melting layer in moderate intensity rainfall, from 1 to
spheric paths over a frequency range of 1 to 300 GHz
10 mm hr1, in Switzerland.
is performed with RADTRAN or similar radiative trans-
fer models* (Falcone et al. 1982). RADTRAN is a
5.4 Passive microwave radiometers
design tool to assess potential environmental impacts
Microwave radiometers operate by receiving thermal
on microwave sensors. In effect, it allows reasonably
energy emitted and scattered by the earth's atmospheric
complete modeling of the atmospheric radiation envi-
constituents (Grody 1997). They are passive instruments,
ronment due to gases and clouds, including polariza-
receiving natural radiation and actively emitting none
tion, prior to actually building a radiometer.
of their own. Radiative energy is emitted, scattered, and
5.4.1 Detecting liquid-water content
absorbed by the atmosphere, and radiometers act as ther-
Cloud liquid water and rainfall rates have been
mometers to measure a narrow spectral portion of this
observed with radiometers over oceans and over land
energy. Passive microwave radiometers are used to
from the ground, from the air, and from satellites.
measure many atmospheric and surface characteristics,
Observing from the ground up to clouds, or from aloft
but their ability to measure atmospheric temperature,
down to clouds over water, is least difficult because
cloud liquid-water content, and attributes of cloud and
clouds contrast well with the thermally cold background
precipitation constituents such as phase is needed for
of space or water surfaces. Sensing cloud water from
estimating icing hazards. Microwave measurement is
aloft toward the ground is more difficult, as indicated
based on the brightness temperature of atmospheric con-
above, because of the radiative diversity of land sur-
stituents. The radiation intensity observed by a radio-
faces. Airport-based radiometers scanning for icing
meter is a function of the temperature, reflectivity, trans-
conditions will scan upward toward the cold of space.
missivity, and emissivity of the emitter and attenuation
The situation may be more difficult for aircraft-mounted
by constituents between the emitter and the radiometer
radiometers because they will scan ahead, above, and
at the specific wavelength of interest.
below the aircraft flight path and, depending upon the
Each atmospheric constituent--gas, liquid, and solid-
aircraft's flight altitude and attitude, they may be look-
--has a unique absorption spectrum. The atmosphere
ing at the horizon, toward space, or toward the earth's
in general absorbs in several narrow wavelength bands
surface, which could be either land- or water-covered.
and allows radiation to be transmitted through several
Thus, sensing cloud water along flight paths may be
broad windows due to the atmosphere's gaseous com-
considerably more complex and difficult than most
position. The primary absorbers are oxygen and water
radiometer sensing to date. In general, cloud liquid water
vapor. Oxygen absorbs and re-emits in the 5060-GHz
can be sensed with microwave radiometers, but ice is
region and at 118 GHz and is used for temperature pro-
difficult to detect. Thus, cloud-water sensing is not com-
filing. Water vapor has peak absorption and re-emis-
plicated by the presence of ice, but the presence of ice
sion at 22, 37, and 183 GHz (Grody 1997). Liquid-wa-
cannot be used to determine whether cloud water is
ter peak absorption and emission occurs near 37 GHz
supercooled.
and 89 GHz.
5.4.1.1 Sensing upward from ground. Westwater
Passive microwave detection of cloud water and pre-
(1978) reviewed the theory and assessed the accuracy
cipitation is practical from the ground because the back-
of determining cloud liquid water from upward-look-
ground of space has a low brightness temperature, pro-
ing radiometers. His analysis yielded a two-wavelength
viding high radiative contrast with clouds. Satellites can
system, operating at 30 GHz (1.0-cm- wavelength) and
detect cloud water and rainfall rates over water bodies
21 GHz (1.4-cm wavelength) to detect liquid water and
because water also provides a low brightness tempera-
water vapor, respectively. Two wavelengths are needed
ture, but land masses are warmer and more radiatively
because, if a single-frequency radiometer is used, varia-
complex, making interpretation difficult. Thus, obser-
tions in water vapor cause apparent changes in cloud
vation direction is more critical for passive radiometry
liquid water (Hill 1991b), though Hill disputes the need
than for radar, because the latter creates the energy that
for water vapor measurements during the winter (Hill
it observes and so contrast is typically more adequate.
1991a). Westwater indicates that knowing cloud temper-
Radiometers are also capable of measuring polar-
ature would improve the accuracy of water vapor and
ized energy. Polarization describes brightness tempera-
cloud water estimates.
ture (radiance) along either a horizontal plane between
Hogg et al. (1983a,b) describe a dual-frequency
the emitter and the radiometer--vertical polarization,
or along a plane orthogonal to that path--horizontal
polarization. Cloud and precipitation drops are usually
* Statement made by T. Lines, Raytheon Corp., Denver, Colorado, at
vertically polarized.
Inflight Remote Sensing Icing Avoidance Workshop, Meteorologi-
cal Panel, 2 April 1997.
Analysis of microwave attenuation for typical atmo-
39
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