tents were high (up to 0.8 g m3), and drops as large as
It is evident that SLDs create uniquely hazardous
200 m in diameter were present. The authors indicate
in-flight icing conditions, yet little is known about the
that supercooled layer clouds that form in clean mari-
phenomenon: its characteristics, its climatology, or what
time air (lacking cloud condensation nuclei) that is
comprises an SLD condition (Shah et al. 1998). Flights
decoupled from the surface could pose a significant threat
during the winters of 19961997, 19971998, and
to aircraft from supercooled drizzle or rain.
19981999 by NASA Glenn Research Center's Twin
Cober et al. (1996b,c) report conditions off the east
Otter aircraft into SLD should help answer some of the
coast of Canada similar to those reported by Hobbs and
remaining questions (Miller et al. 1998). Jeck's (1996)
Rangno (1996). Freezing drizzle was observed in 1100-
report addresses most of the weaknesses in knowledge
m-thick stratiform clouds in temperatures between 11C
about SLDs and is probably the most complete and suc-
and 8C. The maritime air was very clean, with con-
cinct paper on the subject from an aviation perspective.
densation nuclei allowing only a few drops to grow large
and coalesce. Though the MVD was 29 m, cloud drop-
4.3.4 Temperature
lets larger than 40 m exceeded 300 L1, and 500-m-
The thermal environment of an icing event deter-
mines the type, amount, and location of ice formation
diameter drops were measured near the cloud tops. This
on an airframe (Cooper and Sand 1997). The thermal
suggests one mechanism for ZL, that of isolating humid
environment is controlled by radiative, convective, con-
air with few condensation nuclei, allowing coalescence
ductive, latent heat and advective processes of the atmo-
and drop growth to occur.
sphere and the airframe and by the dynamics of the air-
In reports exploring the causes of ZL off the Canadian
craft moving through the atmosphere. When isolated
east coast, Isaac et al. (1996) and Cober et al. (1996b,c)
from the airframe and the thermodynamics of the icing
review the processes that could cause ZL and compare
processes, thermal processes within the atmosphere
them with CFDE measurements. In Newfoundland, ZL
alone determine the temperature of air and of drops.
is associated with easterly and southeasterly winds and
The "source" of cold also affects the amount, type,
rarely with westerly winds. Only about 15% of ZR cases
and shape of ice that forms. For example, droplets
are nonclassical, but 60% of ZL cases are nonclassical.
warmer than 0C may freeze upon a cold-soaked air-
Classical ZR and ZL result from overrunning, such as
frame, but supercooled droplets may not freeze effi-
occurs within warm fronts. Nonclassical drizzle forma-
ciently on an airframe warmed aerodynamically above
tion does not involve overrunning. Mechanisms may
freezing. Supercooled drops impinging upon an air-
include giant aerosol initiation of large drops, wind shear
frame that is colder than 0C will typically produce ice.
leading to entrainment, mixing and coalescence, long
Of the thermal processes operating, the temperature of
drop lifetimes in stratiform clouds that encourage drop
the droplets, or the temperature of the atmosphere sur-
growth, and high supersaturations. Eleven days of flights
rounding the droplets, is typically most important in
in both classical and nonclassical freezing precipitation
determining whether ice will form on an airframe.
situations showed no consistency of mechanism, except
According to Rodert (1951), it is tacitly assumed
for wind direction and the existence of inversions and
that cloud droplets are at the same temperature as the
wind shear near the cloud top.
surrounding atmosphere. This may not always be true
Climatologies of SLD accretions at the surface have
for cloud or for precipitation drops, which typically cool
been developed as a method of assessing where freezing
to the wet-bulb temperature of the surrounding atmo-
rain may be occurring aloft as a hazard to aircraft. Strapp
sphere through evaporation (Cooper and Sand 1997).
et al. (1996), Robbins and Cortinas (1996), and Bern-
Since the relative humidity within icing clouds is typi-
stein and Brown (1997) completed independent climatol-
cally near 100%, the dew point and air temperature will
ogies of the frequency of SLD events in North America
also be similar, especially within stratiform clouds of
to assess where aircraft icing due to ZR and ZL may be
stable air masses. Within cumulus clouds with active
occurring with greater frequency. All maps indicate
updrafts and entrainment of dry air, evaporation and
freezing precipitation at the surface as being most com-
subsequent cooling may be greatest near the outside of
mon east of the Rocky Mountains, with frequency
the cloud where entrainment is most active (FAA 1991).
increasing from the mid-Mississippi Valley to the North-
Therefore, one will find warm cores in clouds with inter-
east and Labrador, with an axis through the Great Lakes
nal updrafts because of reduced evaporation and the
Basin. Ahmed and Brown (1995) produced a climatolo-
release of latent heat as drops grow. In general, cloud-
gy of in-flight ZR globally, with seasonal detail in Great
size droplets reach thermal equilibrium with surround-
Britain and Europe from the U.K. Meteorological Office's
ing air very rapidly, typically within 1 second (Borovi-
numerical model output. Their model-derived climatolo-
kov et al. 1963). Precipitation drops cool to the wet-
gy suggests high frequencies of ZR over the Atlantic
bulb temperature after they have fallen into dry air
and Pacific Oceans.
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