encounters, over 80 km in the winter. Kansas, Mon-
Strapp et al. (1996) and Bernstein and Brown (1997)
tana, Illinois, and Florida had short encounters, most
have created modern climatologies of the occurrence
being less than 36 km in length. In Kansas and Florida
of freezing precipitation. Neither study, however, specu-
there was about a 1% chance that an icing encounter
lates about the spatial extent of individual freezing pre-
would extend more than 10 km. Cooper and his col-
cipitation storms. Bernstein (1996) indicates implic-
leagues point out that only about 10% of encounters
itly that freezing precipitation does occur for hundreds
extended more than 5 km, and the majority of icing
of kilometers, but broken in continuity. As an example,
measured was in cumulus clouds.
he presents a map for 1800 hr on 6 March 1996, illustrat-
ing freezing precipitation as extending in a broken band
Gap encounters are useful for assessing the utility
from New York City through Missouri, with a maxi-
of remote sensing to avoid icing, because aircraft could
mum width of about 250 km.
avoid ice by navigating gaps. Cooper et al. (1982) report
In general, spatial patterns of icing on sub-kilome-
that gaps are typically short, like icing encounters, with
ter to tens-of-kilometers scales, and icing's relationship
50% being less than 5 km in extent. A gap occurred
when liquid-water content was less than 0.01 g m3.
to synoptic and mesoscale weather, are only generally
Gerber (1996) reports liquid-water content gaps, or
understood. The ability to avoid icing is a function of
minima (called turbules), of a much smaller scale em-
its spatial distribution. Large cloud masses that are
bedded within marine stratocumulus clouds. Turbules
homogeneous with respect to icing are difficult to avoid.
are typically a few hundred meters or less across.
Kline and Walker (1951) related icing to synoptic
4.3.6 Mixed-phase clouds
Supercooled liquid water freezes on aircraft struc-
patterns in stratiform clouds during 22 flights from 1948
tures, whereas ice crystals within clouds, snow, and ice
through 1950. In extratropical cyclones, most icing was
pellets typically do not adhere (Riley 1998). Never-
associated with post-cold-frontal situations, with most
theless, clouds composed of mixtures of ice crystals
icing in the southwestern and northwestern quadrants
and supercooled water are of interest for remote sens-
of the storm. Very little icing was found in the overrun-
ing of icing potential for several reasons. First, remote-
ning portions of warm fronts east of the storm center.
sensing systems scanning clouds that are completely
Most icing was typically 300400 km behind cold
glaciated or mixed phase must distinguish successfully
fronts, and north of the center of lows, similar to pat-
between ice and supercooled liquid water and not be
terns reported by Ryerson (1990) at Mt. Washington,
compromised by the presence of ice crystals. A remote-
N.H., and Mt. Mansfield, Vt. These patterns are con-
trary to analyses of pilot reports of icing reported by
sensing system must be capable of quantifying the
amount of supercooled liquid water mixed with ice crys-
Politovich* that indicate that most icing is ahead of
tals or of sensing beyond a frozen cloud in the fore-
warm fronts and near the center of lows, with least icing
ground, for example, to a more distant supercooled
behind warm fronts and cold fronts.
liquid cloud.
Little is known about the horizontal extent of ZL
and ZR, according to Jeck (1996) in a summary of
The second concern for mixed-phase clouds is in
knowledge about the phenomenon. Bennett's (1959)
reference to the need to range-resolve temperature to
report indicated that most freezing rain occurs in over-
determine if sensed liquid water is supercooled. A cloud
running situations, so it is associated with warm fronts
made up of a mixture of liquid water and ice crystals is
in many instances. Freezing rain can extend continu-
likely to contain supercooled liquid water. As a result,
ously or intermittently several hundred kilometers par-
even if a method is not found for range-resolving tem-
allel to a front and short distances perpendicular to
perature, it may be possible to determine whether liquid
water is supercooled by sensing the presence of ice crys-
fronts. It can also be associated with cold fronts, but
tals mixed with the liquid water. Thus, mixed-phase
then it typically is of shorter extent than in warm fronts.
clouds may serve as a surrogate for explicit tempera-
Design values use 160 km as a representative extent.
ture measurement ahead of the aircraft. Mixed-phase
There is no information for the extent of ZL, according
clouds, however, may be less of an icing hazard than
to Jeck, who argues that extent should be related to the
supercooled clouds without ice crystals (Guttman and
time an aircraft must spend below 7000 ft, especially
Jeck 1987, Riley 1998).
on approach and departure. This agrees with Perkins'
Simply seeking ice crystals may not be a reliable
(1952) conclusions that over 50% of icing conditions
solution to determining supercooling, however. Clouds
in general are found during climb or descent.
may be composed completely of supercooled liquid
water and still contain no ice crystals, or they may con-
tain a concentration of ice crystals that is so small as to
* Personal communication, M. Politovich, National Center for Atmo-
be not detectable. The success of using mixed-phase
spheric Research, Boulder, Colorado, 1997.
23
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