square error (rms) in the lowest 3000 m of the atmo-
cate which methods may be most viable for remote sens-
sphere. Radiometers still do not have sufficient accu-
ing of aircraft icing conditions.
racy or resolution with height to replace radiosondes.
When comparing the accuracy of radiosondes, RASS,
5.6.1 Infrared radiometers
and radiometers for temperature profiling, Schroeder
Infrared radiometers are not sounders; temperatures
(1990) determined that the three devices compared well
obtained are an integration of the cloud boundary layer,
in the summer when temperature changes with height were
depending upon cloud optical depth, and the depth from
not rapid, but during the winter the RASS clearly pro-
which contributing radiating energy originates. Exam-
ples of infrared thermal imagers are 10.8- and 11.8-m
vided better resolution.
Decker et al. (1978) constructed a radiometer system
sensors of the NOAA Advanced Very High Resolution
operating around 60 GHz that scanned to an elevation
Radiometer used to determine cloud-top temperatures
angle of 45 from zenith. They indicated that temper-
(Giraud et al. 1997, Lee 1997). Bissonnette* suggests
ature retrievals are possible from elevation angles other
that cloud temperature could be determined in a stand-
than zenith, though they chose not to report on them.
off situation using an infrared radiometer and range
The Solheim and Godwin (1998) microwave radiometer
determined by lidar. This method would not be viable
discussed earlier profiles temperature, water vapor, and
within cloud, however, because the temperature pro-
liquid water at zenith and at low elevation angles.
vided would represent an integration of the cloud mass
Gary et al. (1992) report on an automatic tempera-
only a few hundred meters ahead of the aircraft, depend-
ture profiler operated on the NASA ER-2 high-altitude
ing upon cloud optical depth.
aircraft. The radiometer, operating at 60 GHz, scans in
10 angular steps from 50 below the flight path to 60
5.6.2 Microwave radiometers
above the flight path ahead of the aircraft. Brightness
Microwave temperature sounders detect temperature
temperatures from 15 distinct altitudes span from 2 km
changes with altitude by either sensing from the ground
below the aircraft to 3 km above it. Temperature retrieval
to zenith or from satellites to nadir. Oxygen absorbs and
thus re-emits in the 60-GHz region (10 GHz) and at
accuracy is a function of distance from the aircraft, with
greater distance causing greater uncertainty. Measure-
118.75 GHz. About 45 absorption lines centered on 60
ments are not range gated. However, as the aircraft flies,
GHz are used to determine temperature with height in
if scanning is rapid enough, temperatures measured sev-
the atmosphere by pressure broadening (Grody 1997).
The absorption lines found in the 10 GHz region around
eral kilometers ahead of the aircraft can be assembled
to create a composite temperature map. Such maps have
60 GHz result from decreasing atmospheric pressure
been assembled by the authors to create altitude tem-
with altitude (Elachi 1987). Oxygen molecular collisions
perature profiles and horizontal profiles of temperature.
are frequent enough that at given pressures they reach a
This profiling technique may be promising for detect-
local thermodynamic equilibrium. This results in a shift
ing temperature ahead of aircraft in an icing environ-
in the wavelength of emission around 60 GHz as pressure
ment. It deserves further exploration, but the instrument's
changes with altitude, with lines of maximum emission
capabilities within clouds are unknown.
shifting closer to 60 GHz with altitude. That is, the spec-
trum of wavelength from which oxygen emits radiation
near 60 Hz narrows as pressure decreases with altitude.
5.6.3 Radio acoustic sounding systems (RASS)
RASS is used operationally by NOAA; it operates
As a result, temperature can be retrieved with altitude by
using the expected wavelengths of emission of oxygen
by directing acoustic waves, typically at 900 Hz, verti-
at given altitudes. Measurements are typically made on
cally into the atmosphere (Schroeder 1990, Matuura et al.
either side of 60 GHz.
1986, May et al. 1989). Compression and rarefaction by
Microwave sounders routinely retrieve temperatures
the sound wave alters the air's dielectric constant, allow-
with height (Westwater and Grody 1980, Westwater et
ing radar reflection. A strong reflection is obtained when
al. 1983, Askne 1987, Gary 1989, Solheim and Godwin
the acoustic signal is matched to half of the radar wave-
1998). The primary problem with satellite and ground-
length, creating Bragg scattering (May et al. 1988).
based radiometers is their inability to detect rapid
NOAA uses 404-MHz Doppler radar to track the acous-
changes in temperature with altitude, such as inversions,
tic wave, the same radars that are used for wind profiling
in the lower few kilometers of the atmosphere (West-
(Westwater 1997).
water 1997). However, overall accuracy of ground-based
RASS is generally immune to cloud effects, but there
radiometers is typically better than 1.2C root mean
are other sources of error. Humidity changes the speed
of sound and, if not considered, can cause errors of up
to 2.2C, and vertical wind velocities of 3 m s1 can
* Personal communication, L. Bissonnette, Defence Research Estab-
produce a nearly 7C temperature error (North et al.
lishment, Valcartier, Quebec, Canada, 1997.
45
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