Quantified ice-nucleating ability of AgI-containing seeding particles in natural clouds

Abstract. For decades, silver iodide (AgI) has been widely used for laboratory ice nucleation experiments and glaciogenic cloud-seeding operations due to its ability to nucleate ice at relatively warm temperatures (up to −3 °C). Despite being one of the most well-characterized ice-nucleating substances, gaps remain in the understanding of how its ice nucleation behavior in the laboratory translates to natural clouds. Here, we present, for the first time, measurements of the ice-nucleated fractions (INFs) of AgI-containing seeding particles, derived from in situ measurements of ice crystal number concentrations (ICNC) and seeding particle number concentrations during glaciogenic cloud-seeding experiments. The experiments were performed as part of the CLOUDLAB project, in which we used targeted cloud seeding with an uncrewed aerial vehicle to try to answer fundamental questions about ice-phase cloud microphysics. Data from 16 seeding experiments show strong linear correlations between ICNC and seeding particle concentration, indicating relatively constant INFs throughout each experiment. Median INFs (0.07 %–1.63 %) were found to weakly increase with decreasing cloud temperature at seeding height (range of −5.1 to −8.3 °C). We compare our results with previous key laboratory experiments and discuss the possible freezing mechanisms. This study can help to bridge the gap in understanding of AgI ice nucleation behavior between laboratory and field experiments which further helps to inform future cloud-seeding operations.