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SEPTEMBER 1982 J. B. SNIDER AND D, ROTTNER 1287 to make continuous observations of liquid water in clouds, it was employed in the 1979-80 SCPP. Briefly, the instrument simultaneously but indepen- dently measures the absorption of a 28.5 GHz (wave- length = 1.05 cm) microwave signal radiated by a COMST AR communications satellite and the asso- ciated microwave energy (brightness temperature) emitted by liquid-bearing clouds that pass through the propagation path. Independent, simultaneous measurement of the two quantities is accomplished using a unique design that allows coherent and in- coherent reception with a single instrument. Opera- tion in the two modes is made possible by means of a switched-bandwidth receiver. When measuring the signal from the satellite, a 200 kHz bandwidth is em- ployed to obtain a high signal-to-noise ratio. How- ever, when measuring the: incoherent, noise-like emis- sion from the cloud, a 500 MHz bandwidth is used so that the coherent signal from the satellite contrib- utes a negligible amount t6 the total noise power re- ceived by the radiometer. As a result, the two mea- surements are independent. The receiving system switches between the twOl bandwidths at a 40 Hz rate so that the absorption and emission may be consid- ered to be sampled simultaneously with respect to a relatively slow-moving cloud in the propagation path. The 40 Hz samples are averaged over 1 s intervals before conversion to liquid values. The two measurements are combined first to es- timate an effective temperature of the liquid water in the cloud, and second, to determine the absorption coefficient which is a function of the liquid temper- ature. Once the absorption coefficient has been de- termined, the total amOlunt of liquid is calculated from the measured absorption. The effective temperature Tc of the liquid in the cloud is calculated from T = Tb c 1 - exp(')' c/4.343) , where Tb is the increase in emission, expressed as brightness temperature, occurring when clouds pass through the antenna beam, and 'Y c is the measured absorption (in decibels) caused by the liquid-bearing clouds. The effective cloud temperature is used to compute the absorption coefficient a(T) in dB km-1 g m-3, using a quadratic fit to the absorption-tem- perature relationship reported by Gunn and East (1954): a(T) = 43.164 - 0.287Tc + 0.000482T/. (2) Finally, L, the total amount of liquid per unit cross- section (g m-2) along the propagation path, is cal- culated from L = i'c/a(T). Since the radiometer does not provide information on the dimensions of tht: cloud, it is convenient to I... (1) normalize L to 1 km path length and to unit volume. The quantity L then represents the total liquid (mm) contained in the entire propagation path through the cloud, regardless of its length. If the total path length through the cloud is known from data obtained with other sensors, an equivalent liquid water content (LWC) in g m-3 can be obtained by dividing the total liquid by the path length. For example, a total liquid of 1 rom extending over a 1 km path length is equiv- alent to a LWC of 1 g m-3. The threshold of liquid detection for the system is -0.1 mm. Because the radiometric method is based upon sound, well-understood physical principles, the tech- nique is believed to produce accurate measurements ofliquid water in non-precipitating clouds containing water droplets smaller than 100 Jlm. Indeed, a sys- tematic comparison of cloud liquid measurements made by two independent ground-based systems showed consistent liquid values (within 0.28 mm rms) for clouds observed simultaneously by the two sys- tems (Snider et al., 1980b). An example of the data output from the receiver- radiometer is shown in the lower curve of Fig. 1. Rainfall recorded at the Sheridan site using a weigh- ing bucket raingage is plotted as vertical bars in the upper curve; each bar represents the amount of rain that has fallen in the previous 15 min. The rainfall data are used as flags to indicate when radiometric data may be suspect. Because raindrops are often larger than 100 Jlm, Eq. (1) may not be valid during rainfall. In addition, the larger attenuation in rain can obscure absorption by water droplets in clouds. 3. Ailrborne instrumentation in the SCPP Liquid water content and cloud microphysical data were also measured by means of probes mounted on a Beechcraft King Air 200 cloud physics aircraft (N2UW) operated by the University of Wyoming. Instruments carried on the aircraft for measurement of L WC include a forward scattering spectrometer probe (FSSP) and a Johnson-Williams (JW) hot-wire probe. A series of imaging probes are available for sensing ice particle size and concentration. A de- celerator and slide sampler are used to capture ice particles on oil-coated glass slides for later analysis. Other instruments measure the velocity and position of tht: aircraft, atmospheric turbulence, horizontal and vertical winds, state parameters and ice nuclei. A detailed description of the aircraft's instrumenta- tion is given by Marwitz et al. (1978). A second aircraft was employed in the 1979-80 SCPP to dispense seeding materials and to observe cloud characteristics. The second or "seeder" aircraft was an Aero Commander Turboprop operated by Aero Systems, Inc. of Boulder, CO; the aircraft had the capability of dropping silver iodide (AgI) flares' and dry ice (C02) pellets. A fuselage-mounted acetone AgI gt:nerator was also available. (3)