by stating that they are severely attenuated by rainfall,
portable. However, because of attenuation in moderate
but they provide fine-scale measurements of nonpreci-
to heavy rainfall, they are best used in nonprecipitating
pitating and drizzle clouds because the amount of energy
clouds. Ice crystals can also cause non-Rayleigh scatter-
reflected by cloud droplets and ice crystals sharply
ing, which causes loss of signal. If combined with a
increases as wavelength decreases.
longer-frequency radar, such as X band, differential
There have been many applications of Ka-band radar
attenuation can be used to extract cloud liquid-water
to measuring cloud microphysics, it appears to be the
content (Martner et al. 1991). According to Mead and
most useful of the millimeter wavelengths for aircraft
his colleagues, as of 1994, only five universities and
icing. Ka band, when compared with the shorter-wave-
one government laboratory had operating millimeter-
length W band, can penetrate optically thick clouds with
wave radars. Applications include studying internal
high liquid-water contents, it can detect clouds above
circulations of cumulus clouds, remotely measuring
light precipitation and multiple cloud layers, and Mie
rainfall drop-size distributions, and studying drizzle in
scattering is relatively uncommon (Kropfli et al. 1995).
stratus clouds (Mead et al. 1994).
White et al (1996) indicate that Ka-band radar can be
Bragg scattering caused by atmospheric refraction,
used to obtain the reflectivity, Doppler velocity, and
which is a problem for longer-wavelength radars, is
Doppler spectral width of drizzle drops. Directly related
negligible for millimeter-wavelength radars (Kropfli
to reflectivity are drizzle liquid-water content, flux, and
and Kelly 1996). However, the short wavelengths often
number concentration. Doppler velocity and spectral
restrict the applicability of Rayleigh scattering. Thus,
for Ka-band radar, Mie scattering begins for water drops
width provide estimates of the magnitudes and shape
at about 2.7 mm diameter, and for W-band radar at about
of the drop-size spectrum. Measurements made during
1 mm diameter, which increases the complexity of inter-
the Atlantic Stratocumulus Transition Experiment
pretation. Mie scattering is an even more difficult prob-
(ASTEX) by Ka-band radar provided liquid-water con-
lem for ice crystals and snowflakes, where Mie theory
320 m, and drop concentrations from 0.3 to 700 L1.
has not been developed (Kropfli and Kelly 1996). High
humidity is also a problem with the shortest wave-
Kropfli et al. (1995) further demonstrated in ASTEX
lengths, such as W band, where its attenuation effects
that Ka-band radar detected ice masses as low as 0.003 g
may reduce sensitivity to small drops.
Doppler radar in the millimeter wavelengths excels
strated retrieval of integrated liquid water at zenith, as
in measuring drop fall speeds because of its high fre-
may be applied at airports, and compared it with radio-
quency, which allows high precision. Doppler tech-
meter-derived integrated liquid water (Martner and
niques, as indicated earlier, may be helpful for airport-
Kropfli 1993). Radar and radiometer-integrated liquid-
based sensing systems, but they may not be useful for
water estimates compared well except in drizzle.
aircraft-mounted systems unless they are scanned up
The University of Massachusetts has developed a
and down at large angles (Kropfli and Kelly 1996).
dual-wavelength, ground-based cloud-profiling radar
Polarization measurements may also be used, especial-
system operating at Ka and W bands (Sekelsky and
ly with Ka-band radar, to determine particle shape and
McIntosh 1996). A system with a 1-m-diameter anten-
orientation (Kropfli and Kelly 1996). For example, the
na is used to make polarimetric and Doppler measure-
circular depolarization ratio has been used to distin-
ments of clouds at both wavelengths. The authors state
guish plate-like crystals from aggregates in Colorado
that dual-wavelength millimeter-wave particle sizing
clouds using a ground-based Ka-band radar, and future
(drops and ice crystals) has the potential for more accu-
capabilities may allow distinguishing between other
racy than single-wavelength Doppler methods. In addi-
crystal types.
tion, they indicate that MVD and the shape of drop-
Ka-band radar can determine cloud base and top, and
size spectra can be determined more accurately with
thus thickness, and cloud structure, which may be use-
both wavelengths. They speculate that it also may be
ful for avoiding icing (Politovich et al. 1995) when
possible to discriminate glaciated from liquid clouds
combined with a microwave radiometer capable of
and to estimate particle sizes in fully glaciated clouds.
measuring vertically integrated liquid water. Vertical
W-band radars, operating at about 95 GHz, are
cloud liquid-water profiles may be mapped by adjust-
becoming increasingly popular for cloud microphysics
ing drop concentration to force the two signals to fit.
work (Mead et al. 1994, Kropfli and Kelly 1996). They
Kropfli and Kelly (1996) state that, though W-band
offer even better size, weight, and power advantages
radars are superior for airborne use because of their
than Ka-band radars, but they also suffer more severely
smaller size, the Ka band is less attenuated by water
from attenuation in large drops, humidity, and precipi-
vapor and cloud water, making it more useful. Kropfli
tation. They are used to measure cloud structure and
and Kelly (1996) summarize millimeter-wave radars
excel at observing drizzle, which is a subject of intense
34
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