between icing effects on aircraft performance, a Beech
sition Experiment (ASTEX), the First ISCCP (Interna-
King Air, and the MVDs that produced the ice. Only
tional Satellite Cloud Climatology Project) regional
MVDs larger than 40 m, reaching into the supercooled
experiment (FIRE), and the U.S. Department of Energy
large-drop regime, affected aircraft performance.
Atmospheric Radiation Measurement (ARM) program.
Roebber (1988), in a review of icing potential on
The general characteristics of MVD by cloud genera
helicopters and fixed-wing aircraft off the east coast of
are understood. However, less is generally understood
Canada, presents statistics of drop sizes encountered
about cloud drop size than about cloud liquid-water con-
during icing and reported by the Royal Canadian Air
tent. Controls of drop-size spectra are not well para-
Force. The MVDs of convective clouds were between
meterized, although the general controls are believed to
18 and 21 m and for layered clouds near 12 m, but
be understood. Changes in drop size over time within
MVDs as large as 50 m were generally observed in con-
storms, and diurnally, have been tracked and simulated,
vective clouds, and as large as 40 m in layered clouds.
but general theory explaining drop-size evolution over
Jeck (1989) summarizes MVDs from his FAA/NRL
time is not mature.
icing database for clouds at all altitudes. Mean MVDs
Understanding of drop size has been hindered by the
are 13 m for layer clouds and 17 m for convective
need for improved instrumentation, the three-dimensional
clouds. The range of MVDs within the database, by gen-
complexity of liquid water within clouds, the need for
eral cloud type, are 721 m for layer clouds and 1026
observation flights focusing on drop-size measurements,
m for convective clouds.
and too much emphasis on reporting only MVDs instead
Cober et al. (1995) reported on 31 flights into Cana-
of the full drop-size spectrum.
dian east coast winter storms over the North Atlantic
Ocean and created a high-quality database of those
4.3.3 Supercooled large drops
The existence of large droplets (>50-m diameter)
flights. Flights were made into fronts, low-pressure areas,
was well known to early NACA investigators of the
and stratus clouds. The average MVD for all clouds was
18 m: 16 m for low-level stratus clouds and 20 m
microphysics of icing clouds, but they were not included
in the FAR 25, Appendix C tables, which include drop
for "system" clouds. These measurements compare well
sizes from only 15 to 40 m (FAA 1991). Sand et al.
with earlier measurements in the area, according to
(1984) and Politovich (1989) state that droplets larger
Cober et al. (1995), and with measurements by Sand et
than 30 m in diameter have a greater effect on flight
al. (1984).
than smaller droplets. Hansman (1985) indicates that,
Politovich and Bernstein (1995) investigated the pro-
from model and wind-tunnel tests, large drops present a
duction and depletion of supercooled liquid water in a
much larger threat to aircraft than small drops and that
February 1990 winter storm in the Denver area. Strati-
even a small liquid-water content in large drops may be
form clouds associated with a cold-front passage creat-
ed mean droplet diameters of 1013 m, with droplets
a significant icing threat. Bragg (1996) attributes large-
larger than 50 m in diameter observed.
droplet ice accretions, and the formation of ice ridges
aft of ice-protected areas, as a likely cause of flow sepa-
Small diurnal changes in drop-size spectra occur as
ration, aileron snatch, and loss of roll control. Shah et
a result of changes in cloud dynamics between night
al. (1998) indicate that secondary ice shapes producing
and day. Modeling of marine stratocumulus clouds by
ridges spanwise along a wing can be created by super-
Considine (1997) demonstrated increases in MVD of a
cooled large drops (SLDs), even with a heated leading
few microns in the afternoon and decreases at night, with
edge. The larger drops also strike unprotected areas of
minima in the morning. Much of the effect is due to
the aircraft, such as the underside of the wing, increas-
daytime decreases in dry air entrainment and increases
ing drag (Politovich 1989). Loss of a research aircraft
in entrainment at night.
by the Desert Research Institute in icing conditions may
An active area to watch for advances in information
have been caused by SLDs, typically drops in the 50- to
regarding drop-size spectra, outside of aircraft icing, is
500-m-diameter size range (Telford 1988). Coffey
climate change research. Measurements and models
(1995) describes the hazard of SLDs as observed from
characterizing cloud microphysical properties have
the cockpit of a research aircraft, with advice on how to
become critical for parameterizing the effects of clouds
avoid and exit SLD conditions.
on climate change. Radiative models used to simulate
Droplets larger than about 50 m in diameter do not
potential climate change and isolate the effects of green-
remain suspended in clouds by turbulence effects as do
house gases are very sensitive to cloud drop-size distri-
smaller droplets. Gravitational forces cause them to fall
bution (Choularton and Bower 1993, Telford 1996).
at greater speeds as drop size increases, producing pre-
Experiments analyzing the roles of cloud microphysi-
cipitation. Though long recognized as a hazard, these
cal properties in climate change that are either in progress
large drops have been receiving more attention in recent
or completed include the Atlantic Stratocumulus Tran-
18
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