Y. Yin et al., Seeding convective clouds with hygroscopic flares: Numerical simulations using a cloud model with detailed microphysics, J APPL MET, 39(9), 2000, pp. 1460-1472
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).