Abstract. We have developed a tunable temperature profiler based on a highly stable synthesizer that can observe at multiple frequencies on the shoulder of the 60-GHz atmospheric oxygen feature. We are developing a similar radiometer to obtain the vertical distribution of water vapor by making observations on the pressure-broadened water vapor line from 22 to 29 GHz. Information on cloud liquid water profiles is also contained in these two wave bands. Various mathematical retrieval methods for temperature, water vapor, and cloud liquid water profiles were tested based on these radiometer designs. These include neural networking, Newtonian iteration of statistically retrieved profiles, and Bayesian "most probable" retrievals. On the basis of realistic radiometer errors and performance, very good retrieval capability is demonstrated. The performance of the various retrieval methods are presented and compared. Examples of profile retrievals are also presented. Data were not binned into seasons to reduce computer time; better retrieval results for all methods would be expected with binning. Copyright 1998 by the American Geophysical UnionPaper number 97RS03656.0048-6604/98/97RS-03656 $11.00 393 ments. They also have the capability to profile cloud liquid water, a capability absent in RAOBs and all other systems except for in situ aircraft devices. Applications for this passive radiometric profiling include weather forecasting and nowcasting; detection of aircraft icing and other aviation-related meteorological hazards; determination of density profiles for artillery trajectory and sound propagation; refractivity profiles for radio ducting prediction; corrections to radio astronomy; satellite positioning and Global Positioning System (GPS) measurements; atmospheric radiation flux studies; estimation and prediction of telecommunication link degradation; and measurement of water vapor densities as they affect hygroscopic aerosols and smokes. Microwave Profiling MethodsThe microwave profiling methods discussed here make use of atmospheric radiation measurements in the 20-to 75-GHz region. The zenith path atmospheric absorption spectrum at sea level for a typical midlatitude atmosphere with a 1-km-thick, 0.5 g/m 3 cloud in this frequency band is shown in Figure 1. The feature at 22.2 GHz is a water vapor resonance that is pressure broadened according to the pressure altitude
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