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1286 JOURNAL OF APPLIED METEOROLOGY VOLUME 21 The Use of Microwave Radiometry to Determine a Cloud Seeding Opportunity J. B. SNIDER NOAA/ERL/Wave Propagation Laboratory, Boulder, CO 80303 D. ROTTNER Office of Atmospheric Resources Research, Bureau of Reclamation, Denver, CO 80225 (Manuscript received 31 December 1981, in final form 14 April 1982) ABSTRACT A ground-based combination microwave radiometer-satellite receiver operating at 28.5 GHz (wavelength = 1.05 cm) was employed to measure liquid water in clouds during the 1979-80 field season of the Sierra Cooperative Pilot Project (SCPP) in northern California. We report upon the ability of the instrument to detect small amounts (-0.1 g m -3) ofliquid water in non-precipitating clouds which may not be observed by other remote sensing systems. A successful cloud seeding experiment, performed after cloud liquid was detected by the instrument, is described. 1. Introduction During the winter of 1979-80, the Wave Propa- gation Laboratory of NoAA operated a ground-based microwave radiometric instrument in the Sierra Co- operative Pilot Project (SCPP), a weather modifica- tion experiment conducted by the Bureau of Recla- mation in the northern Sierra Nevada of California. One objective of the radiometric observations was to study the usefulness of the technique as an aid to cloud seeding operations. Because the microwave ra- diometer is sensitive to emission from liquid water at any temperature, but not from ice, the system is able to detect the presence of supercooled liquid water that may respond favorably to seeding, i.e., result in conversion of liquid to ice. In this paper we report on one occasion during the 1979-80 field season when datafrom the radiometer were used, in part, to initiate a successful seeding experiment. 2. Radiometric measurement of liquid water in the SCpp Radiometric measurements of the amount of a substance are based upon the absorption of electro- magnetic energy by the substance. Although the ra- diometer actually measures the energy emitted by a substance, the absorption principle is employed, since emission is created by virtue of the absorption. If the absorption coefficient for the substance is known, the quantity of the substance along the path being ob- served by the radiometer can be readily calculated. The absorption of microwaves by liquid water has received considerable study over the past several years. Accordingly, the theory of absorption and scat- tering of microwaves by liquid water (and water va- por) is well understood and has been verified exper- imentally. An extensive review of the general subject as well as a comprehensive list of references is given by Hogg and Chu (1975). At wavelengths greater than -0.3 cm, the total attenuation caused by liquid composed of droplets with diameters < 100 JLm is due primarily to absorp- tion. This statement may not be true where droplet diameters are> 100 JLm and scattering of energy may be important. The magnitude of the absorption de- pends upon the liquid content and the temperature of the liquid. For example, for liquid temperatures between 265 and 293 K, the absorption decreases by a factor of 2 at a wavelength of I cm (Gunn and East, 1954). The variation with temperature is approxi- mately quadratic. At these wavelengths, the total ab- sorption is proportional to the volume of liquid pres- ent and independent of the drop-size distribution in the liquid. Therefore, the microwave radiometer has an advantage over those sensors whose accuracy is affected by drop-size distribution. Since absorption by ice is two orders of magnitude smaller than that by liquid, ice is nearly transparent to the transmission of microwaves. As a result, the microwave radiometer can detect liquid water at any temperature, but does not respond to ice. The radiometric instrument used in the 1979-80 SCPP is a combination receiver-radiometer that has been previously described by Snider et al. (1980a). The instrument was developed to verify measure- ments of cloud liquid by a passive two-frequency sys- tem which also measures precipitable water vapor (Guiraud et a/., 1979). However, because of its ability