The effect of hygroscopic seeding on warm rain clouds was examined using ahybrid cloud microphysical model combining a Lagrangian Cloud CondensationNuclei (CCN) activation model, a semi-Lagrangian droplet growth model, andan Eulerian spatial model for advection and sedimentation of droplets. Thishybrid cloud microphysical model accurately estimated the effects of CCN oncloud microstructure and suggested the following conclusions for a moderatecontinental air mass (an air mass with a large number of background CCN).(1) Seeding can hasten the onset of surface rainfall and increase theaccumulated amount of surface rainfall if the amount and radius of seedingparticles are appropriate. (2) The optimal radius of monodisperse particlesto increase rainfall becomes larger with the increase in the total mass ofseeding particles. (3) Seeding with salt micro-powder can hasten the onsetof surface rainfall and increase the accumulated amount of surface rainfallif the amount of seeding particles is sufficient. (4) Seeding by ahygroscopic flare decreases rainfall in the case of large updraft velocity(shallow convective cloud) and increases rainfall slightly in the case ofsmall updraft velocity (stratiform cloud). (5) Seeding with hygroscopicflares including ultra-giant particles (5 μm) hastens the onsetof surface rainfall but may not significantly increase the accumulatedsurface rainfall amount. (6) Hygroscopic seeding increases surface rainfallby two kinds of effects: the "competition effect" by which large solubleparticles prevent the activation of smaller particles and the "raindropembryo effect" in which giant soluble particles can immediately becomeraindrop embryos. In some cases, one of the effects works, and in othercases, both effects work, depending on the updraft velocity and the amountand size of seeding particles.
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