Finally, experiments are being conducted by federal,
siderably within clouds and from cloud to cloud. In addi-
state, and private groups to augment precipitation (cloud
tion, temperature may change rapidly within frontal
seeding). This often involves monitoring of cloud micro-
systems and in the vertical, as experienced by aircraft
physics. Efforts should be made to interact with these
when changing altitude rapidly upon departing or
groups and perhaps to conduct coordinated research.
approaching terminal areas.
Characterization requires reliable and accurate instru-
Liquid-water drop-size spectra determine the loca-
mentation for making in-situ measurements. The ideal
tion of drop impingement on airframe structures, the
instrumentation for measuring cloud microphysics
type of ice that forms, the shape of ice that forms, and
would be similar in character to instruments currently
the location of ice on the airframe as a result of run-
used on aircraft for determining airspeed and outside
back. Runback is caused by supercooled large-drop
air temperature: generally small, inexpensive, accurate,
impingement and flow along the airfoil chord causing
robust, maintenance-free, and unobtrusive. A focused
freezing on areas of the airfoil unprotected by deicing
effort is needed to simplify and miniaturize current
equipment. Runback has become a critical problem,
instrumentation, but efforts should also be made to com-
especially with respect to SLDs within the drizzle size
range (typically diameters of 50 to 500 m). Runback
pletely rethink cloud microphysics instrumentation and
to design and develop completely new concepts.
and formation of an ice ridge immediately aft of the
Finally, icing terminology needs improvement. This
boot-protected leading edge is believed to be the cause
is being addressed in the FAA Inflight Aircraft Icing Plan
of the ATR-72 crash in Roselawn, Indiana, in October
(FAA 1997). Currently, icing reports and forecasts are
1994 (NTSB 1996). The overall characterization of
not purely meteorological but include the aircraft.
SLDs vs. smaller drop sizes is relatively poorly under-
Though practical, because pilots observe how icing is
stood, so special emphasis should be placed on charac-
affecting their aircraft, the current terminology is not
terizing the SLD environment.*
effective because it does not utilize purely meteorologi-
Liquid water, drop temperature, and the drop-size
cal information to evaluate icing intensity. The aircraft
spectra are the most important indicators of in-flight
must be separated from weather to evaluate icing con-
aircraft icing potential, and thus the most important
ditions objectively and unambiguously.
conditions to characterize for the development of the
sensing needs of remote-sensing systems. However, it
4.2 Introduction
may also be useful to sense conditions that are not criti-
Information required to assess in-flight aviation icing
cal to icing potential but that may serve as surrogates
hazard is derived from measurements of the atmospheric
for the other conditions. For example, range-resolved
conditions that create ice on aircraft. In order of impor-
remote sensing of air temperature or droplet tempera-
tance, those conditions are cloud or precipitation liquid-
ture may prove to be the most difficult remote-sensing
water content, drop temperature, and drop size.* Of these
challenge. Whether clouds are glaciated or partially
three conditions, liquid water is most important because
glaciated, or mixed-phase, may provide an indication
it is the material that creates ice on the aircraft. Liquid-
of whether liquid-water temperatures are warmer or
water magnitude varies widely, both spatially and tem-
colder than freezing, so the ability to detect ice within
porally, so it must be measured continuously.
clouds may serve as a surrogate binary temperature indi-
Droplet temperature is the second most important
cation of above- or below-freezing conditions. How-
atmospheric condition affecting icing; it determines in
ever, little is known about the glaciation process and
part whether liquid water will freeze on an aircraft struc-
the probability of glaciation at given temperatures below
ture. Air temperature (static and total temperature are
freezing. Nevertheless, characterization of cloud glaci-
not distinguished here) may serve as a surrogate for drop-
ation with temperature, to determine if it would be a
let temperature, especially if droplets are so small that
meaningful surrogate for temperature, may be useful.
their fall speed allows them to maintain a temperature
The magnitudes of liquid-water content, tempera-
nearly that of the surrounding atmosphere. However,
ture, and drop-size spectra must be characterized to
snow or graupel falling into a warm layer may melt or
provide specifications for remote-sensing technology.
partially melt, resulting in particle temperatures colder
The spatial variability of these conditions must also be
than the air in the warm layer. If the particles or droplets
characterized. Characterization is needed at the mesos-
cale (~104106-m scale), synoptic scale (~106-m scale),
then fall into colder air below, they will be warmer than
and global scale, although for different reasons, depend-
the air. Air temperature may be relatively constant over
ing on the scale. Characterization of spatial variability
large horizontal distances, but it may also fluctuate con-
* Personal communication, M. Politovich, National Center for Atmospheric Research, Boulder, Colorado, 1997.
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