radiometer or aircraft support. The February and March
wavelength differential attenuation method allows dis-
cases gave highly variable results (Martner et al. 1993b).
crimination between the solid and liquid phases in a
Measurements were made within upslope clouds in one
nonprecipitating cloud and an estimation of water and
March case with aircraft-measured liquid-water contents
ice contents. The attenuation of ice is negligible com-
of up to 0.4 g m3, a cloud droplet MVD of about 15
pared with the attenuation of water. Thus, after correc-
m, and no ice crystals. However, the radar measured
tion for the temperature of the ice and water particles
no liquid-water content. In another case with drizzle, liq-
and the coefficients of attenuation of water and ice, liquid
uid-water contents were occasionally negative, perhaps
water can be computed from the difference between
because drops fell outside of the Rayleigh regime. In the
the reflectivities. They claim that theory indicates that
April tests, liquid-water contents of 0.2 to 0.6 g m3 were
the technique allows the quantity and location of super-
measured, but no in-situ verification was available.
cooled water to be estimated precisely and easily. An
analysis of paired X and Ka bands, and Ka and W bands,
Martner et al. (1993a,b) have evaluated their meas-
concluded that the X- and Ka-band pair was best for
urements extensively. They identify several meteorologi-
both air- and ground-based systems because attenua-
cal conditions that cause problems and radar deficiencies
tion of the W band is too high. There are problems with
that may be responsible for problems. If non-Rayleigh
Mie scattering, however, if water drops are large. Gosset
scattering occurs due to large water drops or ice crys-
and Sauvageot (1992) indicated that, with a knowledge
tals, there is a reversal of the X Ka-band reflectivity
of drop temperature, the technique is useful in mixed
difference trends, producing negative liquid-water con-
clouds and light precipitation. Since liquid-water con-
tents. Measurements of liquid-water content may also
tent at any range gate is proportional to the local range
be particularly poor in regions of large aggregate snow-
derivative of the reflectivity difference of the two radars,
flakes and just below the melting layer where liquid-
precise absolute calibrations of the radar reflections are
covered ice crystals occur. In addition, false positive and
not needed because only a change in their relative val-
negative liquid-water contents can occur at locations
ues with range is important (Martner et al. 1991).
within clouds where large changes in drop size occur--
Martner et al. (1991; 1993a,b) have done the most
the boundaries between smaller drops and larger drops.
definitive field testing of dual-wavelength X-band and
For example, if droplets change from small to large, X-
Ka-band radars for measuring supercooled liquid water.
band reflectivity may increase rapidly and Ka band does
During the 1991 NCAR Winter Icing Storms Project
not--producing a large false positive liquid-water con-
(WISP), radars were installed northeast of Boulder,
tent. The reverse can occur as the beams move from large
Colorado, and liquid water was measured intermittently
to small droplets, causing a negative liquid-water con-
from February to April. A steerable microwave radio-
tent.
meter to measure integrated liquid water and occasional
Clouds composed of small droplets, typically those
research flights through the radar beams were avail-
of continental origin, are difficult to detect, especially at
able to verify radar measurements. The radars and radio-
long range and if liquid-water content is low. Ice crys-
meter were scanned at an angle of 7.5 above the
tals within the clouds can help make them visible, but
horizon, and the research aircraft flew up the radar
then Rayleigh problems may occur (Martner et al.
beam. Liquid-water content was analyzed with a least-
1993a,b). Higher-power radars may solve the problem.
squares fit between the X- and Ka-band reflectivities
Overall, Martner et al. (1993a,b) were encouraged by
the promise of dual-wavelength differential attenuation
for a 4-km window of gates (53 gates of 75 m each).
to obtain cloud liquid-water content. Most of the prob-
Computed liquid-water content was assigned to the
lems can be solved, and they indicate that more field
center range gate of the 53 gates and then was shifted
trials should be performed with improved hardware and
out one gate, and the computation was repeated. This
in more favorable weather conditions (Martner et al.
was repeated for each gate along the beam from 2.0 to
1993a).
22.6 km from the radar (Martner et al. 1993a). Cloud
Fournier (1993) also proposed a dual X- and Ka-band
temperature, necessary for liquid-water content com-
radar as a terminal aviation weather-sensing system to
putations, was either estimated or available from the
estimate cloud parameters at distances from 20 to 30
research aircraft.
km. He indicates that the radar would be capable of esti-
Test results during WISP were mixed because of
radar design deficiencies due to the low-budget nature
mating, through differential attenuation, cloud type, visi-
of the tests, weather conditions, and inability to obtain
bility, wind fields, median drop diameters, ice vs. liquid-
complete in-situ measurements of the radar-measured
water content, and light and moderate precipitation.
cloud conditions. Seven cases were analyzed from
Development of this system did not continue beyond the
WISP91, five cases in February and March with radio-
proposal stage, however, because of funding cuts at
meter and aircraft support and two in April without
Transport Canada.
36
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