clouds as an indicator of supercooling is thus depen-
Additional information about the utility of using
dent upon the probability of supercooled liquid clouds
mixed-phase clouds for indications of supercooling is
containing detectable ice crystals. Parameterizing nucle-
available from various field programs. Tremblay et al.
ation of water droplets in clouds, and the glaciation
(1996) observed mixed-phase clouds at 4461 points as
process, is one of the classical problems cloud physics
part of CFDE in 1995 off the Newfoundland coast. Plots
has yet to solve.
of the proportion of liquid water vs. ice water within
When a cloud is cooled below 0C, ice crystals could
mixed-phase clouds showed a temperature dependence
between 0C and 10C if liquid-water contents larger
form. However, because there are relatively few ice
than 0.3 g m3 are ignored. As temperature decreased,
nuclei in the atmosphere when compared with conden-
the proportion of ice increased. However, the relation-
sation nuclei, nucleation often does not begin until drop-
lets cool to 10C (Rogers and Yau 1989). Observations
ship is also proportional to the cloud liquid-water con-
tent, with the proportion of cloud water nucleating
of 258 clouds by Hobbs et al. (1974, as cited by Rogers
increasing, at a given temperature, with cloud total
and Yau 1989) showed that glaciation typically does not
begin until cloud-top temperatures cool to about 4C,
liquid-water content. In flights through summer cumu-
lus in southern Missouri, Koenig (1963) found glacia-
after which the percentage of clouds containing ice
tion related to drop size, with clouds with large drops
increases to 100% at a cloud-top temperature of about
20C. Rogers and Yau (1989) state that it is impossi-
rapidly forming high concentrations of ice crystals
regardless of the availability of ice nuclei.
ble to determine at what cloud-top temperature any indi-
Bower et al. (1996) surveyed frontal and maritime
vidual cloud will begin to glaciate or to estimate how
convective clouds from the United Kingdom and con-
much glaciation will occur. Thus, in general, clouds with
tops warmer than about 5C are ice free, and clouds
tinental convective clouds from New Mexico and Mon-
with tops colder than 20C are virtually guaranteed to
tana to refine parameterization schemes for global circu-
lation models. Detailed analyses were done for all
have ice crystals (Riley 1998).
clouds that had been measured for glaciation activity
The first crystals to appear in a cloud must form on
using aircraft-mounted instrumentation. Continental and
ice nuclei (Rogers and Yau 1989). Additional crystals
maritime frontal clouds had very rapid glaciation, begin-
are formed from secondary processes such as the frac-
ning at 0C, with total glaciation typically ocurring at
ture of ice crystals and the shattering or splintering of
temperatures of 10 to 15C. Continental and mari-
drops as they freeze. These crystal fragments then strike
time convective cloud glaciation was much slower and
liquid-water droplets, causing them to freeze though
less complete, with glaciation beginning at 0C, but at
contact nucleation, or the fragments simply serve as
3C, typical clouds were only about 40% glaciated.
deposition nuclei (Houze 1993).
At 15C, some clouds were still 90% supercooled
Overall, glaciation is difficult to predict because it
liquid water.
depends upon cloud type, cloud age, liquid-water con-
It is not clear from these studies whether glaciation
tent, and geographical location--especially as related
begins in earnest at 0C. Characteristics of mixed-phase
to air mass type and availability of icing nuclei (Rogers
clouds must be better defined to determine the proba-
and Yau 1989). Since the probability of glaciation
bility of ice crystals at temperatures below 0C. The
increases with decreasing temperature, it would be
most current and thorough review of mixed-phase
expected to find a monotonically increasing percent-
clouds and aircraft icing is by Riley (1998).
age of cloud water to be frozen at lower temperatures.
This does not appear to occur, however. Instead, once
4.4 In-situ instrumentation
clouds begin to glaciate, freezing occurs rapidly, and
In-situ measurements of cloud microphysics are
the final ice particle concentration is not proportional
needed to support and augment the remote sensing of
to temperature (Pruppacher and Klett 1997).
in-flight icing conditions, and in-situ instrumentation is
Mixed-phase clouds are also not necessarily uni-
needed for improved characterization of icing cloud
formly glaciated. On the basis of a large number of
microphysics. Drop-size distributions are often not cor-
soundings of nimbostratus clouds in Russia, Borovik-
rectly represented by current measurement methods, and
ov et al. (1963) report that three different types of mixed
ice crystals and drops can be confused by the coarse-
cloud structure can occur: clouds can consist of rela-
ness of sensing systems. Two measurements by the same
tively uniform mixtures of crystals and water through-
model of instrument often do not agree when used on the
out, successive layers of water droplets and ice crys-
same aircraft, which indicates repeatability or calibration
tals, or three or four layers of warm water, supercooled
problems. The dynamic range and sensitivity of instru-
water, mixed conditions, and ice. The relative frequency
ments is often insufficient, and gaps in size distributions
of each was observed 52%, 28%, and 20% of the time,
synthesized from combinations of instruments often occur.
respectively, in Russia.
24
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