Geoscience Reference
In-Depth Information
storms. However, it appears likely that clouds with
an abundance of natural ice crystals, or with
above-freezing temperatures throughout, are not
susceptible to rainmaking. Premature release of
precipitation may destroy the updrafts and cause
dissipation of the cloud. This explains why
some seeding experiments have actually
decreased
the rainfall! In other instances, cloud growth
and precipitation have been achieved by such
methods in Australia, China and the United
States. Programs aimed at increasing winter
snowfall on the western slopes of the Sierra
Nevada and Rocky Mountains by seeding cyclonic
storms have been carried out for a number of
years with mixed results. Their success depends on
the presence of suitable supercooled clouds. In a
recent experiment in Wyoming, ground-based-
only seeding began in the winter of 2006-2007 and
in 2007-2008, a plane and 25 ground stations were
involved. Evaluation of the degree of success is
underway. There are at least two reasons why it is
difficult to establish the impact of cloud seeding:
the mismatch between the scale of the impact and
the scale at which seeding operates; and the large
natural variability of precipitation versus the
relativcly minor effect of seeding.
When several cloud layers are present in the
atmosphere, natural seeding may be important.
For example, if ice crystals fall from high-level
cirrostratus or altostratus (a 'seeder' cloud) into
nimbostratus (a 'feeder' cloud) composed of
supercooled water droplets, the latter can grow
rapidly through the Bergeron process and such
situations may lead to extensive and prolonged
precipitation. This is a frequent occurrence in
cyclonic systems in winter and is important in
orographic precipitation (see E3, this chapter).
ever, particles just as easily break up if subject to
collisions. Langmuir offered a variation of this
simple idea. He pointed out that falling drops
have terminal velocities (typically 1-10cm s
-1
)
directly related to their diameters, such that the
larger drops can overtake and absorb small
droplets; the latter might also be swept into the
wake of larger drops and absorbed by them.
Figure 5.9
gives experimental results of the rate of
growth of water drops by coalescence, from an
initial radius of 20mm in a cloud having a water
content of 1g/m
3
(assuming maximum efficiency).
Although coalescence is initially slow, droplets
reach 100-200μm radius in 50 minutes.
Moreover, the growth rate is rapid for drops with
radii greater than 40
μ
m. Calculations show that
drops must exceed 19
m radius before they can
coalesce with others; smaller droplets are swept
aside without colliding. The initial presence of a
few very large droplets calls for the availability
of giant nuclei (e.g., salt particles) if the cloud
top does not reach above the freezing level.
Observations show that maritime clouds do have
relatively few large condensation nuclei (10-50
μ
m
radius) and a high liquid water content, whereas
continental air tends to contain many small
nuclei (~ 1
μ
m) and less liquid water. Hence, rapid
onset of showers is feasible by the coalescence
mechanism in maritime clouds. Alternatively, if a
few ice crystals are present at higher levels in the
cloud (or if seeding occurs with ice crystals falling
from higher clouds) they may eventually fall
through the cloud as drops and the coalescence
mechanism comes into action. Turbulence in
cumulus clouds serves to encourage collisions
in the early stages. Thus, the coalescence process
allows for more rapid growth than simple
condensation, and is, in fact, common in 'warm'
clouds in tropical maritime air masses, even in
temperate latitudes.
Rainmaking in warm clouds has been
attempted through hygroscopic seeding of clouds
that makes use of compounds such as sodium,
lithium and potassium salts. The idea is to
generate larger droplets, either by providing larger
μ
2 Coalescence theories
Theories of raindrop growth use collision,
coalescence and 'sweeping' as growth mecha-
nisms. It was originally thought that cloud particle
collisions due to atmospheric turbulence would
cause a significant proportion to coalesce. How-
