radiometer at NOAA ETL that measures both integrated
icing pilot reports. Westwater and Kropfli (1989) cite
water vapor (20.6 GHz) and cloud liquid water (31.6
Fotino's work as one among several demonstrating the
GHz). Accuracy was determined by comparing with
utility of scanning microwave radiometers at airports
liquid-water estimates derived by microwave transmis-
for detecting aircraft icing conditions. They stress the
sions from a COMSTAR satellite through clouds. A
importance of the 21-GHz and 31-GHz frequencies for
scatter plot comparing the two methods showed a nearly
measuring integrated liquid water. They demonstrate
1:1 relationship, with scatter in the 1:1 relationship
how the radiometers can be used to continuously map
increasing as liquid water increased. They also demon-
integrated liquid water, and they indicate that 90.0 GHz
strate azimuth scans of the radiometer at an elevation
is also a frequency that can be used to detect liquid
angle of 12.5, showing liquid-bearing cloud fluctua-
water. It is six times more sensitive than 31 GHz, and
tions depending upon the antenna direction. In heavy
can measure integrated liquid per unit area ranging in
rains, such as 80 mm hr1, the radiometer saturates due
magnitude from 0.03 to 5.0 mm, which translates to
0.06 to 10.0 g m3 of cloud water. This higher sensitiv-
to excessive radiation. Signals were not affected by water
or snow on the antenna. The authors suggest that an
ity is confirmed by Grody (1997), who indicates, how-
airborne system could scan from the zenith to any angle
ever, that scattering from precipitation droplets is a seri-
forward along the line of flight, suggesting that hori-
ous problem at 8590 GHz.
zontal sensing may be possible.
Hill (1991a, 1992) made some of the first measure-
Gary (1983) describes a microwave system designed
ments comparing radiometer-measured supercooled
to monitor aircraft icing conditions that was demon-
liquid water with in-situ aircraft measurements. The
strated at Buffalo International Airport. Temperature pro-
Utah State University radiometer, a copy of the NOAA
files, water vapor, and cloud liquid water were meas-
ETL scanning radiometer discussed above, operates at
ured with radiometers. Water vapor and liquid-water
20.6 and 31.65 GHz. Field tests were done at Sodus
measurements were made from radiometers operated
Point, N.Y. A research aircraft carrying a Rosemount
at 22.23 and 31.4 GHz, respectively. Water vapor and
ice detector to measure supercooled liquid water was
temperature were verified with radiosondes, but intended
flown in ascending and descending spirals centered over
overflights to verify liquid water did not occur. How-
the radiometer. Measured liquid-water contents were
ever, forecasts of aircraft icing and pilot reports of icing
low, near the limits of the radiometer resolutions, so
did compare well, suggesting that the liquid-water meas-
completely valid comparisons could not be made. Seven
urements were reasonable. This study is the first example
validation tests were made, with two producing spuri-
of a system explicitly designed and tested, using remote-
ously high readings by the radiometer, potentially attrib-
sensing devices, for detecting aircraft icing conditions.
utable to cloud-entrained snowfall that did not adhere
A workshop about remote detection of aircraft icing,
to the ice detector and a melt layer along an inversion.
sponsored by the University of North Dakota (Smith
Hill (1991b) also compared the ability of a dual-
1985), concluded that passive microwave radiometers
frequency radiometer, the Utah State instrument cited
would be useful for determining cloud liquid water.
above, and a single-frequency radiometer operating at
However, since radiometers only provide integrated
31.65 GHz to measure cloud water. A dual-frequency
liquid water, cloud top and base would also have to be
unit corrects cloud water content by accounting for
measured to provide an estimate of cloud liquid-water
changes in water vapor. However, according to Hill,
content in mass/volume units. They recommend that
these changes are very small during the winter and intro-
duce little error if ignored. A comparison between the
radiometers be used in scanning mode but indicate that
scanning is slow and would take about 5 min per 360
two radiometers during winter tests indicated a nearly
scan at one elevation angle. A solution to this may be to
1:1 relationship, suggesting that the winter water vapor
use multiple radiometers, each scanning assigned sec-
correction is not needed. This would simplify cloud-
tors and elevation angles.
water measurements from aircraft, reducing hardware
and computational requirements.
Fotino et al. (1986) related radiometer-derived zenith
measurements of temperature and liquid water near
Stankov et al. (1992) describe the use of four micro-
Denver to pilot reports of icing. Frequencies of 20.6
wave radiometers to measure supercooled liquid water
in the Denver and Boulder area during WISP91. Though
GHz and 31.65 GHz were use to measure water vapor
and liquid water, respectively. Measurements were inte-
the radiometer estimates of liquid water were not explic-
grated over 2-min periods. Comparisons with pilot
itly compared with aircraft measurements, they con-
reports were difficult because they are often inaccurate
clude that the radiometer's ability to measure liquid
water was excellent. The only reported problem with
in time and location, and pilots avoid icing upon hear-
ing reports of the conditions. They found overall strong
the radiometers involved keeping snow from adhering
correlations between the radiometer measurements and
to the sensor windows.
40
To Contents
Previous Page
