particles produce less depolarization than water drops
was not a satisfactory proven method for estimating
or dry ice crystals do, so it is a useful method for locat-
rain rate from radar. That situation seems to have
ing melting layers (Houze 1993, Rinehart 1997).
changed little; NEXRAD needs to utilize rain gauges
Circular depolarizing radars transmit a signal that
for correction (Crum and Alberty 1993, Houze 1993,
rotates one complete revolution orthogonal to the beam
Rinehart 1997).
axis per radio frequency cycle (Toomay 1982, Houze
5.3.1.2 C-band radar. C-band radars have also been
1993). The circular depolarization ratio is the ratio of
used experimentally to obtain rainfall rates, but again
the parallel (transmitted) component to the orthogonal
they are too large for aircraft use, though they could be
(received) component and indicates the sphericity of
used at airports (Gorgucci and Sarchilli 1996a,b; Tian
particles. Polarization may be useful for determining
and Srivastava 1997). Doviac and Zrnic (1984) argue
some elements of the drop-size spectra, especially from
that this ability to measure rainfall rates, along with
airborne radar that is scanning drops and crystals ortho-
Doppler detection of wind shear, would be a useful tool
gonal to their falling direction and thus maximizing
for predicting freezing rain at the surface and aloft.
shape deformation, or long-axis orientation, to the hori-
zontal.
5.3.1.3 X-band radar. X-band radar is small enough
to be carried aboard aircraft and often is. It is useful for
detecting precipitation, although with less accuracy than
5.3.1 Detection of liquid water
longer wavelengths (Rogers and Yau 1989). X band is
5.3.1.1 S-band radar. The ability to detect raindrop-
typically considered a precipitation radar, but it is capa-
sized particles may be needed in a remote ice-detection
ble of detecting cloud liquid water. For example, Paluch
system because freezing rain is a serious aircraft icing
et al. (1996), comparing the use of X- and Ka-band
threat, although it is typically not considered as dan-
radars for detecting cloud liquid-water content, found
gerous as freezing drizzle. Raindrop sizes begin at about
a close, consistent correlation between radar reflectiv-
500 m diameter and extend to about 5 or 6 mm in
ity and cloud liquid-water content in summer cumulus
thunderstorms (Pruppacher and Klett 1997). It is pos-
in Florida. However, they indicated that Bragg scatter-
sible to experience icing in very large drops near the
ing--the susceptibility of longer wavelengths to detect
tops of towering cumulus in the tropics or in the mid-
"angels" caused by turbulence--was a potentially
latitudes in the summer months. However, freezing rain-
greater problem with X-band radar (White et al. 1996).
drops will usually be found at the smaller end of the
They also indicated that there is typically a strong rela-
size spectrum, typically no larger than 3 mm, because
tionship between drop size and reflectivity, which is a
turbulence is small in most freezing rain (Jeck 1996).
source of error in radar liquid-water measurements. In
According to Battan (1973), S-band 10-cm radar can
the Florida observations, however, there was a strong
successfully estimate rainfall rates, and thus liquid-
reflectivityliquid-water content relationship because
water content, for long distances. The National Weather
most of the liquid water was concentrated within the
Service's NEXRAD, for example, is a 10-cm radar with
large end of the drop-size spectrum. This suggests that
a range of about 460 km for reflectivity measurements.
broad drop-size spectra will produce a larger rainfall
NEXRAD does not provide explicit precipitation liquid-
rate estimate error than narrow spectra.
water content at the present time, but it does provide
vertically integrated liquid water from rainfall contained
5.3.1.4 Millimeter-band radar. Millimeter-wave
in a 4 km by 4 km grid (Crum and Alberty 1993). Thus,
radars, principally the Ka and W bands, are the current
precipitation liquid water may be estimated near air-
choices for detecting cloud microphysical properties and
ports from a ground-based remote detection system.
precipitation (Mead et al. 1994, Kropfli and Kelly 1996).
Doviac and Zrnic (1984) indicate that accurate estimates
Since the backscattering cross-section of a drop is
of rainfall rates and rainfall liquid water require a knowl-
inversely proportional to the fourth power of the wave-
edge of the raindrop size distribution, which, if unknown,
length, long-wavelength radars, despite their great range
can cause rainfall rates to differ by a factor of four. In
and power, are at a disadvantage for detecting cloud
their review of the extensive work that has been done
droplets. They can compensate with larger antennas and
more power, but at high cost (Martner and Kropfli
estimating rainfall rates from radar, they indicate that
1993), and, though they may be useful at airports for
spatially detailed measurements of precipitation liquid
scanning for icing conditions, they do not fit on air-
water require knowledge of drop sizes unless the drop
craft. Kropfli and Kelly (1996) indicate that millimeter-
sizes are reasonably uniform. Curvature of the earth
wavelength radars are sensitive to small hydrometeors,
also causes radar to observe different portions of storms
have excellent spatial resolution, minimal ground clut-
with distance and thus different drop-size distributions.
ter problems (which allows observation of weakly
Doviak (1983), in a review of rain rate estimation
reflecting cloud with high resolution), and are easily
methods using radar, indicated that at that time there
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