shape of the ice accretion will vary considerably. Low
the MVD alone does not describe the shape of the drop-
liquid-water contents at low temperatures and small
size distribution, nor does it adequately describe where
drop size tend to create rime ice, and larger liquid-water
collection efficiency effects will cause ice accretion on
contents in warmer temperatures and larger drops tend
the airframe (Newton 1979, Cooper et al. 1982).
to produce clear ice (FAA 1991). Rime tends to pro-
Early measurements of drop-size spectra were made
duce an ice surface that conforms generally to the shape
either with oiled or soot-covered slides or with the use
of the airfoil. Clear ice may create a smooth surface, or
of rotating multicylinders. Slides were difficult to use in
it creates "horns" near the leading edge that have a large
high wind speeds, though they were occasionally used as
impact on drag and airfoil lift. As drop size increases
late as the 1960s (Warner 1969). Usually, rotating multi-
within clear-ice conditions, the size of the accretion
cylinders, developed in the early 1940s at Mt. Wash-
increases, the impingement limits increase in area, and
ington Observatory, were used instead of slides, and
the horns tend to form farther back on the airfoil (FAA
they are still in use (FAA 1991, Howe 1991). Multi-
1991). Overall, according to Sand et al. (1984) and Polito-
cylinders provide an indication of the shape of the drop-
vich (1989), drops larger than 30 m in diameter have
size spectra by utilizing curves developed from theory
a greater effect on flight than smaller droplets.
by Langmuir and Blodgett (1946) using the collection
A third effect of drop size is runback. Although run-
efficiency of various-diameter cylinders for given wind
back may occur over a large range of cloud drop sizes,
speeds and drop sizes. From these curves, and the
depending upon temperature and liquid-water content,
amount of ice collecting on each cylinder, the MVD
runback becomes more serious when the drizzle-size
can be estimated. However, serious errors in MVD esti-
regime is entered, at about 50 m. Here, all water does
mation could occur with multicylinder use in large-
droplet situations, where MVDs approach 30 m or
not freeze near its impingement location--some runs
back and freezes beyond ice-protected areas of the lead-
larger (Jeck 1980).
ing edge. This often creates an ice ridge or roughens
Drop-size spectra were measured coincidentally with
wing surfaces, significantly altering airfoil aerodynam-
liquid-water content in most experiments. The database
ics and aircraft performance.
created by Jeck (1980) at the Naval Research Labs, in
Cloud droplet size varies by cloud genera, from cloud
cooperation with the FAA, is probably the most compre-
to cloud, by season, and with location within clouds.
hensive available. Jeck (1983) and Masters (1983) sum-
For example, the largest drops in growing, nonprecipi-
marized older and modern measurements both below
tating cumulus clouds typically occur near the center
3049 m and at all levels of the atmosphere. Jeck (1983)
and top of the cloud within updrafts. Smaller drops are
indicates that below 3049 m, average MVDs measured
found near the cloud base and near the cloud perimeter
with multicylinder and newer optical instruments, for
supercooled layer clouds, are about 13 m and for con-
where dry air entrainment causes evaporation of drops
vective clouds they are 18 m. MVD also shows temper-
(FAA 1991). Overall, drop size is controlled by evapo-
ature dependence, with MVD increasing from 10 m to
ration, collisioncoalescence, curvature and solute
about 30 m in stratiform clouds as temperature
effects, the Bergeron process, and the number and type
increases from 25C to 0C. Jeck (1982) also observed
of cloud condensation nuclei present (Miller and Anthes
1980). As an example, maritime clouds of a given gen-
that MVD generally increases with altitude in single-
era typically exhibit broader drop-size spectra than do
layer clouds below 3049 m.
continental clouds due to differences in the type, num-
Jeck (1983) questions the use of a minimum MVD
of 15 m in FAR 25, Appendix C, considering analy-
ber, and size of cloud condensation nuclei (Rogers and
Yau 1989).
ses of the database of cloud properties below 3049 m
Cloud drop-size spectra are typically characterized
(Jeck 1980). Masters (1983) and Jeck (1983) both pro-
by the median volume diameter (MVD), the drop size
vide diagrams from this database showing MVDs in
icing clouds well below 15 m.
where one-half of the spectrum's water volume resides
within smaller-diameter droplets and the other half
A summary of five years of cloud measurements by
resides within larger droplets. Internal cloud dynamics
the University of Wyoming (Sand et al. 1984) showed
MVDs ranged from 5 to 40 m, with a characteristic
may create bimodal drop-size distributions, observed in
MVD of about 15 m. The smallest MVDs were meas-
most cloud types in most climatic regimes (Pruppacher
and Klett 1997, Politovich and Vali 1983). Bimodal dis-
ured during the winter over the Great Lakes and the
tributions are not properly represented by a single MVD,
Great Plains, with the largest MVDs in the summer over
however, which relies on a unimodal distribution. The
the Great Lakes and Illinois and in the winter over Flor-
average collection efficiency of a drop-size spectrum
ida. Droplets were smaller in the Great Lakes and Illi-
around a median volume diameter is generally quite
nois areas because of low liquid-water contents, accord-
close to the collection efficiency of the MVD. However,
ing to Sand et al. (1984). No relationship was found
17
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