and the reverse-flow thermometer were both accurate
In-situ instrumentation would be useful onboard all
within clouds, and the Rosemount was not (Lawson and
aircraft for sensing when icing conditions have been
Cooper 1990).
entered and for near-real-time calibration of onboard
Marillier et al. (1991) constructed an ultrasonic
remote sensors. Instrumentation intended for use on all
thermometer that measured temperature ahead of an air-
aircraft may not require the accuracy of research instru-
craft to avoid cloud-wetting effects and to obtain temper-
ments. Ease of use, maintenance, cost, and size are more
ature at 100 Hz. Measurements were typically accurate
important factors in these applications.
to within a few tenths of a degree Celsius of a colocated
Most current aircraft-mounted instrumentation suited
Rosemount probe.
for cloud microphysical measurements is intended for
Lawson and Rodi (1992), Friehe and Khelif (1993),
research applications. Very few instruments, notably air-
and Haman and Malinowski (1996) have constructed
temperature measurement devices, are sufficiently inex-
very fast response thermometers in an attempt to match
pensive and robust for general field use. The following
the response time of a forward-scattering spectrometer
is a brief review of available instrumentation and their
probe (FSSP). These thermometers are very delicate,
general applicability. This review does not discuss
with either platinum wires 12.5 m in diameter (Law-
instrumentation for detecting ice on aircraft surfaces,
son and Rodi 1992), tungsten wires 2.5 m in diameter,
either preflight or in-flight. A review of in-flight ice
or small thermistors (Friehe and Khelif 1993, Haman
detectors is available in the FAA Aircraft Icing Hand-
and Malinowski 1996). Both wire probes are accurate
book (FAA 1991) and in an SAE document (SAE 1995).
but delicate and broke easily when stressed by high-
Cloud microphysics measurement instruments are
speed airflow and precipitation. All three instruments
typically designed to measure temperature, liquid-water
are too delicate for general operational use, and the ther-
content, and elements of the drop-size spectrum. Early
mistor instrument had calibration difficulties.
instrumentation was manually operated, but most mod-
ern instruments are electronic.
4.4.2 Liquid-water content measurement
The first common instrument for liquid-water meas-
4.4.1 Temperature measurement
Temperature cannot be measured with a standard
urement was the rotating multicylinder, developed at
outside air-temperature probe because, in addition to
Mt. Washington Observatory between 1940 and 1945
accuracy and exposure problems, it is wetted by cloud
(Lewis et al. 1947, 1953; FAA 1991; Howe 1991). In
water. A wet thermometer measures the wet-bulb tem-
the 1930s, a single rotating cylinder was used. The rotat-
perature. Lawson and Cooper (1990) provide a detailed
ing multicylinder, still in use at Mt. Washington Observ-
analysis of the problem. Thus, the most important task
atory but no longer used on aircraft, is used to deter-
is to protect the thermometer from cloud droplets with-
mine cloud liquid-water content and the shape of the
out disturbing the measurement.
drop-size spectrum. The multicylinder typically con-
A common temperature measurement instrument is
sists of six cylinders, stepped in diameter. The shape of
the Rosemount total temperature probe (Haman et al.
the drop-size spectrum is determined by comparing the
mass of ice on each cylinder to curves of expected ice
1997). It is used by the U.S. Air Force (Glass and Gran-
tham 1981), by NCAR (Sand et al. 1984) on its King
accretion on each cylinder for a given shaped drop spec-
Air, and on the Canadian Convair (Cober et al. 1996b).
trum, after theory developed by Langmuir and Blodgett
The instrument measures the resistance of a platinum
(1946). Cloud liquid-water content is related to the mass
wire in a bridge circuit. Accuracy is claimed to be from
of ice on the cylinder; the smallest cylinder with the
0.5C (Sand at al. 1984) to 1C (Cober et al. 1996b),
highest collection efficiency is typically used for the
but wetting in clouds is a problem.
calculation.
NCAR and the AES also use reverse-flow tempera-
Rotating multicylinders are manually operated and,
ture probes. According to Cober et al. (1996b), they are
depending upon cloud liquid-water content and rela-
as accurate as the Rosemount and prevent wetting of
tive wind speed, are exposed from a few minutes to
the probe.
more than 20 minutes (Kline 1949, Howe 1991).
Jeck (1980) thoroughly reviews problems with the
There has been considerable effort to improve the
accuracy, resolution, and response time of thermome-
multicylinder method, especially with regard to runoff
ters to measure small-scale cloud features. Lawson and
near the Ludlum limit. The advantage of the multi-
Cooper (1990) analyzed the Ophir radiometric thermo-
cylinder method is that, with a skilled operator, it is
meter to improve reliability within clouds and to
cheap, easy to use, and reasonably accurate in colder
improve response time. However, the sample volume
temperatures, smaller drop sizes, and moderate liquid-
is quite large (10 m in depth), and the instrument is
water contents.
large, expensive, and complex. The Ophir thermometer
The only other early method of measuring liquid-
25
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