to icing, they may be penetrated, but the hazard is that
5.4.2 Drop-size spectra and cloud phase
The only research found that described detection of
clouds beyond cannot be sensed if there is no cloud-
cloud drop size with microwave radiometers was that
free space ahead of them, so aircraft flying IFR cannot
of Savage et al. (1999), described above. Generally,
use lidar to maximum advantage. Lidar could indicate
cloud ice particles cannot be detected by microwave
to night VFR aircraft whether cloud lies ahead, and it
radiometers because ice is transparent to microwaves.
could indicate whether liquid precipitation that could
However, Wu (1987) applied four channels of NASA's
be freezing also lies ahead below clouds, giving the
Advanced Microwave Moisture Sounder, flown on
aircraft advance warning. Thus, lidar appears to have
high-altitude aircraft, to the problem of detecting the
the greatest utility to night VFR pilots who have multi-
ice-water content of clouds in the microwave frequen-
ple reasons to avoid clouds and may need to avoid pre-
cies of 92-, 183- (2), 183- (5), and 183- (9) GHz
cipitation. Lidar is of greatest potential utility for avoid-
bands. A microwave radiativex transfer routine was
ing icing and of least potential utility for escaping icing.
developed that allows detection of the ice-water con-
Lidar can operate over a range of wavelengths, it
centration through mixed-phase clouds by observing
can be polarized, and it can be used in single-scattering
the changes in brightness temperature of each frequency.
and multiple-scattering modes where multiple-field-of-
Some success was claimed by comparing computed ice-
view lidars can utilize the information. Its most typical
water contents with observations made in the near-
cloud uses are for determining liquid-water content,
infrared during the Cooperative Convection Precipita-
phase, drop number, mean drop size, and optical thick-
tion Experiment. The study also showed that cloud
ness. Lidar is also widely used to determine ceiling
brightness temperature at each frequency depended not
height, though there is often difficulty with optically
only on total ice-water content of a cloud, but also on
thin clouds, virga, and precipitation.
its distribution within the cloud. Further developments
at 10.6 m, Eberhard (1993) developed theory and
of Wu's technique have not been published in the last
demonstrated retrieval of the mean radius of cloud drop-
decade, so it is not clear whether the technique is fully
size distributions. The lidar determines the extinction-
viable.
to-backscatter ratio, which is then fitted to a variety of
5.5 Lidar
expected drop-size distributions until a fit is obtained.
Lidar, or light detection and ranging, is the optical
The method is valid for distributions with drop sizes
falling between 1 and 17 m. Data from fair-weather
equivalent of radar, operating in the visible and infra-
red wavelengths. Unlike radar, however, wavelengths
cumulus clouds at Cape Kennedy provided reasonable
used for lidar suffer rapid extinction in optically thick
results, although no in-situ measurements were avail-
able for validation. Eberhard indicated that an 11-m
clouds, so their use for sensing cloud properties is lim-
ited. Cloud scattering rapidly attenuates the signal, pre-
wavelength may provide better results.
venting most lidars from penetrating dense clouds for
Bissonnette and Hutt (Hutt et al. 1994; Bissonnette
more than a few hundred meters. However, multiple
and Hutt 1995a,b) at the Defence Research Establish-
scattering of lidar returns from clouds can be used to
ment at Valcartier, Quebec, Canada, have used the back-
scattered power from a 1.06-m multiple-field-of-view
advantage for interpreting elements of cloud composi-
tion.
(MFOV) polarized lidar to characterize cloud, fog, and
aerosols. The system measures the backscatter from a
Overall, lidar may be able to contribute to aircraft
central beam with 1.5-m-long pulses and multiscattered
icing avoidance by remotely sensing cloud conditions,
return signal intensity at three or more coaxial fields of
but only in very specific and limited ways because of
view with a maximum of ten possible fields of view.
the extinction problem. An ideal supercooled liquid-
The amount of scatter returned is proportional to the
water sensor will range-resolve liquid water and drop
number density of drops in the cloud. Fitted to a multiple
size many kilometers ahead of an aircraft, even if the
scattering lidar equation, the measurements provide a
aircraft is flying within clouds. A lidar operates effec-
scattering coefficient and a droplet effective radius.
tively only when the aircraft is flying within a nearly
From this, and an assumed gamma drop-size distribu-
cloud-free atmosphere. Lidar can sense through cloud-
tion, liquid-water content and extinction coefficients
free atmosphere to the nearest clouds and determine
are computed. Range-resolved droplet size distribution
the properties of the first few hundred meters of those
(1 to ~100 m), liquid-water content to 1.0 g m3, and
clouds, but it cannot penetrate them to determine what
extinction coefficient measurements have been made
lies beyond. Therefore, lidar can only help aircraft avoid
at ranges to 1000 m and verified in situ (Bissonnette et
icing conditions by determining whether there are
al. 1998). The linear polarization ratio, between paral-
clouds in the immediate flight path and if they are con-
lel and perpendicular polarization, is about 35 to 40%
ducive to airframe icing. If the clouds are not conducive
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