Effects of hygroscopic seeding on raindrop formation as seen from simulations using a 2000-bin spectral cloud parcel model

A 2000-bin cloud spectral parcel model is used to investigate the effect of hygroscopic seeding on warm rain formation under different thermodynamic conditions. Simulations show that utilization of commercial hygroscopic flares (“French”, “South African”, New AI and D383) increases raindrop production in those cloud parcels where the natural warm rain process is inefficient. The most effective flare was found to have a maximum fraction of large seeding cloud condensational nuclei (SCCN). An optimum seeding particle radius, which provides the maximum raindrop production under a given mass of the seeding reagent varies from 1.5 to 2.5 μm and slightly depends on the reagent mass, as well as on the dynamic properties of cloud parcels. The existence of the optimum size of seeding particles is important from both a business perspective and an environmental perspective. In the presence of natural, large CCN, the seeding effect decreases due to the efficient collision process initiated by them. The decrease depends on the concentration of large, natural CCN. Thus, to evaluate the effects of hygroscopic seeding one needs to know the properties of large, natural CCN in the region of seeding activity. It is shown that, when a reagent consisting of CCN of the optimum radius is used, a significant increase in the raindrop production can be achieved even when large, natural CCN are present as well. to the limitations of warm rain cloud parcel models in representing the microphysical and dynamical properties of real clouds, the results presented in this study should be verified using more complicated multidimensional models with spectral microphysics.