raindrops. They conclude that the combination of fall
Lhermitte (1987) uses W-band Doppler radar to
speed and the circular depolarization ratio will yield more
extract information about the rainfall drop-size spectra
information than either can individually.
in clouds. Using Doppler velocity measurements of drop
fall speeds, and Rayleigh and Mie scattering, drop sizes
5.3.2.3 Polarization radar techniques. Reinking et
were detected in field tests in Florida and Colorado. He
al. (1996) proposed and tested a method of differentiat-
proposes that combining several radar wavelengths, such
ing drizzle from rain and ice crystals using elliptical
as W, Ka, and X bands, would provide the capability of
depolarization ratios (EDRs) and linear depolarization
detecting the full range of raindrop sizes using Doppler
ratios (LDRs) with the NOAA ETL scanning Ka-band
techniques. Lhermitte (1988) also presents a method of
radar. Drizzle drops are spherical and do not polarize
backing out air vertical velocities from raindrop fall
the signals, raindrops are nonspherical and will depolar-
speeds to improve measurement of drop-size spectra.
ize the radar signal, and depolarization by ice crystals
The technique relies upon Mie backscattering oscilla-
depends upon their shape and orientation. Scattering cal-
tions around Mie scattering maxima and minima caused
culations, and measurements during WISP in 1993, indi-
by raindrops of various sizes (Lhermitte 1987). The tech-
cate that EDR provides a good capability to distinguish
nique would be useful in providing vertical profiles of
between ice crystals of various habits, drizzle, and rain.
drop-size spectra within clouds and could be applied to
If the requirement is simply to distinguish drizzle from
S-band radars such as NEXRAD.
ice, then LDR is better and could be applied to NWS
5.3.2.2 Doppler and polarization radar techniques.
NEXRAD radars for use at airports. Matrosov et al.
Wilson et al. (1997) developed a method of determining
(1996) and Reinking et al. (1997) have further differen-
parameters of the drop-size distribution of rainfall to
tiated ice crystal types using EDR and LDR with drizzle
estimate rainfall rates. This is accomplished, using S-
drops as a reference. They are able to discriminate hydro-
band radar, by measuring differential Doppler velocity
meteor types within cloud systems, which will help deter-
(DDV), the difference between Doppler velocities at
mine the presence of cloud ice and drizzle.
vertical and horizontal polarization. DDV is indepen-
Pazmany et al. (1994) described the development of
dent of turbulence and most shear. It is used with reflec-
a new W-band dual-polarized Doppler radar at the Uni-
tivity and differential reflectivity to produce a three-para-
versity of Massachusetts. The radar flies on the Univer-
meter gamma fit to the drop-size distribution. The radar
sity of Wyoming King Air, as described earlier, and oper-
beams must have elevation angles of 10 to 40 for accu-
ates pointing either horizontally, along the flight path,
rate measurements. The technique is not affected by drop
or vertically. Early tests indicated the ability to detect
oscillations and can detect ice crystals and determine
melting bands and hydrometeor type. Flights with the
the location of melting layers. Because of the radar wave-
radar in 1992 and in WISP94 provided airborne in-situ
length, the technique would be usable only with ground-
and radar measurements of snowstorms, needle ice crys-
based radars, such as the NEXRAD.
tals, and melting layers. Most observations were made
Takahashi et al. (1996) utilized a Doppler and dual-
at vertical incidence to clouds above the aircraft. The
polarized X-band radar system to detect the drop-size
paired radar and in-situ measurements will be used to
distribution of rainfall in isolated cumulus clouds in
develop relationships between the two.
Japan. Drop-size distribution was computed from ter-
minal fall velocities of the drops and the differential
5.3.2.4 Neural net and other radar techniques.
Mead and Pazmany (Mead at al. 1998, Koenig et al.
reflectivity and horizontal polarization. Drop sizes larger
than 140 m were detectable. The linear depolarization
1999) proposed using a three-band radar consisting of
X, Ka, and W to detect liquid-water content and elements
ratio and the correlation between the vertical and hori-
of the drop-size spectrum, using a neural net for post-
zontal polarized waves were used to determine mixed-
processing the radar returns. This technique, described
phase layers, such as the bright band. Two parameters
of an assumed exponential drop-size distribution were
earlier, shows excellent potential for estimating elements
returned by the radar. Measurements made of rainfall in
of the drop-size spectrum, but it cannot detect mixed-
isolated cumulus clouds were plausible, but no in-situ
phase conditions.
verification measurements were made.
Using C-band radar, Huggel et al. (1996) improved
In one of the first modern uses of a Ka-band radar,
estimates of rainfall rates by more accurately estimating
Kropfli et al. (1982) used a Doppler radar scanning at a
drop-size distribution. They argue that most liquid pre-
high elevation angle, and circular depolarization, to dis-
cipitation is formed in a bright band where falling ice
tinguish falling precipitation forms. Fall speed distin-
crystals melt and coalesce into raindrops. By develop-
guished graupels, aggregates, and dendrites into five cat-
ing a relationship between reflectivity within the "bright
egories within a squall line, and the circular depolari-
band" and measured drop sizes with disdrometers, they
zation ratio was able to distinguish frozen particles from
were able to predict drop-size distributions below the
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