liquid-water content and drop-size distribution. How-
when clouds are composed primarily of ice crystals.
The 1.06-m-wavelength is not eye-safe, but an eye-
ever, because Raman systems are large and expensive
safe 1.54- or 2.0-m lidar could be built with 1.5- to
and can only be used at night, their application to air-
craft icing is unlikely.
3.0-m resolution and a 100-Hz pulse repetition rate.
In general, lidars have limited use for remotely
Temperature at the cloud could also be determined by
ranging with the lidar and sensing temperature with an
detecting aircraft icing conditions because of their lim-
IR radiometer. Evidence of supercooling could also be
ited ability to penetrate clouds. Many ground-based and
assessed using polarization techniques to determine if
airborne lidars are currently used for boundary-layer
research and wind-shear monitoring (Targ et al. 1991,
the cloud is mixed phase.
Benayahu et al. (1995) developed a method for
Hannon and Henderson 1995). Canadian organizations
retrieving drop number and drop-size distribution from
interested in aircraft icing have proposed a remote-
sensing system for detecting icing conditions that may
clouds, and potentially liquid-water content, assuming
use lidar as a principal component (EWA 1996).
multiple scattering occurs at all times. A total scatter
signal and multiple scattering signal, simultaneously
Therefore, though not as promising for penetration of
received from two separated receivers, contained infor-
clouds, lidar does have demonstrated capability and may
mation about the shape of the drop-size distribution and
be a candidate technology for limited and specialized
applications.
the mean droplet radius. A field test was conducted on
a marine stratus cloud off the coast of Israel with concur-
5.6 Temperature measurement
rent lidar and in-situ aircraft measurements, with good
Drop temperature, or a surrogate for drop tempera-
agreement between the measurements.
Eberhard (1995), working with a CO2 laser, argues
ture, is necessary to determine if liquid cloud water is
that depolarization in longer wavelengths cannot be
supercooled. In most cases, drop temperatures will be
used to discriminate ice from water in clouds. However,
nearly the same as the air temperature, neglecting radi-
the ratio of backscatter between two different wave-
ative exchanges, but droplets can be considerably dif-
lengths can indicate the presence of ice. Seven wave-
ferent from air temperature in several situations. Snow
lengths between 10.4 and 11.5 m were selected and
falling into warm air, such as in overrunning, will melt.
paired to test the theory, and the backscatter ratio
The temperature of snow and ice crystals will rise to
0C until melting is complete, and then they will con-
between water and ice was shown to range from 2 to 5
depending upon the frequency pairs chosen. The method
tinue to warm. These drops may then fall into colder
demonstrated the feasibility of detecting ice vs. water
air below and remain warmer than the air until conduc-
without polarization and estimated an approximate pro-
tion, convection, radiative exchange, and evaporation
portion of ice vs. water. No field tests were conducted
cools them to the dew-point temperature.
to verify the method, though they were planned. The
There likely will be little choice whether to measure
technique is simple and eye safe.
drop temperature or air temperature, because some sens-
When a laser beam is incident upon water droplets,
ing technologies may preferentially sense water drops
most of the energy is scattered away from the drop with-
or ice crystals and others may preferentially sense air
out change but scatters a small portion of the light at
temperature. A surrogate for air-temperature measure-
different wavelengths. The scattering of light at differ-
ments is the detection of ice crystals. If a cloud is mixed
ent wavelengths is Raman scattering, actually an
phase, then it is likely that any liquid water is super-
exchange of energy between a photon and a molecule
cooled. Ice crystals may be detected by radar and lidar.
with a resulting change in the energy--and thus wave-
Ice detection methods were discussed above, so they
length--of the photon (Carey 1987). Raman scattering
will not be dealt with here.
has become a useful technique for atmospheric sensing
There are at least four types of sensors for measur-
of nitrogen, oxygen, and water vapor and for tempera-
ing temperature profiles:
ture profiling. Melfi et al. (1997) report on a potential
Microwave radiometers
use of Raman scattering for measuring the liquid-water
Infrared radiometers
content of clouds. During water-vapor measurements
Radio acoustic sounding (RASS)
with a XeF laser centered at 0.35 m, they detected
Raman lidar.
two thin cloud layers as they passed over the lidar. Melfi
et al. (1997) indicate that Raman lidar techniques hold
Radiative techniques are infrared and millimeter
promise as a new method for remotely measuring cloud
wave, acoustic systems are based on tracking the speed
of sound by radar, and lidars detect Raman scattering.
The purpose of this review is to indicate how tempera-
* Personal communication, L. Bissonnette, Defence Research Estab-
ture is measured remotely in the atmosphere and to indi-
lishment, Valcartier, Quebec, Canada, 1997.
44
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