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Abstract
Surface-based radiometric sensing of tropospheric parame-
ters has a long history of providing useful measurements of
temperature, water vapor, and cloud liquid. In this tutorial,
a general overview of physical fundamentals, measurement
techniques, and retrieval methodology is given. Then sever-
al contemporary instruments are discussed and representa-
tive results are presented. Recent and promising develop-
ments include multi-frequency radiometers, scanning
observations of clouds, and combined active-passive
remote sensing. The primary applications of these new
technologies are weather forecasting and climate, commu-
nications, geodesy and long-baseline interferometry, satel-
lite data validation, air-sea interaction, and fundamental
molecular physics.
Introduction
A more extensive review is given in [11] and some of
the material in this tutorial has been extracted from this
document.
16
IEEE Geoscience and Remote Sensing Society Newsletter March 2005
EDUCATIONAL TUTORIAL
Surface-based Microwave and Millimeter wave
Radiometric Remote Sensing of the Troposphere:
a Tutorial
Ed R. Westwater
Cooperative Institute for Research in Environmental Sciences, University of Colorado/NOAA
Environmental Technology Laboratory
325 Broadway MS R/E/ET1, Boulder, CO 80305 USA
Tel: 303-497-6527
FAX: 303-497-3577
email: Ed.R.Westwater@noaa.gov
http://www.etl.noaa.gov/~ewestwater
Susanne Crewell
Meteorologisches Institut, Universitaet Muenchen
Theresienstr. 37 80333 Muenchen,Germany
Tel: +49 (0) 89 / 2180-4210
FAX: +49 (0) 89 / 2805508
email: CREWELL@meteo.physik.uni-muenchen.de
http://www.meteo.physik.uni-muenchen.de/
Christian Mätzler
Institute of Applied Physics, University of Bern
Sidlerstr. 5, CH-3012 Bern, Switzerland
Tel:: +41 31 631 45 89
FAX: +41 31 631 37 65
email: matzler@iap.unibe.ch, http://www.iapmw.unibe.ch 2. General Physical Principles
The basic ideas of radiative transfer and thermal emission
are given in [12] and their application to microwave radio-
metric remote sensing is outlined in [13] . From the con-
cept of an ideal black body and Kirchoffs law, it is known
that the emission from a black body depends only on its
temperature and that the higher the temperature of the
body, the more is its emission. The idea is made quantita-
tive by calculating the spectral distribution of a blackbody
emission from Plancks law, which expresses the radiance
B
V
(T) emitted from a blackbody at temperature T and fre-
quency v as
B (T) = 2h
3
c
2
1
(exp(h/kT) 1) ,
(1)
where h = Plancks constant, and k = Boltzmans constant.
The radiance expresses the emitted power per unit project-
ed area per unit solid angle per unit frequency interval.
The second consideration is to relate the emission from a
real body, sometimes called a grey body, to that of a
blackbody at the same temperature. If the fraction of inci-
dent energy from a certain direction absorbed by the grey
body is A(v), then the amount emitted is A(v) B
V
(T). For a
perfectly reflecting or transmitting body, A(v) is zero, and
incident energy may be redirected or pass through the body
without being absorbed. In the situation considered in this
tutorial, namely upward-looking radiometers viewing a
non-scattering medium, the equation that relates our pri-
mary observable, brightness temperature, T
b
, to the atmos-
pheric state is the radiative transfer equation (RTE) [13]
B (T
b
) = B (T
c
) exp( )+
+ 0
B (T(s)) (s) exp(
s
0
(s )d s )d s,
(2a)
where s = path length in km, T(s) = Temperature (K) at the
point s, T
c
= Cosmic background brightness temperature of
2.75 K, T
v
= opacity = total optical depth along the path s
= 0
(s)ds,
(2b)
where a
v
(s) = absorption coefficient (nepers/km) at the
point s. The use of the blackbody source function in (2a) is
justified by the assumption of local thermodynamic equi-
librium in which the population of emitting energy states is
determined by molecular collisions and is independent of
the incident radiation field [12]
Equation (2) and its Rayleigh-Jeans approximation are
discussed in [13], and its more general form including
scattering is discussed in [14]. Scattering, although
neglected here, may arise from liquid, ice, or melting liq-
uid depending on the size distribution of the particles. For
our purposes, we note the dependence on the temperature
profile T(s) and the implicit dependence on pressure,
water vapor, and cloud liquid through
(s). For a plane
parallel atmosphere, the path length s and the height h are
related by s sin(
) = h, where is the elevation angle.
