definitions are needed that are defined in standard terms
More work is needed to perfect existing information-
(Auld 1989, Newton 1979). A graduated, parametric
retrieval methods and to make them more robust. Dop-
method of describing icing severity should be developed
pler and polarimetry techniques may be able to provide
that is meaningful to meteorologists, aircraft designers,
information about drop-size distributions. Most cloud
microphysical work occurs in the X, Ka, and W bands,
regulators, and operators (Erickson et al. 1996).
most ground-based precipitation measurement occurs in
A remote-sensing system must, at minimum, supply
the S and C bands, and in X band from aircraft. A detailed
information to the pilot with regard to the icing poten-
sensitivity analysis is needed to find the optimal mix of
tial of a given volume of atmosphere in the flight path.
requirements for detecting liquid-water content and
A cockpit display must relay to the pilot information that
drop-size information from ground-based and airborne
enables a risk-management decision to be made. This
systems.
means the display must either provide information that
Radar has shown modest success at measuring atmo-
is uniformly applicable to all aircraft, but to which the
spheric precipitation and cloud water content--both liquid
pilot can reference his aircraft, or the system must directly
and frozen. Rainfall rates may be estimated with S- and
indicate the hazard potential to that particular aircraft.
C-band radars. NWS NEXRAD radars are an example
According to Newton (1977, 1979), the most tenable
of a system with some rainfall rate capabilities that may
solution with current definitions is to use a form of the
be useful for detecting freezing rain and perhaps freezing
old NACA icing intensity curves. The FAA is address-
drizzle conditions around airports. Although not practi-
ing terminology problems as part of the FAA Inflight
cal on aircraft because of size, using S band to detect
Aircraft Icing Plan (FAA 1997).
uation techniques to detect cloud water, or multiband
5.0 TECHNOLOGY DEVELOPMENT
X-, Ka- and W-band neural network techniques may be
5.1 Summary
optimal combinations at airports. In addition, Doppler and
polarization techniques may provide some indication of
A remote-sensing system, operated either from the
the drop-size spectra and the presence of ice crystals.
ground or from an aircraft, must sense the three-
A possible problem of radar, especially of the differ-
dimensional fluctuation of atmospheric liquid-water con-
ential attenuation methods using two or more wave-
tent, drop size, and temperature both from outside of icing
lengths, is the need to know droplet temperature to
conditions and from within icing conditions. In addi-
extract an accurate liquid-water content measurement
tion, the sensing system must have a range, resolution,
because of the dependency of backscatter on drop tem-
and refresh rate sufficient to satisfy pilot needs. The capa-
perature (Rinehart 1997). In addition, in military appli-
bilities of different technologies are often vastly differ-
cations radar could place an aircraft that is sensing icing
ent, depending upon whether scanning is toward space,
conditions at tactical disadvantage. Radars with stealth-
toward the earth's surface, or in the horizontal plane
like capabilities should be investigated.
ahead of an aircraft. In addition, it is likely that no sin-
In general, for ground-based systems, the focus
gle sensing technology is capable of providing all sens-
should be on S- or C-band systems for determination of
ing needs--a fusion of technologies will be necessary.
precipitation rate and precipitation liquid-water content
Radar is the most viable technology for providing
estimations and on X, Ka, and W bands for determin-
range-resolved cloud liquid-water content through large
ation of cloud water content. Doppler and polarization
expanses of cloud. Its scan rate is rapid, orientation is
techniques are available and practical in ground-based
generally not a limitation (unless observing below the
systems, but for airborne systems they may not be use-
horizon where earth reflections may be a problem), and
ful. For general aviation aircraft, (relatively) inexpensive
there are a variety of bands from which to choose. Radar
Ka- or W-band radar could be used to simply indicate
is a reasonably mature technology, and its basic back-
the location of cloud bases and tops if pointed to zenith
scatter capabilities may be augmented by Doppler and
and nadir--a useful tool to escape icing.
polarization techniques.
Microwave radiometry has the potential to remotely
An important issue that must be addressed in the use
sense icing conditions and, like radar, has been proposed
of radar for remotely detecting icing conditions is to
and used for several icing studies. Microwave radiome-
develop accurate and robust techniques for retrieving
ters are capable of integrating or profiling liquid-water
liquid water, drop size, and temperature information from
content and water vapor and profiling temperature. These
radar backscatter. Developments in this area include the
capabilities have been demonstrated for zenith and in-
differential attenuation method of liquid-water retrieval
termediate angles.
demonstrated by NOAA-ETL (Martner et al. 1991;
There are several ways radiometers could be used to
1993a,b) and the neural net retrieval of liquid water and
detect icing conditions. Ground-based airport systems
drop size by Quadrant Engineering (Mead et al. 1998).
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