Numerical Simulation of Distribution and Evolution of Supercooled Liquid Water in Seeding Stratiform Cloud
层状云催化后过冷水分布与演变规律的数值模拟

Two materials are usually used for cold cloud seeding agents.One is coolant agent such as solid,liquid CO_(2)or N_(2),and they can generate ice crystals by strong cooling,the other agent,such as AgI,can be ice-forming nuclei.The important advantage of the coolant agent is that a number of generated ice crystals is nearly independent of the temperature. Since dry ice pellets have high fall velocity and have to be dropped from high altitudes,Fukuta(1996a,b) suggested a method to seed the liquid CO_(2) horizontally at the lower level of the supercooled portion of cloud.The objectives of this study are to compare the effect of the newly proposed cloud(seeding agent-liquid CO_(2) with current widely used seeding agent-AgI on cloud dynamics,cloud microphysics and precipitation evolution,especially the distribution and evolution of supercooled liquid water in seeded stratiform cloud.A one-dimensional rain category model developed by Guo Xueliang et al.(1999) is employed in this study with ice multiplication available.The contact and deposition nucleation processes of the seeding agents are considered as only inertial impact and Brownian collection are considered as a possible mechanism for contact nucleation.As a cooling agent,the cooling process due to vaporization and heat conduction between the seeded air and the liquid CO_(2) droplets are also included into this model to explore the cooling effect of liquid CO_(2) on the seeded cloud.Seeding rate is set to 0.06 g/s for both AgI and liquid CO_(2) in all seeded cases and seeding was starting at 170 min.Both AgI and liquid CO_(2) were released continuously for 10 min,20 min and 30 min in 52005600 m and 26003000 m levels,respectively.The simulations indicate that the liquid CO_(2) and AgI seeding in 52005600 m levels have nearly the same seeding effect which can reach to 11.1%,while that of liquid CO_(2)seeding at 26003000 m levels can reach 14.2%;and the cooling effect of liquid CO_(2) is found to be very faintness at the same time;when it comes to the seeding effect on cloud microphysics,growth of super cooled water begin at 200 min after depleted by seeding process;and the vapour content reduces 0.5 g/m~(3)2 g/m~(3) after seeding and contribute to the growth process of super cooled water;the concentration of rain droplet reduces 73% at 210 min and increases 309% at 230 min as compared to the condition without seeding.The conclusions can be made that:(1) Growth process of supercooled cloud water appears at 200 min after depleted with the vapour content contributed to the growth process;(2) better seeding effect can be achieved by seeding liquid CO_(2) in the region with more supercooled liquid water.