true range of supercooled liquid-water contents that may
distances were 18.5 km, so the values do not represent
be encountered. Though general values are known, most
maxima encountered during flight.
Telford (1988) analyzed the causes of instability and
older measurements were made over rather long averag-
ing distances. Nearly instantaneous measurements, on
the loss of a Desert Research Institute research aircraft
the order of one measurement each second (150 m for
measuring layered cloud properties in the Sierra Nevada.
Extreme icing was associated with the crash, and liquid-
a 300-kt research aircraft), provide high resolution.
water contents were measured from 0.2 g m3 to a maxi-
Though the granularity of cloud liquid water is undoubt-
mum of 1.4 g m3 prior to the crash. Measurements
edly finer, higher resolution would not be necessary
were instantaneous and were made with an FSSP and a
for developing remote-sensor specifications.
JohnsonWilliams probe.
Liquid-water content has been measured during field
In a detailed study of a winter storm and shallow
programs with modern instrumentation at as fine as 1-
cold-front passage in the Denver area, Politovich and
second intervals. Examples include ASTEX, the Atlan-
Bernstein (1995) measured unusually high liquid-water
tic Stratocumulus Transition Experiment; FIRE, the
contents for that area in stratiform clouds of 0.6 g m3.
First International Satellite Cloud Climatology Project
Severe icing was also reported by the research aircraft.
Regional Experiment; the U.S. DoE ARM campaign,
During the second Canadian Atlantic Storms Pro-
Enhanced Shortwave Experiment; and many other
gram (CASP), Cober et al. (1995) reported 3745 super-
smaller programs. These data could be reanalyzed for
cooled liquid-water content measurements within an
remote-sensing purposes if they could be acquired. In
800-km radius of Halifax, Nova Scotia, within strati-
addition, new flights should be made with better instru-
form and "system" clouds such as through cold fronts,
mentation.
warm fronts, and low-pressure areas. Clouds of oceanic
and continental origin were included. Though few com-
4.3.2 Cloud drop-size spectra
Liquid water is delivered to aircraft surfaces as dis-
parisons of liquid water in maritime vs. continental
crete drops varying in diameter from only a few microns
clouds have been conducted specifically with regard to
at the smallest diameter to over 4000 m in rain drops
aircraft icing, in general, liquid-water contents are sim-
(Fletcher 1962, Pruppacher and Klett 1997, Rogers and
ilar for continental and marine clouds of a given gen-
Yau 1989, Willis and Tattelman 1989). Drop size has
era (Rogers and Yau 1989). Cober et al. (1996a) also
several important roles in aircraft icing.
found that liquid water varied little between cloud types,
One effect of drop size on airframe icing is its influ-
except for larger liquid-water-content standard devia-
tions in system clouds. Median supercooled liquid-water
ence on the amount of water collected. The amount of
content was 0.11 g m3, with supercooled liquid-water
liquid water delivered to an airframe surface is a func-
content exceeding 0.94 g m3 only 0.01% of the time,
tion of the collection efficiency of that surface as
similar to conditions measured by Sand et al. (1984)
affected by the relative speed between the surface and
the drop, the radius of the surface, and the drop diameter.
over the Great Lakes and California in winter. Stewart
As relative wind speed increases, drop size increases,
et al. (1996) measured supercooled liquid-water con-
and as surface radius decreases, droplet collection effi-
tents of warm and cold fronts off the Nova Scotia coast.
ciency increases. As a result, smaller drops are carried
Cloud types are not provided, but the measurements
over an airfoil surface to impact aft of the leading edge,
were made with modern optical instruments. Maximum
whereas larger drops impact closer to the leading edge.
supercooled liquid-water contents were not greater than
0.9 g m3, and typically they were less than 0.3 g m3.
Objects with a large radius are preferentially impacted
Pruppacher and Klett (1997) summarize character-
by large drops rather than by small drops. As a result,
istic liquid-water contents found in clouds by genera,
the amount of liquid water delivered to a specific por-
warning that liquid-water content typically varies
tion of an airframe surface is a function of the liquid
strongly from cloud to cloud. Early-stage cumulus typi-
water residing within that portion of the total cloud drop-
size spectrum striking the surface. This ignores run-
1.0 g m3, and stratus and stratocumulus 0.1 to 0.5 g
back and other effects that occur after drops impinge
upon the surface.
contents up to 5.0 g m3. Though these measurements
A second effect of drop size is upon the type and
shape of ice that forms on the airframe surface (Hans-
were made in warm and cold conditions, the tops of
man 1985, FAA 1991, Shah et al. 1998). Depending
cumulus clouds typically have the highest liquid-water
contents, which, in the tropics, are often supercooled
upon a variety of factors, including the amount of liquid
and produce significant icing.
water impinging on a portion of airframe over a unit of
It is evident that modern measurements taken at short
time and the collection efficiency of the icing surface
time intervals are necessary to evaluate properly the
as a function of the drop-size distribution, the type and
16
To Contents
Previous Page
