overall, and a larger drop spectral width. They attribute
years. The crash of an ATR-72 at Roselawn, Indiana, in
the formation of drizzle drops in stratus clouds to areas
October 1994, focused the attention of aviation icing
where updraft velocities are greater, which causes differ-
researchers on SLDs and their unique hazard to aircraft
ent percentages of cloud condensation nuclei at the
(NTSB 1996, Broderick 1996).
cloud base to be activated. They suggest that updraft
Early research reports describing the results of flights
velocity can be used to predict drop concentration and
measuring the microphysical properties of icing clouds
the width of the drop-size spectrum.
have mentioned supercooled drizzle drops, for example,
Kline (1949), but relatively few reports focused on
In one of the most ambitious drizzle measurement
programs to date, Cober et al. (1995; 1996a,b,c) describe
SLDs. Rodert (1951) and Lewis (1951) both indicated
freezing drizzle measurements made in the Canadian
the importance of freezing drizzle and freezing rain as
Freezing Drizzle Experiment (CFDE) off the New-
aircraft icing hazards. However, most work until recent
foundland coast. Freezing drizzle was encountered in
years has been in response to needs to understand rain-
four research flights within thick (~1000-m) stratiform
drop-formation mechanisms for cloud physicists, rather
clouds. In these four encounters, liquid-water content
than for aviation needs (Fletcher 1962, Hobbs and Deepak
varied between 0.05 and 0.2 g m3 when MVDs were
1981, Cotton and Anthes 1989, Rogers and Yau 1989,
larger than 40 m. MVDs as large as 950 m were
Houze 1993, Young 1993, Pruppacher and Klett 1997).
measured in freezing rain below the cloud base. Within
Isaac and Schemenauer (1979) found supercooled
clouds, MVDs often exceeded 500 m. When combined
large drops near the tops of cumulus clouds near Yellow-
knife, NWT. Many cloud tops between 0C and 8C
liquid-water contents and MVDs were compared to FAR
25, Appendix C, 34 of 147 data points fell outside the
had concentrations of supercooled drops larger than 70
m. About twice as many clouds had concentrations of
envelopes. They conclude that freezing drizzle may be
a frequent phenomenon in East Coast winter storms and
large water droplets as had concentrations of ice crys-
a significant aviation hazard.
tals. Large drops were associated with low liquid-water
contents, and droplets larger than 150 m never had a
Jeck (1996) published the most comprehensive
concentration of more than 1 L1. The authors could
review to date of the state of knowledge about freezing
not explain why the drops existed and did not relate
rain (ZR) and drizzle (ZL) with regard to aviation. He
them to aircraft icing since the purpose of the research
indicates that few instrumented aircraft have flown in
was related to precipitation enhancement.
ZL and ZR, and that little is known about the meteoro-
Politovich (1989) describes icing from large droplets
logical conditions and geographic locations of SLD
on a research aircraft flying in California and Arizona.
occurrence. Elevated ZL and ZR, encountered by air-
Eleven flights are characterized within a narrow tem-
craft in flight, are a hazard that may not be experienced
perature range, between 5.5 and 9.4C and drop con-
at the surface if they freeze as sleet before reaching the
centrations of generally less than 100 cm3. Conditions
ground. Though techniques have been proposed for
had the greatest effect on aircraft performance when
detecting ZL and ZR from radiosondes, no reliable
fewer than 0.1 1 cm3 droplets occurred in a size range
methods of prediction are available, especially for ZL,
from 30 to 400 m. Politovich indicates that the fre-
which can occur without the traditional warm layer often
found in ZR. ZR and ZL are typically lower-altitude
quency of these occurrences is low but not rare. Ample
phenomena, with most occurring below 3811 m agl,
moisture and time, accompanied by lift, must be avail-
making them a distinct hazard to nonpressurized air-
able to create these large drops. She suggests that envi-
craft, helicopters, and all aircraft on approach and depar-
ronments most likely to experience SLDs are orographic
ture. Little is known about the frequency, depth, and
and upslope in warm fronts and within the warm sector
horizontal extent of ZR and ZL layers, the causes of
of cyclones where adequate moisture and lift are avail-
able.
ZL, and the full range of meteorological conditions asso-
Feingold et al. (1996) argue, from numerical simu-
ciated with each. Jeck indicated that the use of MVD to
lations, that the production of drizzle within clouds is
characterize drop spectra associated with SLDs is not
related to droplet residence time and within-cloud turbu-
appropriate because the MVD provides no indication
lence. Vigorously growing clouds produce more driz-
that SLDs exist.
zle because they allow longer in-cloud drop dwell times,
Hobbs and Rangno (1996) observed supercooled
prolonging the collisioncoalescence process. Their
drizzle drops with very high liquid-water contents off
arguments are similar to that of Politovich (1989).
the Washington coast. The stratocumulus clouds were
Hudson and Svensson (1995) measured drizzle-drop
trapped above an inversion, preventing cloud conden-
concentrations off the Southern California coast as part
sation nuclei from the marine boundary layer below
of the FIRE experiments and associated drizzle drops
the inversion from reaching the clouds. As a result, drop
concentrations were low (~ 500 L1), liquid-water con-
closely with lower drop concentrations, larger droplets
19
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