WMOD00299 ‎(2)‎

• Title: Scientific Weather Modification Experimentation in the United States
• Date: 8/25/1984
• Weather Modification - Doc Type: Report

concentrated on observing winter storm behavior and making measurements of
supercooled liquid water. A team of scientists and technicians have established
a year-round facility in the project area that consists of a project office and
a ground based observi ng faci 1 i ty known as the Grand Mesa Observatory. The
observatory has the capabil ity to measure state parameters at the surface and a
height of about 20 meters. Other equipment at the observatory includes a 5-cm
radar, a dual wavelength microwave radiometer to measure liquid water, precipi-
tation gages, and Particle Measuring System's 2-dimensional cloud particle
probe. Measurements above the surface of the Mesa are made using an instru-
mented cloud physics aircraft. Initial investigations (Holroyd and Super, 1984)
have confirmed the presence of supercooled liquid water in sufficient quantities
to i ndi cate that cloud seedi ng coul d potentially be successful. Terrai n, rather
than ice particle presence, appears to be the dominant influence in determining
the distribution of liquid water over the slopes and top of Grand Mesa (see
Figure 5). The water appears to be concentrated in the lowest levels and is
present in sufficient significant amounts only for limited periods of time (mean
= 3 h, upper standard deviation = 6.5 h). The presence of the highest liquid
water in the lowest levels may make ground seeding most practical.

6. Related research

a. Microwave radiometer

In the description of the CRADP and SCPP programs, an instrument was described
which appears to have a high potential for detecting clouds which are seedable.
The instrument is called a dual wavelength radiometer and remotely senses the
amount of water vapor and supercooled liquid water that is present in nearby
clouds. It does this by passively measuring incoming microwave radiation at
wavelengths appropriate to water vapor (20.6 GHz) and to liquid water (31.6
GHz). The brightness of the radiation is a measure of the integrated water in
that phase along the line of sight. The computer-controlled antenna which
receives the signal can be operated in a stationary or a scanning mode, and the
data are recorded on disk and/or on tape.

A cross-section of the physical layout of the system is shown in Figure 6. The ·
electronics for both the 20.6 and 31.6 GHz radiometers are located in a single
package. The minicomputer is also located inside the trailer. Equal beamwidths
(2.50) are produced at both frequencies. Mobility of the instrument is achieved
by using an enclosed trailer as both a small laboratory and an antenna platform.
A more detailed description of the radiometer unit is found in Hogg et al.
(1983).

Several analyses have been performed to determine the veracity of the radiometer
data. Westwater (1978) found the accuracy of vapor values to be better than 15
percent for a wide range of cloudy conditions. Guiraud et al.(1979) showed that
the accuracy in measurement of precipitable water vapor is about the same or
somewhat better than that of operational radiosondes. He also found that a con-
siderable amount of vapor often passes overhead unobserved by sondes. The
liquid measurements have been compared against cloud physics aircraft data
(Holroyd, 1984) and with a more accurate combination receiver-radiometer that
utilizes absorption of a signal from the COMSTAR satellite as well as the
absorption of microwave energy by liquid water (Snider et al., 1980),.

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