Numerical Simulation on the Formation of Large-size Hailstones

Although large-size hailstones may cause damages to agriculture, human life and properties, the formation mechanism of large-size hailstones has not been completely understood. In order to further understand the formation of large-size hailstones, the three-dimensional compressible non-hydrostatic hailstorm model with hail-bin microphysics that can simulate different sizes of hailstones developed by Institute of Atmospheric Physics, Chinese Academy of Sciences, is used to investigate the formation process of a heavy hailstorm in Beijing on 16 July 2014. The observed maximum diameter of hailstones on the ground is up to 7 cm. The convection effective potential energy is 1785.3 J·kg^-1 and the moisture content is high in the lower layer and low in the upper layer, indicating that the atmosphere is strongly instable and is conducive to the formation of strong convection. The simulated hail cloud-top height is about 13 km, which is consistent with that observed by the S-band radar in Beijing. The simulated maximum updraft is up to 30 m·s^-1, indicating that the hailstorm is strong and severe. Moreover, the storm has an obvious tilting dynamic structure due to the strong wind shear at middle and upper levels, which makes the separation of falling path of hailstones and raindrops from the main updraft and causes the long duration of hailstorm. The simulated microphysical process of the hailstorm has some obvious characteristics, one of the most prominent properties is that there is an accumulation zone of high supercooled rain water with 12-16 g·kg^-1 located between -35℃—-10℃. The main process of embryos production for hailstones is the collision between cloud ice and supercooled raindrops, and the production rate may be up to 10^-2 g·kg^-1·s^-1. And the hailstone growth process strongly depends on the accretion of supercooled cloud water by hailstones, and the growth rate is the same as that of production rate of embryos of hailstones. This research shows that the supercooled rainwater accumulation zone may exist in the formation process of large hailstones in Beijing. However, the model is not able to simulate the size of hailstones up to 7 cm, the simulated maximum sizes of hailstones are usually about 2-3 cm. Causes are not clear, one important cause might be related with the melting process of hailstones in the model, and the initial atmospheric field used in the model. The issue needs to be further clarified and the microphysical processes relevant to hailstones need to be improved in the future study.