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.
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.
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
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
3. Pitch, roll, and yaw and aircraft speeds prevent
6. Lidar currently used for operational onboard wind
RASS use on aircraft.
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.
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-
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
TECHNOLOGY KNOWLEDGE WEAKNESSES
1. Ground (vertical scanning) and airborne (horizon-
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,
for example, increases the need for a multiple-
4. Millimeter-wavelength radars are suited to air-
borne cloud studies because of their small size,
niques require temperature for accurate liquid-
low ground-clutter susceptibility, high resolution,
4. Gossett and Sauvageot (1992) theoretically dem-
5. Radar scans rapidly.
onstrated that X and Ka bands are the best for
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
6. Dual-frequency radar at X and Ka bands cannot
detect small drop diameters to a long range, nor
3. Cloud phase may be possible with polarimetry.