the production of ice crystals and water in rising air.
Martner et al. (1993a), as part of the Lake Ontario
The process operates only in clouds with updrafts. Air
Winter Storms project, installed Doppler radar, wind
temperature and Doppler radar measurements of
profilers, and microwave radiometers to measure inte-
updrafts are used to describe the rate of production of
grated liquid water on the eastern shore of Lake Ontario.
water vapor in excess of saturation with respect to ice.
A freezing rainstorm on 15 February 1990 was contin-
uously monitored by the microwave radiometers at 20.6,
The radar reflectivity is inverted to determine the profile
31.65, and 90.0 GHz. Vertically pointing scans and
of ice-particle size distribution. From the ice-particle
scans at a 7.5 elevation angle were made. The radio-
size distribution, the rate that water vapor can be con-
sumed by pure deposition is determined. The differ-
meter could not observe snow and ice, but it immedi-
ence between the rate of water vapor production and
ately signaled the onset of melting aloft at the begin-
deposition on ice crystals is the production of super-
ning of liquid precipitation. Some overestimates of
cooled liquid water at each level within the cloud. This
liquid water may have occurred because water-coated
technique was simulated, but no field studies have been
snowflakes appeared to the radiometer to be completely
conducted to demonstrate its viability.
liquid water as they melted. In addition, high emission
Solheim and Godwin (1998) describe the develop-
during rainfall occasionally saturated the radiometer
ment of a passive microwave radiometer that profiles
signal. The radiometers were valuable for their ability
atmospheric temperature, water vapor, and liquid cloud
to observe the immediate onset of freezing precipita-
water. Operating at two frequencies, the radiometer pro-
tion aloft.
files water vapor by frequency scanning near 22 GHz
Huggins (1995) describes the use of a dual-wave-
and temperature and liquid-water profiles by scanning
length radiometer system (20.6 and 31.65 GHz), similar
near 60 GHz. Initial use demonstrates radiometer pro-
to the ETL system but mounted on a truck for mobile
files using a variety of retrieval algorithms compared
use. The system was used to monitor the spatial distri-
with radiosonde profiles. The instrument, although it
bution of liquid water over the Wasatch Plateau of Utah
needs additional testing, holds promise for providing
for cloud seeding. A unique spinning mirror system was
temperature and liquid-water profiles at airports to
used to keep snow and rain off the radiometer mirror.
assess aircraft icing conditions.
Most microwave radiometers measure integrated
The effects of rainfall on integrated cloud liquid-
liquid water; they provide no indication of the distribu-
water measurements also demand attention because
tion of liquid water vertically through a cloud. The dis-
rainfall causes cloud liquid water to be underestimated
tribution of liquid water within a cloud layer, and among
in the 2090-GHz range. Sheppard (1996) examined
layers if there is more than one layer, is needed for accu-
the effect of rainfall rate on vertically pointing micro-
rate aircraft icing estimates. Stankov et al. (1995) melded
wave radiometer measurements of integrated liquid
climatological and iterative techniques that had been
water. Using a radiative transfer model, the amount of
used unsuccessfully in the past into a new technique
error in estimated cloud liquid water increased with
using Ka-band radar, wind profilers, and ceilometers.
rainfall rate. The theory was tested against measure-
Their goals were to determine cloud base height, cloud
ments made in CASP. Sheppard indicates that a future
top, and cloud layers. Then, using temperature profiles
study should investigate the effects of solid precipita-
and three different assumptions about the distribution
tion.
of water within clouds, they parsed integrated water
measured by the radiometers through the clouds. These
5.4.1.2 Sensing from satellites or aircraft over
experimental techniques were compared with aircraft
water or land. Passive microwave sensing of cloud
measurements as part of WISP. Results were mixed:
liquid water from a satellite or aircraft over land or water
some profiles compared well with aircraft measure-
is more difficult than viewing toward zenith from be-
ments, and others compared poorly within the same
low clouds. When observing toward the earth's surface,
storm. Stankov and his colleagues indicated that more
as explained earlier, the brightness temperature of water
comparisons are needed. This work demonstrates a fun-
and land surfaces must be considered. When viewing
damental need for methods of distributing liquid water
toward space, background temperature is near absolute
zero (~3K) and clouds emit strongly in contrast. When
through clouds after total integrated values are meas-
ured. This is important for determining the concen-
viewing toward water bodies from above clouds, the
trations of supercooled liquid water in each cloud profile.
water bodies appear cold, with brightness temperatures
of about 140K at 37 GHz (this can vary by 15K or
Sauvageot (1996) has developed a method for con-
structing vertical profiles of liquid and ice water in
more), because the emissivity of water bodies is about
mixed-phase clouds using a microwave radiometer to
0.5 (Jones and Vonder Haar 1990, Greenwald et al.
determine total integrated liquid water, a Doppler radar
1993). Land surfaces typically have larger brightness
to determine updraft velocities, and theory to explain
temperatures than water because land surfaces are often
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