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Graham Feingold and Zev Levin

Abstract

Use of the lognormal form of raindrop size distributions in simulations of differential reflectivity (ZDR) measurements is investigated. Using two remotely measured variables and an empirical relation, the three parameters of the lognormal distribution can be deduced and the spectrum integrated to obtain rain rate. This is demonstrated by a simulation of the ZDR method using ground-based drop size distributions. Drop axis ratio and sampling time effects are also investigated and results compared to those obtained using a gamma distribution. It is shown that the lognormal representation is easily adaptable for use in the ZDR method. Using our dataset, we show that the lognormal size distribution provides lower average absolute deviations of theoretically determined rain rates from actual ones (10.7%) than those obtained using either the exponential (41.0%) or gamma distributions (11.8%).

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Graham Feingold and Christian J. Grund

Abstract

This paper addresses the feasibility of using mulliwavelength lidar measurements to differentiate both qualitatively and quantitatively between the relative concentrations of hygroscopic and nonhygroscopic aerosol particles. The proposed technique utilizes the fact that hygroscopic particles undergo a size increase and refractive-index change with increasing relative humidity and that different wavelengths respond to these changes in different ways. The lidar wavelengths considered are 0.289, 0.355, 0.532, 0.694, 1.064, and 2.02 µm and the 9–11.5-µm range. It is shown that under certain conditions, a judicious choice of lidar wavelengths can provide a differential backscatter, sufficient to provide information on the size and percentage number concentration of the hygroscopic aerosol and, consequently, cloud condensation nuclei concentration. The presence of a mode of coarse particles (median radius greater than 0.3 µm) produces ambiguous results and limits application of the technique to regions sufficiently distant from coarse mode sources (e.g., in the free troposphere). The authors have identified a pair of wavelengths in the infrared region that provides a clear indication of the existence of these particles. The potential benefits of distinguishing hygroscopic particle concentration from nonhygroscopic particle concentration are great since remote measurement can provide good temporal and spatial coverage of these properties and valuable information for climate monitoring.

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Shelby Frisch, Matthew Shupe, Irina Djalalova, Graham Feingold, and Michael Poellot

Abstract

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 high-sensitivity 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 high-sensitivity 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−3. 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.

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