud physics from perspectives of mechanism, technology, scientific experiments and effect evaluation of hail suppression for the purpose to gain in-depth knowledge of domestic and international development of hail suppression in both theoretical and practical fields, improve our understanding of scientific problems in hail suppression, and provide references for promoting theoretical research and technological progress of hail suppression in China. Major results as follows: (1) The "theory of zone of accumulation" and the "theory of cyclic growth" are the most common theories of hail formation. Limited by the early radar observation technology and the lack of complete numerical models for hail simulation, the early knowledge of the hail formation mechanism has certain limitations. (2) Hail embryos are generally divided into frozen drop embryos and graupel embryos. Frozen drop embryos are formed by the freezing of supercooled raindrops while graupel embryos are formed by the growth of collision and freezing of ice crystals and snowflakes. What type of hail embryo is dominant in the hail cloud mainly depends on temperature of the cloud base. The development of hail clouds depends on key factors such as water vapor condition, dynamic instability condition, and vertical wind shear, etc. (3) The mechanism of hail suppression mainly follows two technical lines, "hail suppression by seeding" and "hail suppression by explosion". "Competing interests" and "early rainfall" are the two most widely used theories of seeding among the six common hypotheses of hail suppression on which hail suppression operations are designed. (4) Technically, the hail suppression operations mainly include seeding hail clouds with artificial ice nuclei by aircraft, rocket launcher, ground generator, etc., or launching shells with artificial ice nuclei by ground artillery, which can affect the growing process of hail to suppress or weaken the growth of hail. (5) A large number of field experiments of hail suppression have proved that there are regional differences in operation effect of hail suppression. It is necessary to formulate and develop regional hail suppression technology systems adapted to local conditions according to the characteristics of hail clouds and hailstorms in different regions. (6) Evaluating the effect of hail suppression is still a bottleneck problem that limits the development of hail suppression technology. The methods commonly used to assess the effect of hail suppression mainly include statistical, physical, and numerical simulation evaluations, which need further improvement. Due to the rapid changes in hail clouds and short hailstorm processes, there exits great difficulties in the timeliness of implementation of hail suppression operation and the effect evaluation of hail suppression. It will start from carrying out fine detection of hail clouds based on a variety of observation equipment and comprehensive field experiments of hail suppression with scientific design. Statistical, physical and numerical simulation approaches should be combined to evaluate the effect of hail suppression and promote further development of hail suppression technology in future.