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project area that consists of a pr9ject office and a ground based observing facility known as the Grand Mesa Observatory. The observatory has the capability to measure state parameters at the surface and a height of ~bout 20 meters. Other equipmenc at the observatory includes a 5-cm radar, a dual wave 1 ength mi crowave radi ometer tv measll re liquid water, precip1tation 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 ~uantities to indicate that cloud seeding could potentially be suc- cessful. Terrain. 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 7). The water appears to be concentrated in the lowest levels and is pre- sent in sufficient significent amounts only for limited periods of time (mean = 3 h. upper standard deviation =6.5 h). The pre- sence 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 seedab1e. 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 6Hz). 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 3. The electronics for both the 20.6 and 31.6 GHz radiometers are located in a single package. The mini- computer is also located inside the trailer. Equal beamwidths (2.50) are produced at both fr~quencies. Mobility of the instrument is ~chieved 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 ale '(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 a1.(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 considerable ,amount of vapor often passes overhead unobserved by sondes. The liquid measurements have been compared . _ __.q(Q.1St~, ijO 70 :, J() c:.. Ji 60 :, 30 > .... '"' E ::l u o km -~; T""~ I ,---~ 30 70 100 :r. ~ rr. :L <'l C. '~ :: 50 ..., c (lJ i:: OJ c. Key both Figs. rua..__ 0,2.4 gm-3 3.7 km P"SSE'S o ;.. :G: -50 J~ - 30 0 km te r.r"in 30 SO FIGURE 7 -- DISTRIBUTION OF LIQUID WATER OVER GRAND MESA; TOP: ALONG AXIS. BOTTOM: CROSS-AXIS (AFTER HOLYROYD AND SUPER. 1984) Double Mylar, Cowling t Window '\ -- : ~~ Elevation " ./ ,....1....--, I ,... AZlO'uth "15-:'" 7:' I 11'" - Flat Flal . /', 11-,r-r.- 4:1 and and -:' : ~ ~~:,.Ii' ;:-"-~:-- - Bearing 6ea~lng,,~ _~ ( ! __ ,]*~~ _., :; -- - ----10~:-c--.-~~--:- - ~~~;o3~e~e~8HZ t ~ ,--- -~- r----j ; j II - -1 bo~:;~,., t~' I?J_'j~ L~:JJL __" }l::tt~~ld _8 ---:.;::-~-'- - -- ------- -- f / -,-~,--'\ ~~~J~) , Side I L FIGURE 8 -- SIDE VIEW OF ~ STEERABLE DUAL-CHANNEL RAD IO'IETER SHOWl NG ~IECHANICAL LAYOUT; A CENTRAL RAY FROM ZENITH IS SHOWN AS DASHED LINES WITH ARROWS (AFTER HOGG ET AL. 1983) ..J. , ... l,'. .,-~, ~ a :3 . 't f~~n?"t~w ~'."'fi~J,