A Numerical Seeding Simulation of Convective Precipitation in Zhejiang, China

To change precipitation amount and distribution through artificial cloud seeding is one target of weather modification, especially for some important events. Cloud numerical simulations are important ways in research of weather modification activities. A 3-D convective model is used to do simulation for a convective rainfall case in Zhejiang on 1 September 2016. The 3-D convective model calculates 27 microphysical processes, which includes condensation, deposition, evaporation, collection, ice nucleation, ice multiplication, melting and freezing, auto conversion of cloud to rain, ice to graupel and graupel to hail. AgI seeding parameterization is based on cloud chamber results of ice forming processes by AgI which can be identified as deposition, contact freezing, condensation freezing and immersion freezing nucleation. Salt seeding scheme considers the micro-physical process between the salt particle and liquid and ice particles. Using the salt powder and AgI seeding scheme, a series of seeding simulations are designed with salt powder seeding, AgI seeding, and both of them, on seeding height levels, seeding rates, starting seeding times and the size of salt powder, to simulate seeding effects of warm cloud seeding, cold cloud seeding, and mixed cloud seeding schemes.Results show that salt powder seeding is mainly manifested by seeding effects at first rain-increasing then rain-reducing. The seeding mechanism is characterized by salt-dissolved droplets growth through colliding with cloud droplets, collected by raindrops, both of which fall to ground to increase precipitation. The rain enhancement effect is better when seeding in the ascending flow region with 12.5/L of salt powder amount of 30 μm particle size, the precipitation can be increased by 17.8%. AgI seeding is carried out, which basically shows an effect of increasing rainfall after rain reduction. The more silver iodide seeded, the greater the amount of rain reduction will be. For different seeding effects of salt powder and AgI, seeding effects are influenced by their amount of these two seeding agents. With 12.5/L of salt powder of 30 μm particle size, along with 100/L AgI agent, the precipitation can be increased by 19%. These results can be used to guide the field seeding experiment of weather modification with hygroscopic seeding agent and glycogenic seeding agent.