Geoscience Reference
In-Depth Information
composed of supercooled water droplets, the latter can
grow rapidly by the Bergeron process and such situa-
tions 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).
3 Solid precipitation
Rain has been discussed at length because it is the most
common form of precipitation. Snow occurs when the
freezing level is so near the surface that aggregations
of ice crystals do not have time to melt before reaching
the ground. Generally, this means that the freezing level
must be below 300 m. Mixed snow and rain ('sleet' in
British usage) is especially likely when the air temper-
ature at the surface is about 1.5°C. Snowfall rarely
occurs with a surface air temperature exceeding 4°C.
Soft hail pellets (roughly spherical, opaque grains of
ice with much enclosed air) occur when the Bergeron
process operates in a cloud with a small liquid water
content and ice particles grow mainly by deposition of
water vapour. Limited accretion of small, supercooled
droplets forms an aggregate of soft, opaque ice particles
1 mm or so in radius. Showers of such graupel pellets
are quite common in winter and spring from cumu-
lonimbus clouds.
Ice pellets may develop if the soft hail falls through
a region of large liquid water content above the freezing
level. Accretion forms a casing of clear ice around the
pellet. Alternatively, an ice pellet consisting entirely of
transparent ice may result from the freezing of a rain-
drop or the refreezing of a melted snowflake.
True hailstones are roughly concentric accretions of
clear and opaque ice. The embryo is a raindrop carried
aloft in an updraft and frozen. Successive accretions
of opaque ice (rime) occur due to impact of supercooled
droplets, which freeze instantaneously. The clear ice
(glaze) represents a wet surface layer, developed as
a result of very rapid collection of supercooled drops
in parts of the cloud with large liquid water content,
which has subsequently frozen. A major difficulty in
early theories was the necessity to postulate violently
fluctuating upcurrents to give the observed banded
hailstone structure. Modern thunderstorm models
successfully account for this; the growing hailstones
are recycled by a traveling storm (see Chapter 9I). On
occasions, hailstones may reach giant size, weighing
up to 0.76 kg each (recorded in September 1970 at
Coffeyville, Kansas). In view of their rapid fall speeds,
hailstones may fall considerable distances with little
melting. Hailstorms are a cause of severe damage to
crops and property when large hailstones fall.
2 Coalescence theories
Theories of raindrop growth use collision, coalescence
and 'sweeping' as the growth mechanisms. It was
originally thought that cloud particle collisions due
to atmospheric turbulence would cause a significant
proportion to coalesce. However, particles break up just
as easily if subject to collisions. Langmuir offered a
variation of this simple idea. He pointed out that falling
drops have terminal velocities (typically 1 to 10 cm 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 20 mm in a cloud having a water
content of 1 g/m
3
(assuming maximum efficiency).
Although coalescence is initially slow, droplets reach
100 to 200 µm radius in 50 minutes. Moreover, the
growth rate is rapid for drops with radii greater than 40
µm. Calculations show that drops must exceed 19 µm
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 avail-
ability 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 µm radius) and a high liquid
water content, whereas continental air tends to contain
many small nuclei (~ 1 µm) 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 more rapid growth than
simple condensation and is, in fact, common in 'warm'
clouds in tropical maritime airmasses, even in temperate
latitudes.