Information on meteorological variables is obtained from
measurements of T
b
as a function of v and/or
. Equation
(2) is used: (a) in forward model studies in which the rel-
evant meteorological variables are measured by
radiosonde in situ soundings, (b) in inverse problems and
parameter retrieval applications, in which meteorological
information is inferred from measurements of T
b
, and (c)
in system modeling studies for determining the effects of
instrument noise on retrievals and optimum measurement
ordinates, such as v and
. Calculations of T
b
for a warm
(surface temperature T
s
=293 K) atmosphere are shown in
Figure 1. We note the transmission windows near 30-50,
70-100, and 130-150 GHz. Radiometer measurements
IEEE Geoscience and Remote Sensing Society Newsletter March 2005
17
Figure 1. Calculated brightness temperatures (K) from 20 to 220
GHz for clear and cloudy conditions. The clear calculations are
based on a standard atmosphere with the surface values (S) of
P
S
=1013 mb, T
S
=293 K, S
= 10 gm
3
, and IWV = 2.34 cm. The
cloudy atmosphere contains 1 mm of integrated cloud liquid with a
cloud layer of liquid density of 0.1 gm
3
between 1 and 2 km. The
absorption models used are given in Figure 2. near these windows are used primarily for remote sensing
of clouds and water vapor. The strong absorption features
near 60 and 118 GHz are used for temperature sensing.
Finally, the strong absorption region near 183 GHz can be
used to study very low amounts of water vapor such as are
found during Arctic winter conditions.
3. Microwave Absorption and Emission
The principal sources of atmospheric emission and
absorption are water vapor, oxygen, and cloud liquid. In
the frequency region from 20 to 200 GHz, water-vapor
absorption arises from the weak electric dipole rotational
transition at 22.235 GHz and the much stronger transition
at 183.31 GHz. In addition, the so-called continuum
absorption of water vapor arises from the far wing contri-
butions from higher-frequency resonances that extend
into the infrared region. Again, in the frequency band
from 20 to 200 GHz, oxygen absorbs due to a series of
magnetic dipole transitions centered around 60 GHz and
the isolated line at 118.75 GHz. Because of pressure
broadening, i. e., the effect of molecular collisions on
radiative transitions, both water vapor and oxygen
absorption extend outside of the immediate frequency
region of their resonant lines. There are also resonances
by ozone that are important for stratospheric sounding
[15]. In addition to gaseous absorption, scattering,
absorption, and emission also originate from hydromete-
ors in the atmosphere. Our focus in this article is on non-
precipitating clouds for which emission and absorption
are of primary importance.
3.1 Gaseous Absorption Models
Detailed calculations of absorption by water vapor and
oxygen were first published by J. H. Van Vleck [16, 17].
The quantum mechanical basis of these calculations,
including the Van Vleck-Weisskopf line shape [18],
together with laboratory measurements, has led to increas-
ingly accurate calculations of gaseous absorption. Both
these historical- and recent- developments are discussed
in [19]. Currently, there are several absorption models that
are widely used in the propagation and remote-sensing
communities. Starting with laboratory measurements that
were made in the late 1960s and continuing for several
years, H. Liebe developed and distributed the computer
code of his Microwave Propagation Model (MPM). One
version of the model [20] is still used extensively, and
many subsequent models are compared with this one.
Liebe later made changes to both water-vapor and oxygen
models, especially to parameters describing the 22.235
GHz H2O line and the so-called water vapor continuum
[21]. More recently, Rosenkranz [5a, 5b] developed an
improved absorption model that also is extensively used in