ment on aircraft must be capable of sensing horizontally
instruments, from multicylinders that provide integrated
ahead of the aircraft and above, below, and to the sides
measurements to current electro-optical systems that
of the flight path, so they should be tested in the same
provide measurements with high temporal resolution.
position as they will be used aboard aircraft to provide
Thousands of research flight hours have resulted in a
confident results (Ryerson et al., 2000). Mountain-top
general understanding of the magnitude of liquid-water
facilities that have potential testing capabilities include
contents, and their spatial patterns, that can be encoun-
the Desert Research Institute's Storm Peak Laboratory;
tered by aircraft. For example, it is understood that
Elk Mountain, operated by the University of Wyoming;
higher liquid-water contents are generally found in
Whiteface Mountain Observatory, operated by the State
cumuliform rather than in stratiform clouds, that sum-
University of New York, Albany; and Mt. Washington
mer supercooled liquid-water contents are highest, and
Observatory, N.H. (Ryerson et al., in prep.). An advan-
that liquid water is generally more "cellular" than homo-
tage of mountain-top facilities is the availability of natu-
geneous over thousands of square kilometers. Studies
ral icing conditions. Likewise, a disadvantage of natu-
have also demonstrated that icing conditions, for liquid
ral icing is the lack of control over conditions. Research
water and drop size, are extremely variable and diffi-
aircraft available for testing may include the NASA
cult to generalize. It is also recognized, though not nec-
Twin Otter, the NRC Twin Otter and Convair 580, the
essarily widely, that FAR 25, Appendix C, conditions
NCAR King Air, and aircraft from the University of
are only representative values for engineering design
Wyoming, the University of North Dakota, and a vari-
purposes and are not intended to represent the actual
ety of private companies (Marcotte et al. 1996). The
character of the icing atmosphere as encountered by an
Air Force tanker spray rig has been removed from ser-
aircraft.
vice, but the Army still operates its Helicopter Icing
The characterization of the icing atmosphere has
Spray System (HISS) from Fort Rucker, Alabama.
been accomplished somewhat randomly because of the
Flight simulators may also provide information use-
cost of airborne research projects. Each program has a
ful for establishing sensor system characteristics. For
specific focus, so flight hours are typically consumed
example, pilots' abilities to react within given warning
trying to answer the primary research questions of the
times provides information for establishing sensing
project. A large-scale monitoring program dedicated to
range and update frequency.
the characterization of icing conditions would allow
large geographic areas, with weather conditions experi-
enced by most of the nation, to be sampled consistently
4.0 METEOROLOGICAL SENSING
and frequently to produce information that is statisti-
REQUIREMENTS
cally valid. Work by Cooper et al. (1982) and Sand et
4.1 Summary
al. (1984), by the Canadian Freezing Drizzle Experi-
The development of systems to measure icing poten-
ment (CFDE), and by the NASA Glenn Research Center
tial remotely and in situ requires an understanding of
during the winters of 19961997, 19971998, and
the medium being sensed: the atmosphere and its ther-
19981999 (Miller et al. 1998) have come closest to
mal and liquid characteristics must be understood with
the ideal of covering large geographic areas with mod-
regard to the absolute magnitude of conditions and their
ern, carefully calibrated instrumentation. One way to
spatial distribution. This information is needed to evalu-
do this would be to instrument commercial or military
ate the feasibility of sensing and avoiding icing poten-
aircraft that fly large numbers of hours, as Perkins
tial, to design instruments to sense conditions, and to
(1952) did, enabling a representative sample of icing
develop methods of avoiding and exiting icing condi-
conditions to be made nationwide and reported through
tions. The atmosphere must be carefully characterized
a system such as ACARS (Aircraft Communications
with regard to icing potential to develop sensors and
and Reporting System). Ground-based remote-sensing
training protocol, to create terminology for advising
systems installed at airports to protect terminal areas
pilots, and to provide better icing forecasts. Character-
may also be able to provide characterization informa-
ization is needed at all scales from the submesoscale to
tion similar to that of in-flight programs. This is another
the global scale, although the synoptic scale is proba-
argument for accelerating airport-based remote-sensing
bly best understood with regard to icing.*
icing-avoidance systems. Finally, icing radiosondes are
Attempts to characterize the icing atmosphere,
available for measuring supercooled liquid water with
liquid-water content, drop size, and temperature have
height within clouds (Hill 1994). Such radiosondes,
been conducted since the 1940s with a large range of
fielded nationally by the NWS, could improve icing
forecasts and the characterization of supercooled cloud
water.
* Personal communication, M. Politovich, National Center for Atmo-
Cloud liquid water is generally better understood
spheric Research, Boulder, Colorado, 1997.
11
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