Lidar
4. A scanning liquid-water radiometer under devel-
1. Scatter proportional to number density of drops.
opment provides range resolution.
2. High spatial resolution.
5. Passive technology has an advantage for cost,
3. Multiple-field-of-view lidars can indicate effec-
size, weight, power, general aviation, and mili-
tive drop diameter, but distribution must be assumed.
tary applications.
4. Liquid-water content can be measured.
6. Model with RADTRAN or its successors.
5. Can estimate relative amount of ice crystals from
7. Radiometers can be small and use little power.
amount of polarization.
Lidar
6. Current MFOV lidars not eye-safe, but could be
at 1.54 or 2.0 m with 1.5- to 3-m resolution.
1. Pulses can be only 2 to 3 m long, yielding very
7. Could be placed on an aircraft.
high spatial resolution.
8. Detect to clouds through clear air, but cloud extinc-
2. Multiple field-of-view lidars can indicate effec-
tion allows only few hundred meters penetration.
tive drop diameter and liquid water content.
3. Can retrieve relative amount of ice crystals from
Temperature measurements
amount of polarization.
1. RASS profiles temperature to radiosonde accura-
4. Multiple field-of-view lidars could be eye safe at
cy to over 3.5 km, even inversions.
1.54 or 2.0 mm with 1.5- to 3-m resolution.
2. RASS has not been used on moving vehicles or in
5. Five-watt power demand, 6- to 8-in. receiving lens,
100 pulses s1, 1-m3 volume, could be placed on
the horizontal.
an aircraft.
3. Pitch, roll, and yaw and aircraft speeds prevent
6. Lidar currently used for operational onboard wind
RASS use on aircraft.
shear alert.
4. RASS acoustic source could be aircraft engine
7. Small and inexpensive.
noise, but turbulence and relative wind could cause
8. Rapid scanning possible.
loss of signal for up to 1 min depending upon air-
craft speed and heading.
Temperature measurement
5. Radiometers may sense temperature in horizon-
1. RASS sounds temperature with radiosonde accu-
tal. Might try tunable system operating in the vicin-
racy, even through inversions.
ity of the oxygen absorption band, with tuning
2. Radiometers are used operationally to create ver-
providing range resolution.
tical temperature profiles.
6. Radiometers provide lower-resolution vertical
3. Radiometers may sense temperature in horizon-
temperature profiles.
tal. Might try tunable system operating in the
7. Lidar not applicable.
vicinity of the oxygen absorption band, with tun-
ing providing range resolution.
TECHNOLOGY KNOWLEDGE STRENGTHS
Radar
TECHNOLOGY KNOWLEDGE WEAKNESSES
1. Ground (vertical scanning) and airborne (horizon-
Radar
tal scanning) systems are possible.
1. Doppler techniques to detect droplet sizes not
2. Successful attempts have been made to acquire
possible with horizontally scanning systems.
range-resolved liquid water from clouds.
2. Wider ranges of drop sizes in precipitating clouds,
3. Polarization techniques can be used to detect ice
for example, increases the need for a multiple-
vs. water.
wavelength (more than two) radar system.
4. Millimeter-wavelength radars are suited to air-
3. Dual-wavelength differential attenuation tech-
borne cloud studies because of their small size,
niques require temperature for accurate liquid-
low ground-clutter susceptibility, high resolution,
water content.
and sensitivity to small hydrometeors.
4. Gossett and Sauvageot (1992) theoretically dem-
5. Radar scans rapidly.
onstrated that X and Ka bands are the best for
Passive radiometers
detecting water in clouds, but that other wave-
length pairs are possible. Modeling is needed.
1. Operationally used to measure zenith and nadir
5. It may not be possible to uniquely define cloud
temperature profiles, and possibility in horizontal.
drop-size distributions with radar.
2. Integrated liquid-water path sensed in vertical with
scanning possible.
detect small drop diameters to a long range, nor
3. Cloud phase may be possible with polarimetry.
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