Theoretical comparisons with measurements are
over specularly reflecting surfaces for the 11000
good at 35 and 94 GHz, although Weinman em-
phasizes the need for access to more comprehen-
handling of complex precipitation events. The at-
sive data.
mospheric portion of the model is described by
In general, efforts to model realistically com-
Gasiewski and Staelin (1990); a wealth of details
plex meteorological events in a natural setting are
on the formulation, sensitivity, and exploratory
hampered by our inability to characterize the me-
runs and comparison with measurements is also
dium (lack of "air" truth, as it were). Despite
provided by Gasiewski (1993). In general, this RT
these difficulties we single out three sources of
model assumes Rayleigh or Mie scattering from
spheres. Given the expected sparse distribution
simulation that have seen some notable applica-
of scatterers, models simpler than the Mie formu-
tion: the work by Gasiewski at the Georgia Insti-
lation are typically used to determine the form of
tute of Technology, the RADTRAN program, and
the phase matrix, which is less influential than
the MPM program.
the other parameters. Both iterative and quadra-
The most comprehensive and authoritative
ture solutions are pursued, and both mono- and
data measurement and modeling basis for atmo-
polydispersed hydrometeor scattering and extinc-
spheric attenuation/delay is provided by the
tion are considered. Isotropic, Sobelev (two-term),
MPM model of Liebe and his colleagues (Liebe
Rayleigh, HenyeyGreenstein, and two-stream
1989, 1992; Liebe et al. 1989; Manabe et al. 1989).
phase function formulations are offered. Ground
The MPM model is the generally acknowledged
surface reflectivity is considered to be a minor
standard for predicting profiles of complex re-
influence in the applications considered and is
fractivity of the neutral atmosphere from 1 to
1000 GHz with contributions from dry air, water
typically assumed to be a convenient fixed value,
vapor, suspended water droplets, and rain. For
e.g., 5%. Downward-looking brightness tempera-
clear air, the local line base (44 O2 plus 30 H2O
tures were calculated for flight paths over a rela-
tively complex summertime convective cell cou-
lines) is complemented by an empirical water
plet, with concomitant data from weather radar
vapor continuum. On the whole, the basis for the
reflectivity used to determine hydrometeor size
relationships in the MPM program and its testing
distribution and phase profiles (Gasiewski 1993).
lend it very high credibility. Possible shortcom-
Agreement between observed and computed val-
ings reside in missing trace gas spectra and the
ues is impressive. It appears that the model would
empirical watervapor continuum. The physical
benefit from the inclusion of some polarimetric
origin of the latter is basically understood, but its
effects under certain precipitation conditions. This
empirical construction could lead to errors in pre-
may be accomplished to some degree by includ-
dictions for some atmospheric window ranges.
ing more asymmetric particle geometry and scat-
Even with these caveats, we note that the preci-
tering; more general polarimetric ground surface
sion this model offers surpasses that of any other
reflectivity is also planned.
model considered here. The most approximate
The computational vehicle designated as
component may be the treatment of rain effects.
RADTRAN has been developed from work origi-
No attempt is made to engage in elaborate calcu-
nally done at the Air Force Geophysical Labora-
lations for individual drops, which would require
tory to model atmospheric transmission and emis-
drop shape and size distributions, variable per-
sion in the 30300 GHz range (Falcone et al. 1979,
mittivity of the water, and so on; similarly, no
1982). The package is oriented toward passive
complicated RT-type computations are employed
measurements and contains six each of clear at-
with an assumed or empirical phase matrix.
mosphere, cloud, and rain models as well as eight
Rather, the refractivity of the rain is expressed
humidity models in an attempt to cover a world-
simply by a formula, with constituent terms and
wide variety of conditions. The original RT for-
factors developed from a least-squares fit to Mie
mulation has been upgraded more recently by
calculations based on a MarshallPalmer drop-
Isaacs and his co-workers at Atmospheric and
size distribution. Overall, this gratifyingly simple
Environmental Research, Inc. (Isaacs et al. 1988,
code is easily implemented and can feed into the
1989a,b,c). The enhancements allow RT simula-
more complex modeling vehicles discussed be-
tion of polarized multiple scattering for cases that
low.
involve various kinds of precipitation and sur-
Gasiewski has produced a program for calcu-
face emissivities. Jin and Isaacs (1987) show ex-
lating brightness temperature and associated sta-
ploratory computations of hypothetical cases with
tistics for planar stratified scattering atmospheres
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