Design of a Cloud Chamber and Quantitative Seeding Experiments for Warm and Cold Clouds

Abstract. Cloud chambers provide controlled thermodynamic environments for investigating cloud microphysical processes and their responses to aerosol perturbations. In this study, we developed a cloud chamber system for controlled observations of cloud microphysics and weather-modification processes. The system comprises two vessels with volumes of 2.7 and 9.0 m³. It provides adjustable temperature and pressure over approximate ranges of −40 to 40 °C and 30 to 1110 hPa, respectively, and is designed to regulate relative humidity across nearly the full 0 to 100 % range. The measurement system covers particles from submicron aerosols to cloud and precipitation particles, with approximate size ranges of 10–700 nm for submicron aerosols and 2–1550 μm for cloud droplets, larger hydrometeors, and ice particles. The chamber performance was evaluated using temperature-control, pressure-control, and humidity-control tests. Representative paired background–seeded experiments were then conducted in chamber B under warm-cloud and cold-cloud conditions. In the warm-cloud experiment, the introduction of 10 g of hygroscopic seeding powder increased the aerosol number concentration and was accompanied by a rapid increase in 3–10 μm droplets. Relative to the background case, the seeded case had a lower concentration of droplets ≥12 μm, with the mean concentration decreasing from 1357 to 478 cm⁻³, and showed shorter cloud persistence.

The relative-humidity decline rate was higher in the seeded case than in the background case, increasing from 0.47 to 0.75 % RH min⁻¹, consistent with enhanced water-vapour uptake after the introduction of hygroscopic particles. In the cold-cloud experiment, 0.5 g of AgI was ignited, and the resulting AgI-containing combustion aerosol increased the submicron particle concentration to above 2 × 10⁴ cm⁻³. The AgI-seeded case showed identifiable ice-crystal signatures together with a rapid decrease in liquid-droplet concentration. The total droplet concentration reached a short-lived peak of approximately 2.1 × 10³ cm⁻³ and decreased to less than one-third of this peak within about 1 min, while droplets larger than 10 μm were strongly reduced within about 2 min. These representative experiments show that the chamber system can support controlled observations of aerosol, droplet, and ice-particle responses associated with warm-cloud and cold-cloud seeding materials.