The role of competition effect in the raindrop formation

Large cloud condensational nuclei (CCN) affect droplet spectrum evolution in two ways: first, droplets growing on these nuclei are larger than others and accelerate the collision of cloud droplets having larger collision kernel; second, the condensation growth of large soluble aerosols between cloud base and the level of supersaturation maximum decreases the magnitude of the maximum which prevents activation of the smallest or less soluble nuclei and decreases the concentration of nucleated droplets. The latter process is known as competition effect. This effect is usually assumed to be an important factor leading to the rain enhancement by hygroscopic seeding. This study tests the hypothesis that the magnitude of the supersaturation maximum is reduced significantly when large CCN are present. A 2000 bin 1-D cloud parcel model was used to test this hypothesis. The model calculates diffusion aerosol growth and corresponding decrease in air relative humidity both below and above cloud base. It explicitly simulates droplet nucleation without any parameterization. The model calculates droplet concentration at the level of the supersaturation maximum under different concentrations and sizes of large soluble nuclei. Collisions are calculated by solving a stochastic kinetic equation for collision. The results of our investigation indicate that the previous studies have overestimated the efficiency of competition effect. The reasons of the overestimation of the effect in earlier studies are presented. The main effect of large soluble aerosols is the formation of large droplets which accelerate droplet collisions.