In situ samples of cloud droplets by aircraft in Oklahoma in 1997, the Surface Heat Budget of the Arctic Ocean (SHEBA)/First ISCCP Regional Experiment (FIRE)-Arctic Cloud Experiment (ACE) in 1998, and various other locations around the world were used to evaluate a ground-based remote sensing technique for retrieving profiles of cloud droplet effective radius. The technique is based on vertically pointing measurements from highsensitivity millimeter-wavelength radar and produces height-resolved estimates of cloud particle effective radius. Although most meteorological radars lack the sensitivity to detect small cloud droplets, millimeter-wavelength cloud radars provide opportunities for remotely monitoring the properties of nonprecipitating clouds. These highsensitivity radars reveal detailed reflectivity structure of most clouds that are within several kilometers range. In order to turn reflectivity into usable microphysical quantities, relationships between the measured quantities and the desired quantities must be developed. This can be done through theoretical analysis, modeling, or empirical measurements. Then the uncertainty of each procedure must be determined in order to know which ones to use. In this study, two related techniques are examined for the retrieval of the effective radius. One method uses both radar reflectivity and integrated liquid water through the clouds obtained from a microwave radiometer; the second uses the radar reflectivity and an assumption that continental stratus clouds have a concentration of 200 drops per cubic centimeter and marine stratus 100 cm Ϫ3. Using in situ measurements of marine and continental stratus, the error analysis herein shows that the error in these techniques would be about 15%. In comparing the techniques with in situ aircraft measurements of effective radius, it is found that the radar radiometer retrieval was not quite as good as the technique using radar reflectivity alone. The radar reflectivity alone gave a 13% standard deviation with the in situ comparison, while the radar-radiometer retrieval gave a 19% standard deviation.

An evaluation of surface currents measured by HF radar during November 29, 1983, to January 31, 1984, with radar sites at Jupiter and Stuart on the Florida east coast is carried out in comparison with currents and transports measured by moorings and submarine cable. While an earlier analysis of currents measured in summer 1983 with radars located at Palm Beach and Jupiter (Schott et al., 1985) found significant northward shear in the northward radar currents about 20 km offshore leading to concerns about a possible bias in the radar currents, this effect was not observed in the second application farther north. It is possible that the shear in the summer 1983 field might have been real and related to the topography in the southern part of the 1983 radar field where no intercomparison current data had been available. Concerning the usefulness of radar currents as Florida Current transport indicators, which was the prime intention of their application in the context of the Subtropical Atlantic Climate Studies, this second study finds much more encouraging results than the one based on the observations of summer 1983. While the first study was inconclusive because only small transport fluctuations occurred during the summer 1983 observation period, this second study finds significant correlation. Florida Current transport fluctuations had a total range of 15 x 106 m3/s during the second observation period, and correlation with downstream radar currents, averaged zonally across the center of the radar field, was 0.85. Coherence was significant for periods longer than 5 days. Highest correlation with transport was found for radar currents farthest out, to the right of the axis of the stream. S. A. Frisch, NOAA/Wave Propagation Laboratory, 325 Broadway, Boulder, CO 80307.