Simulating Aerosol–Cloud Interactions during Hygroscopic Seeding

Hygroscopic seeding introduces salt particles into warm clouds via burning flares or fine sprays. It has been conducted over the globe since last century for the purpose of rain enhancement. The concept of hygroscopic seeding is that by introducing large hygroscopic particles, the activation of small natural aerosols is suppressed and the collision-coalescence process is promoted. It has been hypothesized that seeding provides raindrop embryos and broadens the droplet size distribution (DSD) at early cloud stage, and thus accelerates or enhances the rain formation over the seeded area.

However, until now, the direct effect of the hygroscopic seeding is still inconclusive due to the complexity of the problem involving different seeding materials, various microscopic processes, different meteorological factors, and different assessment methods. And at the same time, an increasing demand for weather modification over the arid or semi-arid area is observed in the context of changing climate.

This study looks at the impact of hygroscopic seeding at the fundamental level. We conducted a series of simulations in both direct numerical simulation (DNS) and parcel model to investigate the microscopic impact and the macroscopic impact of the hygroscopic salt particles on the aerosol activation, droplet size growth, and evolution of the local moisture field during the air parcel ascends. With the mono-disperse pristine background aerosol condition (dry radius R=0.1μm, number concentration N=100 cm^-3, and hygroscopicity k=0.3), various seeding scenarios (R=1-10μm and N=1-10 cm^-3, and k=1.2) are investigated and compared with the non-seeded scenario. It is shown that both the hygroscopic property and seeded aerosol loading are important to the early DSD development. In contrast, seeding results does not depend on the hygroscopicity of the small ground aerosol. By comparing DNS and parcel model, the importance of aerosol-turbulence interaction on modulating the moisture field and thus modifying the activation rate of the background aerosol is also investigated.