Evaluating hygroscopic cloud-seeding effects by in-situ observation and real-case simulation

Evaluating the efficacy of cloud-seeding operations presents a complex challenge in atmospheric science, due to the multifaceted interactions between artificial nuclei, natural cloud processes, and local meteorological conditions. In this study, to assess cloud-seeding effects, several field experiments were conducted at Dongyanshan site (elevation 840 m) in northern Taiwan during northeast monsoon seasons from 2019 to 2022. The experiments utilized hygroscopic cloud seeding, leveraging the site's advantages of well-established instrumentation and a semi-closed, in-cloud environment. Our observational results indicate that the seeding agents can strengthen the competition effect, which causes an increase in liquid water content (LWC) and a decrease in water vapor mixing ratio, as well as the tail effect, which widens the drop size distribution (DSD). This important phenomenon of increased raindrop number concentration after cloud seeding was further confirmed by DSD observations at the upstream neighboring site, Xiayunping (elevation 340 m), where no cloud seeding was executed. Additionally, real-case model simulations were performed to further investigate the microphysical processes of cloud seeding. Simulation results indicate that cloud seeding enhances the cloud activation process (Pcact) while leading to the development of smaller cloud droplets, which decreases the auto-conversion process (Praut) of rain. Concurrently, an intensified accretion process (Pracw) increases raindrop diameter. These findings provide valuable insights into the mechanisms of cloud seeding, potentially improving the design and implementation of future weather modification strategies in similar atmospheric conditions.