Numerical Simulation on Feasibility of Rain Enhancement by Hygroscopic Seeding over Kochi Area, Shikoku, Japan, in Early Summer.

In this study, we investigated the feasibility of rain enhancement by cloud seeding over a target area (the Sameura Dam catchment area, Kochi Prefecture) in early summer. The effects of salt micro-powder (MP) and hygroscopic flare (HF) seeding on the initial cloud microphysical structures were investigated using a detailed bin microphysics parcel model with background atmospheric aerosol data collected from ground-based observations conducted on the windward side of the target area and seeding aerosol data collected from the coordinated flights of seeding helicopter and in-situ measurement aircraft. Numerical seeding experiments showed that the size distributions of cloud droplets were broadened, and the onset of raindrop formation was accelerated by MP and HF seeding, although MP seeding showed more notable seeding effects than did HF seeding. MP seeding increased the mean droplet size and decreased the total number concentration of cloud droplets, whereas HF seeding had the opposite effect. Based on the relationship between the increase/decrease ratio of the cloud droplet number concentration and increase/decrease ratio of the surface precipitation by hygroscopic seeding obtained in previous studies, MP seeding had a positive seeding effect, whereas HF seeding had a negative effect. In the numerical seeding experiments, a range of variations in the number concentration and hygroscopicity of background aerosol particles, updraft velocity near the cloud base, the amount of seeding material applied, and the change in the physicochemical properties of the seeding aerosols to improve seeding effects were also considered. However, the outline of the results described above remained unchanged. These results demonstrate the possibility of increasing surface precipitation by MP seeding over the catchment. However, seeding a large amount of MP (NaCl) is necessary to enhance precipitation substantially. Simultaneously, considering the environmental impact is essential, as shown in our study. [ABSTRACT FROM AUTHOR]