Supersaturation Fluctuation Modulates Warm Rain Formation in Cumulus Clouds: Implications for Hygroscopic Cloud Seeding

Recent observational studies have demonstrated that supersaturation fluctuations (SS’) in clouds induced by turbulence at scales smaller than 100 m can significantly affect microphysical processes such as aerosol activation and droplet condensation. However, subgrid‐scale SS’ is often ignored in the current model. To investigate the impact of SS’ on warm rain and hygroscopic cloud seeding, this study conducted numerical experiments using the Weather Research and Forecasting large‐eddy simulations for an isolated cumulus cloud in a relatively clean environment. Subgrid‐scale SS’ is represented by Gaussian probability distribution functions. Sensitivity experiments were made to investigate the impact of SS’ on aerosol activation and condensation growth, as well as the seeding effect. The results indicate that subgrid‐scale SS’ affects warm rain processes through different dominant mechanisms: when SS’ acts solely on the activation, the droplets concentration increases significantly while their radius decreases, delaying the onset of precipitation; when SS’ acts simultaneously on activation and condensation, the large droplets are formed faster, which causes precipitation to begin earlier. For the case investigated in this study, hygroscopic seeding near cloud base results in a delay in precipitation formation due to competition between seeded and natural aerosols for water vapor, regardless of seeding fine or coarse aerosols. The subgrid‐scale SS’ could in part offset this delay, suggesting SS’ can modulate the seeding‐induced rain formation. This study reveals the key mechanisms by which subgrid‐scale SS’ influences the rain formation and emphasizes the importance of considering SS’ in understanding the hygroscopic seeding effect using model simulations.