effects prior to sensing clouds. A two-stage algorithm,
warmer, their emissivity is close to 1, surface rough-
developed by Jones and Vonder Haar (1990), estimates
ness and vegetation contribute to variability, and, most
ground transmittance on clear days, and then, after clouds
importantly, soil moisture varies widely and radiates
move over, removes the effects of ground transmittance.
strongly (Jones and Vonder Haar 1990). This larger and
Greenwald et al. (1997b) further advanced the technique
more variable emmitance over land requires different
by using the polarization differences of the brightness
retrieval techniques than over water. The above expla-
temperature effective over some land surfaces. The ad-
nation also suggests why, in principle, sensing cloud
vantage of polarization is that it improves the ability to
liquid water from the ground should be, and is, more
estimate the liquid-water content of low-lying clouds.
accurate, with values typically being correct within
Comparisons with ground-based radiometer measure-
15%, as demonstrated by Hill (1992).
ments in Colorado were generally good. This method
5.4.1.2.1 Liquid-water retrieval over water. Retrieval
would be difficult to apply to in-flight detection of cloud
over water is relatively direct, and techniques for retriev-
liquid water because a priori ground radiance informa-
ing cloud liquid water over the oceans have been in use
tion would be difficult to obtain.
for about 20 years with some of the original techniques
developed by Grody (1997). Schemes to retrieve liquid
5.4.1.2.3 Liquid-water retrieval from the horizontal.
water from microwave emissions are statistical, semi-
Savage et al. (1999) have developed a technique for
statistical, and semiphysical (Greenwald et al. 1993).
locating and estimating cloud liquid-water content using
Greenwald and his colleagues developed a simple physi-
two frequencies, 37 and 89 GHz, and three viewing
cal technique that is accurate and has been rigorously
angles. A radiometer placed on the nose of an aircraft
would scan horizontally ahead of the aircraft and 2
verified, unlike many other methods. Verification was
against ground-based microwave radiometers looking
above and below the flight path. In a clear-sky condi-
tion, the +2 beam sees colder temperatures, observing
skyward detecting cloud liquid water at four oceanic
toward cold space, than does the 2 beam observing
locations. Relative errors in the algorithm range from
20 to 40%, and occasionally to 50%, with the largest
toward the warmer surface of the earth. As the aircraft
errors in areas with low liquid-water content and thin
approaches a cloud, the temperature of both beams con-
clouds. The algorithm is also only valid for nonprecipi-
verges toward that of the horizontal beam. During this
tating clouds, because liquid precipitation radiates
process, the horizontal beam provides an estimate of
strongly. Frozen precipitation does not cause problems
the cloud temperature. An estimate of liquid-water con-
because ice is typically not visible at microwave wave-
tent magnitude is obtained by comparing the bright-
lengths.
ness temperatures of the 37- and 89-GHz beams in the
+2 orientation. Since the 37-GHz beam penetrates far-
Lee et al. (1994) applied the Defense Meteorological
Satellite special sensor microwave/imager (DMSP
ther than the 89-GHz beam, it will be colder than the
SSM/I) in an attempt to predict aircraft icing over ocean
89-GHz beam if there is little liquid water, because it
areas. They utilized a statistical retrieval algorithm to
can detect the cold of space through the water. As liquid-
water content increases, the +2 89- and 37-GHz bright-
convert transmittance to cloud water using the 37-GHz
and 85-GHz bands. Though heavy precipitation con-
ness temperatures converge as cold space is obscured.
taminates 37-GHz retrievals, according to Lee and Clar-
Savage et al. (1999) also believe that the presence of
ke supercooled water and significant precipitation are
drizzle-size drops can be detected by sensing polarized
mutually exclusive and thus reduce the magnitude of
radiation scattered from the earth's surface by large
the problem. Most analyses, however, were performed
drops. The Savage techniques are being evaluated using
with the 85-GHz band because it has higher resolution.
information gathered during the MWISP field project.
Aircraft icing was expected when liquid water was
These examples of microwave radiometer capabili-
greater than 0.2 kg m2 and temperatures were between
ties over land and water surfaces represent only a small
0C and 20C.
portion of all work accomplished, but cloud water
5.4.1.2.2 Liquid water retrieval over land. Microwave
retrieval over land is new and is not yet available opera-
retrievals over land are possible if emittance from the
tionally.* Precipitation retrieval from satellites over
land surface can be accounted for. In addition, the
water and land has been developed even more than
retrieval wavelength is changed over land to 85.5 GHz,
cloud water retrieval; it is presented in papers by Spen-
which is more sensitive to cloud liquid water than are
cer et al. (1989), Petty and Katsaros (1992), Vivekanan-
other microwave channels, and surface effects become
dan et al. (1993), and Ferraro and Marks (1996).
less important as cloud liquid water increases and atmo-
spheric attenuation obscures the ground at this frequency.
Jones and Vonder Haar (1990) and Greenwald et al.
* Personal communication, J. Vivekanandan, National Center for
(1997a) developed methods of subtracting surface
Atmospheric Research, Boulder, Colorado, 1997.
42
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