could use scanning radiometers to measure liquid water
and some aspects of the drop-size spectra and cloud phase
and temperature profiles with height. Integrated water
composition using polarization. However, lidar's inher-
could be distributed through the atmosphere to deter-
ent liability, its inability to penetrate cloud of large opti-
mine volumetric cloud water content if combined with
cal depth more than a few hundred meters, is severe.
cloud base, top, and layer information from a Ka radar.
Lidar may be a useful, inexpensive technique for allow-
However, this can now be accomplished without radar
ing aircraft, especially those flying night VFR (visual flight
by the Radiometrics profiling radiometer. The ability of
rules), to avoid icing, but it cannot provide guidance to
radiometry to scan liquid water at airports is nearly a
escape icing. Its potentially low cost, high scan-rate capa-
mature technology, with the greatest concerns being the
bility, and small size may make it a practical tool for
need to keep sensors free of moisture and improving
small helicopters and light aircraft, especially aircraft
scanning rates. Long scan time is one of the most seri-
operating in the Far North with no ice protection in often
ous radiometer problems because they are passive
limiting weather and few winter hours of daylight. Devel-
devices. At airports, this can be overcome by using multi-
opment along these directions is being made in Canada.
ple radiometers, each assigned to a different sector of
Canadian needs for this VFR cloud and icing avoidance
sky, but it is a greater problem for airborne systems.
capability may be great in the northern territories.
Airborne radiometry of cloud liquid water does not
Temperature measurement is needed to determine if
appear to have been tried from aircraft. It is theoretically
cloud water or liquid precipitation is supercooled. It is
possible for airborne radiometers to measure integrated
also needed for radar retrieval of cloud water content.
liquid water scanning in the horizontal, and that feasi-
Two mature technologies are available for sounding
bility is being analyzed using measurements made from
temperature from the ground: RASS (radio acoustic
the summit of Mt. Washington at the Mt. Washington
sounding systems) and microwave radiometers. RASS
Icing Sensors Project (MWISP). However, it may be
provides greater temperature resolution and thus accu-
more feasible at 85 GHz than at the more commonly
racy, which is especially needed during the winter when
used 37 GHz because of greater sensitivity to cloud water
inversions are common. However, RASS's range is gen-
at 85 GHz. This needs to be explored.
erally limited to altitudes of less than 3 km agl. RASS
As an alternative to horizontal scanning, or in addi-
and microwave sounders could be used together at air-
tion to it, radiometers might be able to sense vertically,
ports to provide the resolution needed at lower altitudes
at zenith and at nadir, from an aircraft, as is now done
and temperatures above the terminal airspace.
from the ground and from satellites, respectively. The
At the present time, there is no explicit capability to
amount of liquid water above and below the aircraft
range-resolve temperature ahead of an aircraft. Radio-
could then be determined, and radar might be used to
metry is the most promising possibility. The NASA Jet
simply determine cloud tops and bases. The slow inte-
Propulsion Lab scans air temperature at high altitudes
gration time of radiometers, however--many seconds--
ahead of an aircraft, with temperature provided at a rel-
would provide only an averaged or integrated liquid-
atively fixed distance. Though not range-resolved, air-
water content. This may be useful information, espe-
craft motion effectively range-resolves the temperature
cially in level flight within relatively uniform clouds,
and thus may provide an interim solution. The efficacy
but during climb-out and descent, and within fluctuat-
of this system needs additional exploration for icing
ing clouds such as cumuliform, this technique may not
applications; its performance within clouds is unknown.
be as useful because of the long integration times and
RASS has been found not to be practical for sensing tem-
limited spatial resolution of microwave radiometers. In
perature from aircraft.
addition, nadir-viewing aircraft radiometers would have
There is a possibility of range-resolving air tempera-
the complexity of radiation from the earth's surface, and
ture above and below aircraft, at zenith and nadir, with
varying altitude above the surface, making cloud-water
microwave radiometers using the same techniques that
retrieval more difficult. This suggests that although
are used by ground-based and satellite-based microwave
ground- and satellite-based radiometers may be able to
radiometer temperature sounders. The scan times
measure integrated liquid water successfully now, air-
required may cause smearing of temperature due to air-
borne systems are presented with greater difficulties that
craft motion, approach and descent altitude changes may
will require more retrieval technique modeling and field-
prevent accurate temperature retrieval because of nec-
work. In addition, there is no indication that radiome-
essary scan times, and ground radiation may cause prob-
ters can detect characteristics of the drop-size spectra
lems when sensing to nadir. A zenith/nadir sensing sys-
or detect ice, though it has been theorized that drizzle-
tem does not indicate temperature ahead except through
size drops may be identified by polarization effects.
extrapolation, although some scanning capability may
Lidar, especially multiple-field-of-view techniques,
be possible. However, it would indicate where warm tem-
has promise for measuring cloud liquid-water content
peratures may exist for escape from icing.
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