Seeding Convective Clouds with Hygroscopic Flares: Numerical Simulations Using a Cloud Model with Detailed Microphysics

Numerical experiments were conducted to evaluate the role of hygroscopic flare seeding on enhancement of precipitation in convective clouds. The spectra of seeding particles were based on measurements of the particles produced by hygroscopic flares used in field experiments in South Africa. The seeding effects were investigated by comparing the development of precipitation particles and rain production between the seeded and unseeded cases for clouds with different cloud condensation nuclei (CCN) concentrations and spectra. The South African hypothesis that the introduction of larger and more efficient artificial CCN below cloud base at the early stage of cloud development would influence the initial condensation process in the cloud, resulting in a broader droplet spectrum and in acceleration of the precipitation growth by coalescence, was tested. The results show that the largest seeding particles broaden the cloud droplet distribution near cloud base, leading to an earlier formation of raindrops, graupel particles, and, therefore, stronger radar echoes at a lower altitude. The results also show that the large artificial CCN prevent some of the natural CCN from becoming activated. It was found that seeding with the full particle spectrum from the flares could increase rainfall amount in continental clouds having CCN concentrations of more than about 500 cm−3 (active at 1% supersaturation). Seeding more maritime clouds resulted in reducing the integrated rain amount, although in some cases rain formation was accelerated. The physical mechanisms responsible for these results were explored by investigating the relative importance of different segments of the size spectrum of the seeding particles to precipitation development. It was found that, out of the full spectrum, the most effective particles were those with radii larger than 1 μm, especially those larger than 10 μm; the particles smaller than 1 μm always had a negative effect on the rain development. The sensitivity of seeding effects to seeding time, seeding height, and seeding amounts also was tested. The biggest precipitation enhancement was obtained when seeding was conducted a few minutes after cloud initiation and above cloud base. The radar reflectivity at that time period was lower than 0 dBZ. Rain enhancement also increased with the increase in the concentration of the large seeding particles in the spectrum (at least for the amounts tested here).