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204 JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY VOLUME 4 Field Evaluation of a Dual-Channel Microwave Radiometer Designed for Measurements of Integrated Water Vapor and Cloud Liquid Water in the Atmosphere MARK HEGGLI Bureau of Reclamation. Sierra Cooperative Pilot Project. Auburn, CA 95603 ROBERT M. RAUBER* Colorado State University, Department of Atmospheric Science. Fort Collins, CO 80523 J. B. SNIDER NOAAjERLjWave Propagation Laboratory. Boulder, CO (Manuscript received 31 January 1986, in final form 17 September 1986) ABSTRACT The dual-channel microwave radiometer is evaluated in regard to the measurement of integrated water vapor and supercooled liquid water. The study includes comparisons of integrated vapor content measured by the radiometer and rawinsondes as part of the Sierra Cooperative Pilot Project in the central Sierra Nevada. In addition, two radiometers with virtually identical characteristics were brought together on the Colorado Orographic Seeding Experiment at Steamboat Springs, Colorado, in order to study the stability and comparability of integrated vapor and liquid measurements. Comparison of vapor measurements by the radiometer and by rawinsonde yielded a correlation coefficient of 0.94, and a rms difference of 0.08 em. There were no significant differences between results for paired data gathered in storms in the presence of supercooled liquid water and for paired data gathered on clear days under more ideal conditions. The collocated-radiometer experiment showed slightly closer agreement than did the comparisons of radiometer data with rawinsondes. Comparisons of paired data from the vapor channel yielded a correlation coefficient of 0.95 with a rms difference of 0.05 em, while the liquid water channel data yielded a correlation coefficient of 0.99 with a rms difference of 0.02 mm. The study lends further credence to other theoretical estimations of the accuracy of the radiometer measurement, in that the measured values of integrated vapor are probably within 15% of truth. Measurements ofSupercooled liquid water are reproducible and very stable. 1. Introduction The dual-channel microwave radiometer (Hogg et aI., 1983) has become an important study and evalu- ation tool for weather modificaton research, particu- larly for wintertime storms that occur over the western United States (e.g., Holroyd and Super, 1984; Long and Walsh, 1984; Heggli, 1985; Sassen et aI., 1986; Rauber et aI., 1986). The radiometer provides contin- uous measurements of both liquid water and vapor integrated through the depth of the atmosphere. At present, there are only two dual-channel radiometers with full scanning capability in the United States: one operated by the Bureau of Reclamation (USBR) and the other by the National Oceanic and Atmospheric Administration (NOAA) Wave Propagation Labora- . tory. A third unit is currently under. construction at the University of Nevada, Desert Research Institute. * Present affiliation: Electronic Techniques, Inc., Auburn, CA 95603. @ 1987 American Meteorological Society Before radiometric measurements can be confidently applied to analyses, field evaluations of performance specifications must be undertaken, so that limitations and deficiencies of radiometric measurements can be recognized and understood. This paper presents results from field evaluations conducted during the Sierra Co- operative Pilot Project (SCPP) and the Colorado Or- ographic Seeding Experiment (COSE). The SCPP has incorporated a microwave radiometer to study the potential for precipitation enhancement of wintertime storms in the central Sierra Nevada. The COSE program is a similar study designed to evaluate the precipitation augmentation potential of wintertime cloud systems that occur over the northern Colorado River Basin. Both of these programs have used micro- wave radiometric measurements extensively to inter- pret the physical processes in these cloud systems (Heggli, 1985; Rauber et aI., 1986). As part of this re- search, these programs have taken steps to compare measurements made by microwave radiometers with independent in situ instrumentation.