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 fl are seeding on enhancement of precipitation in convective clouds. The spect ra of seeding particles were based on measurements of the particles produce d by hygroscopic flares used in field experiments in South Africa. The seed ing effects were investigated by comparing the development of precipitation particles and rain production between the seeded and unseeded cases for cl ouds with different cloud condensation nuclei (CCN) concentrations and spec tra. The South African hypothesis that the introduction of larger and more effic ient artificial CCN below cloud base at the early stage of cloud developmen t would influence the initial condensation process in the cloud, resulting in a broader droplet spectrum and in acceleration of the precipitation grow th by coalescence, was tested. The results show that the largest seeding pa rticles broaden the cloud droplet distribution near cloud base, leading to an earlier formation of raindrops. graupel particles, and, therefore, stron ger 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 hares could increase rainfall amount in continental clouds having CCN concentrations o f more than about 500 cm(-3) (active at 1% supersaturation). Seeding more m aritime clouds resulted in reducing the integrated rain amount, although in some cases rain formation was accelerated. The physical mechanisms respons ible for these results were explored by investigating the relative importan ce of different segments of the size spectrum of the seeding particles to p recipitation development. It was found that, out of the full spectrum, the most effective particles were those with radii larger than I mu m, especial ly those larger than 10 mu m; the particles smaller than I mu m always had a negative effect on the rain development. The sensitivity of seeding effects to seeding time, seeding height, and see ding amounts also was tested. The biggest precipitation enhancement was obt ained when seeding was conducted a few minutes after cloud initiation and a bove cloud base. The radar reflectivity at that time period was lower than 0 dBZ Rain enhancement also increased with the increase in the concentratio n of the large seeding particles in the spectrum (at least for the amounts tested here).