Response of a liquid water cloud to in situ hygroscopic seeding

Abstract. Precipitation enhancement may augment water resources, yet questions still remain as to the effectiveness of current cloud seeding strategies. One such strategy, hygroscopic seeding, is designed to accelerate collision-coalescence and eventually precipitation by creating a large droplet tail in the size distribution. In the traditional approach, particles are released at cloud base, typically through burning a flare. The effect of directly injecting a hygroscopic powder into a preexisting cloud has not been experimentally tested in controlled conditions. We performed experiments in the Michigan Tech Pi Chamber to determine how a steady-state liquid water cloud responds to injection of hygroscopic powders. Three materials were tested: jet-milled NaCl, a newly developed NaCl-TiO2 core-shell material, and Arizona test dust as a non-hygroscopic control. Injection of the powders produced a local increase in liquid water content and stimulated formation of droplets up to 60 microns in diameter—significantly larger than the background cloud droplet population. Analysis of relevant timescales indicates that NaCl particles with dry diameters between 4 and 25 μm would be suitable for this application. These results demonstrate that in-cloud hygroscopic seeding can successfully generate large droplets that could accelerate warm rain processes, which supports reconsideration of in-cloud injection as a viable precipitation enhancement strategy alongside traditional cloud-base release, particularly for warm clouds in arid and semi-arid environments.