Numerical Simulation Of Hygroscopic Seeding Effects On Convective Clouds And Rainfall Reduction

A two-dimensional slab-symmetric cloud model of Tel Aviv University is used to simulate the warm convective cloud precipitation process in autumn in East China.The height of the strong radar echo center and the range of high radar echo intensity are basically consistent with the observed values.The cloud seeding experiments with hygroscopic nuclei are conducted in order to consider the sensitivities of seeding effects to seeding time,seeding height,hygroscopic nuclei size ranges and seeding amounts of particles respectively.Furthermore,the initial CCN conditions are changed to simulate the cloud and precipitation under different pollution conditions,as well as the hygroscopic seeding with hygroscopic nuclei of different sizes.The results show that:1)the early seeding with full-spectrum hygroscopic nuclei in the cloud development stage can lead to more significant effect on rainfall suppression.The earlier the seeding time is,the stronger the inhibition of the growth of large particles.As the seeding time goes backwards,the particle size segment with the most significant inhibition is shifted to smaller particle size.In the early stage of cloud development,the small particle size hygroscopic nuclei are seeded to achieve the same rain-reducing effect as the full-spectrum hygroscopic nuclei,while the mid-range particle size or large particle size hygroscopic nuclei are seeded to obtain no obvious influence on the precipitation.It is indicated that the small particle size hygroscopic nuclei play a dominant role in reducing rainfall.2)The effect of rainfall suppression is more obvious when seeding the small particle size hygroscopic nuclei is carried out just below the area with large supersaturation in the center of cloud.The reduction rate of ground accumulated precipitation reaches to 23.3%,when the seeding concentration is 350cm^-3.With the increase of seeding amounts,the precipitation suppression effect is more significant,and the rain is even eliminated.Therefore,seeding small particle size hygroscopic nuclei properly in warm convective clouds can achieve expected results of reducing or eliminating rain.3)With the increase of pollution degree,that is,the increase of the initial CCN number concentration,the development of cloud is obviously weakened,and the precipitation is also significantly reduced.4)With the increase of pollution degree,that is,the increase of the initial CCN number concentration,the dominant suppression precipitation role of the small-size hygroscopic nuclei in full-spectrum hygroscopic nuclei is gradually weakened,while the effect of large-particle hygroscopic nuclei in full-spectrum hygroscopic nuclei to promote precipitation is gradually enhanced.When the initial CCN number concentration is increased to 2010 cm^-3,seeding with the full-spectrum hygroscopic nuclei can not reduce the precipitation.At this time,only the small-size hygroscopic nuclei can be seeded to achieve the effect of reducing rain.5)Under different pollution degrees,that is,under different initial CCN concentration conditions,the mid-range particle size hygroscopic nuclei in the full-spectrum hygroscopic nuclei seeded have no significant effect on precipitation.