been encouraging. The instrument has been demon-
4.4.3 Drop-size measurement
strated to be reliable in large-drop environments and in
Measurement of cloud drop-size spectra has been
large liquid-water contents. It also does not appear to
important since early aircraft icing research, primarily
because of their impact on ice shape, ice type, collection
be disturbed by ice crystals, but independent tests must
efficiency, and runback. However, the increased inter-
be done before the full accuracy and reliability of the
est in SLDs in recent years has placed renewed emphasis
instrument is known.
on drop-size measurement. Fewer instruments are avail-
The forward-scattering spectrometer probe (FSSP),
able for measuring drop-size spectra than for measuring
developed by Knollenberg (1981) and manufactured by
liquid-water content, but many of the instruments
Particle Measuring Systems (PMS), is the most com-
described above for measuring liquid-water content have
monly used optical probe; it is found on nearly all
dual uses and measure both. In the case of optical instru-
research aircraft (Sand et al. 1984, Cober et al. 1996b,
ments, liquid-water content is typically derived from the
Baumgardner et al. 1993, Thomas and Marwitz 1995).
Intended for measuring drop sizes from 2 to 47 m in
measured drop-size spectrum and drop concentration.
diameter (94 m in extended mode), cloud liquid-water
Except for oiled or soot-covered slides, the rotating
multicylinder described earlier was the first widely used
content can be computed by integrating the spectrum
instrument for obtaining the shape of the drop-size spec-
of drop sizes. However, liquid-water measurements are
trum and MVD (Lewis et al. 1947, 1953; FAA 1991;
prone to large error due to over- or undersizing of drop
Howe 1991). The drop-size spectrum shape is deter-
sizes. The instrument operates by forward-scattering
mined by, in effect, fitting the accreted ice weights of
light through cloud droplets as they pass through a nar-
the six cylinders to a series of curves, each represent-
row laser beam to a detector that records a drop size
ing a different droplet spectrum shape. The rotating multi-
proportional to the flash of light. Drops are classified
into fifteen 3-m-wide bins. Sources of error in the
cylinder method provides only a general indication of
the breadth of the droplet size distribution, in part
instrument are a small sample volume, false sizes from
because it can be fit to only a finite set of curves and
ice crystals, ice accretion and fogging of the optics,
because some clouds have bimodal or multimodal dis-
blockage of airflow through the instrument by ice, and
tributions. Howe (1991) states that accuracy in deter-
saturation of the instrument's electronics at high air-
mination of liquid-water content and droplet size is
speeds and large particle concentrations (FAA 1991).
better than 10% when cloud drop-size distributions
In addition, large drops are typically incorrectly meas-
are narrow or moderately broad. When drop-size distri-
ured. As a result of drop-size measurement errors,
butions are extremely broad, accuracy is reduced to
liquid-water contents typically have up to 34% error
about 20%.
(FAA 1991, Baumgardner 1983). Ide (1996) found
Gerber's PVM (1991, 1996; Gerber et al. 1994) pro-
liquid-water contents computed from an FSSP to be
overestimated by 50% in MVDs up to 60 m, and by
vides the effective drop radius of clouds. Few compari-
sons have been made with other instruments, but a com-
100 to 150% in larger MVDs at NASA's Glenn Research
parison with the FSSP (Gerber 1996) shows a linear
Center Icing Research Tunnel. Baumgardner et al. (1993)
relationship between the two instruments, though not a
and Brenguier (1993), however, have successfully
1:1 relationship. Gerber suggested that the mismatch,
modified the FSSP to measure the microstructure of
with the FSSP providing smaller drop sizes than the
clouds at the centimeter scale.
PVM, was due to errors in the FSSP.
The Phase Doppler Particle Analyzer (PDPA), devel-
The FSSP was described in the liquid-water discus-
oped by Aerometrics, Inc. with assistance from NASA
sion above, where some if its problems of measuring
Glenn Research Center (NASA 1997), measures drop
diameters from 0.7 to 125 m but can be extended to
drop size were also discussed. Overall, the instrument
2000 m (Aerometrics 1997). Droplet sampling is made
tends to broaden the drop-size spectra and, in drops
larger than about 45 m, measurements may not be
in a small sample volume at the intersection of two laser
trustworthy (FAA 1991).
beams. Droplets passing through the beams create an inter-
Optical array probes (OAPs), manufactured by Par-
ference fringe pattern that is projected into several detect-
ticle Measuring Systems (PMS), measure drop size by
imaging (Knollenberg 1981, Oldenburg and Ide 1990a,
to the droplet's velocity and size. Droplet number density,
FAA 1991). OAPs image droplet shadows onto an array
and thus liquid-water content, are also computed. Cali-
of photodiodes by allowing drops to flow through a
bration is not necessary. Models of the instrument have
laser beam. The loss of light on an individual array ele-
been developed for both wind-tunnel and aircraft use.
ment is detected by a logic circuit that measures the
However, the PDPA typically does not appear on equip-
shadow size. The drop size is a function of the shadow
ment lists of primary cloud research aircraft.
27
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