development for retrieving liquid-water content and
size and the optical elements between the drop and the
drop-size spectra.* The second instrument, the CDS,
detector array. If the measurement is made along one
measures the angular resolution of forward-scattered
dimension of the particle, the probe is called a 1-D probe.
light from an ensemble of cloud drops. A 256-photo-
Only shadows fully within the array are accepted, so large
diode array measures the scattered light, and liquid-
droplets--droplets not fully within the array--are
water content is computed from the angular measure-
rejected. The range of drop sizes measured is typically
20 to 300 m, or 300 to 4500 m; and other ranges are
ments of the forward-scattered light, which expresses
available (Knollenberg 1981, FAA 1991).
the collective drop sizes of the drop ensemble. Liquid-
water content measurements have been successfully
OAP 2-D probes are similar to 1-D probes optically,
compared with other instruments in a wind tunnel, and
but enhanced signal processing speed allows the photo-
drop-size spectra measurements have been tested in the
diode array to be scanned faster, retrieving, in effect, a
laboratory, aboard an aircraft, and at Mt. Washington
shadow of particles as they traverse the laser beam. The
Observatory. No data have been published in the open
result is an image of particles indicating the shape of
literature demonstrating the CDS's ability to measure
ice crystals and the size of drops. Particle sizes are
liquid water or the drop-size spectrum.
binned into 64 size classes. Two-D probes have been
configured to detect particles up to 6400 m in diame-
4.5 Terminology
ter (Marcotte et al. 1996). The accuracy of OAPs has
A remote-sensing system designed to detect and map
been scrutinized in recent years with the renewed inter-
icing potential within a projected flight path will sense
est in SLDs. Problems of aircraft speed and location of
the meteorological conditions that create ice on an air-
the drop within the imager depth of field can cause
craft: liquid-water content, temperature, and drop size.
OAPs to miss smaller drops and oversize large drops
Then the remote-sensing system may utilize expert sys-
(Lawson et al. 1996, Morrison et al. 1997). These prob-
lems are detected when FSSP and OAP ranges overlap
tem or fuzzy logic to create information for a cockpit
display that the pilot can use if icing is entered. Since
and when OAPs are compared with other instruments.
ice cannot occur on the aircraft until the aircraft enters
New array-processing techniques and algorithms have
the icing conditions, only an explicit numerical model
been suggested to correct these problems (Hobbs et al.
operated with the remote-sensing system, a set of guide-
1996, Korolev et al. 1996).
lines that relate icing potential to expected performance,
A comparison of the PDPA with the FSSP and a PMS
would solve the problem of how to relate meteorolog-
OAP (Oldenburg and Ide 1990a,b) indicates that all
ical conditions to the pilot. Bragg et al. (1998) propose
three instruments generally agree well. Disagreements
occurred in drop sizes smaller than about 10 m because
a smart icing system that recognizes how aircraft sys-
tems should respond to icing and advises the pilot after
smaller droplets were suspected of freezing, and each
the aircraft has entered icing. A similar system could
instrument treats ice crystals differently because of the
be activated before an aircraft enters icing and act in
differing technologies, causing mismatched sample sta-
response to remotely sensed icing conditions ahead of
tistics. They also disagreed for MVDs larger than 30
m because of the configuration of the PDPA, which
the aircraft.
Current terminology used to categorize or classify
could be reconfigured to detect larger drop sizes. The
icing intensity, or severity, is often inconsistent among
PDPA has the advantage of being a smaller instrument
government agencies, difficult to interpret and, at times,
than the PMS probes, and it has the capacity of sensing
contradictory (Erickson et al. 1996; Green 1995; Auld
a wide range of drop sizes with one instrument, whereas
1989; Newton 1977, 1979). Currently, the National
the PMS probes require two instruments, the FSSP and
Weather Service defines icing intensity with descrip-
an OAP.
tions that are related only to aircraft and contain no
Lawson et al. (1996, 1998) and Lawson and Cor-
meteorological criteria (Auld 1989, Newton 1979).
mack (1995) describe new optical probes that solve
According to Newton (1979), NWS definitions of trace,
problems with the FSSP and OAPs. The new instru-
light, moderate, and severe are only reporting defini-
ments, which are in near-production stage for both
tions and contain no meteorological information that
ground and airborne use, are the cloud particle imager
can be used to forecast icing.
(CPI) and the cloud droplet spectrometer (CDS). The
CPI creates images of cloud particles at a rate of 30 s1,
Newton (1979) also indicates that the National
but at a potential rate of 240 s1. Image detail as small
Weather Service definitions are not related to FAA icing
as 2 m is possible, with maximum possible size limited
regulations (FAA 1991). Uniformly understood icing
by sample volume. Shadowgraph-type images of ice
crystals can be made of nearly photographic quality. Im-
* Personal communication, R. Paul Lawson, Stratton Park Engineer-
ages can be obtained in real-time, and techniques are in
ing Corp., 1999.
28
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