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Kenneth Sassen

The development of the polarization lidar field over the past two decades is reviewed, and the current cloud-research capabilities and limitations are evaluated. Relying on fundamental scattering principles governing the interaction of polarized laser light with distinctly shaped hydrometers, this remote-sensing technique has contributed to our knowledge of the composition and structure of a variety of cloud types. For example, polarization lidar is a key component of current climate-research programs to characterize the properties of cirrus clouds, and is an integral part of multiple remote-sensor studies of mixed-phase cloud systems, such as winter mountain storms. Although unambiguous cloud-phase discrimination and the identification of some ice particle types and orientations are demonstrated capabilities, recent theoretical approaches involving ice crystal ray-tracing and cloud microphysical model simulations are promising to increase the utility of the technique. New results simulating the single and multiple scattering properties of precipitating mixed-phase clouds are given for illustration of such methods.

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Kenneth Sassen

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The design and operation of a cloud-sampling device for laboratory applications is described. The cloud particles are collected in a continuous or sequential fashion by impaction, with volatile particle shape and size preservation accomplished by making permanent casts in a thin plastic layer which hardens after exposure. With the aid of impaction theory and an experimental calibration, the sampling characteristics are shown to be well-suited for determining the composition of the super-micron constituents of the water and ice particle clouds artificially produced in the laboratory.

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Kenneth Sassen

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Ground-based polarization lidar measurements have been obtained in conjunction with a commercial cloud seeding program to evaluate the potential of pulsed laser remote sensing techniques for increasing the effectiveness of orographic cloud seeding operations. The lidar measurements, supplemented by microwave radar cloud top and rawinsonde data, are shown to aid in the determination of seeding criteria. Our real-time seed, no-seed decisions based on lidar polarization measurements of cloud ice-water balance compare favorably with the declarations issued locally by the project meteorologists. Moreover, the lidar has detected changes in cloud layer structure and ice-water balance brought about by aerial seeding operations which are quite distinct from the background conditions, indicating that lidar observations may also find application as part of a physical evaluation method of seeding effects. It is concluded that despite the range limitations of lidar, the application of the remote sensing techniques can make important contributions to orographic cloud seeding operations and research.

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Kenneth Sassen

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Ice crystal habit identification is shown to be accomplished through an analysis of the backscattered signals from single ice crystals (sampled in a small cloud volume) which relates the relative signal strengths of specular reflections and the scattering interactions which yield depolarized energy. Additionally, evidence is presented which indicates that the monitoring of the strength and frequency of specular reflections can be used to obtain estimates of the size distribution and concentration of populations of relatively minute ice crystals. These results in combination with previously reported depolarization measurements of larger and more complex hydrometeors suggest that the optical backscatter depolarization technique displays a significant potential for in situ cloud physics research.

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Kenneth Sassen

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The results of a field program using a polarization diversity lidar system to study high plains thunderstorm precipitation are given. The presence of “reflectivity” bright bands of the general form predicted by a numerical simulation indicates that ice prune precipitation processes were predominantly acting in the storm studied. Depolarization profiles through the melting region, evaluated on the basis of CW laser scattering measurements, are shown to aid in the discrimination of the ice particle types causing the bright bands. Depolarization and returned energy signatures due to graupel were present primarily during the developing stage, consistent with recent findings in northeastern Colorado. However, melting snowflake signatures were predominantly observed during the mature thunderstorm stage, suggesting that the contribution to thunderstorm rainfall from the ice crystal aggregation process has been generally underestimated.

It is also significant that the use of lidar in this study in a manner similar to microwave radar indicates the potential of the lidar depolarization technique for future operational programs of atmospheric research.

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Kenneth Sassen

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The results of a field program of coordinated lidar and aircraft observations of orographically induced clouds are reported. An evaluation of polarization diversity lidar for cloud physics research applications on the basis of the air-truth measurements and earlier findings indicates that the remote sensing technique can monitor some kinds of cloud microphysical information currently measured only through in situ sampling methods. In particular, lidar polarization measurements are indicated from the observations reported here to be sensitive to the composition of mixed phase clouds, including a measure of cloud ice balance, and to permit observations of the ice particle riming process in precipitation formation. Detailed vertical cross sections of hydrometeor content are derived from height versus time displays of lidar returns in combination with the polarization analysis, revealing some interesting features of the structure of complex orographic cloud systems. As a further aid in the evaluation of lidar, the operational restrictions of typical lidars during field operations are also discussed. It is concluded that polarization diversity lidar displays the potential for important applications in cloud physics and modification research programs involving the study of particles in the ice phase.

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Kenneth Sassen

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An apparatus is described which measures the linear depolarization ratios of vertically polarized laser light (6328 Å) backscattered by mixed phase particle clouds grown in a laboratory cold chamber. Variations in linear depolarization ratios during the phase transitions of undercooled clouds seeded by dry ice and silver iodide agents have been monitored by simultaneously measuring the returned signal intensities in the parallel and orthogonal planes of polarization. The experimental results yield linear depolarization ratio values of approximately 3% for pure water clouds (3 to 10 μm diameter droplets), to 50 ± 2.5% for pure ice crystal clouds (5 to nearly 200 μm maximum dimensions) for particle concentrations up to ∼104 particles cm−3. The values for mixed particle clouds were found to be related to the ratio of ice to water particle concentrations. Variations in linear depolarization ratios with the elevation angle of the laser receiver system have amounted to only a few percent of the average values in most cases. It is believed that these results are applicable to the lidar probing of atmospheric clouds with similar particle spectra, provided that lidar transmitter and receiver beamwidths are restricted to values which can limit depolarized energy resulting from photon multiple scattering.

The linear depolarization ratios of individual signal spikes were found to vary from at most a few percent when plate crystals were present, up to 100% for hollow columnar crystals. The onset and intensity of these events in the recorded data are also related to ice crystal size, indicating that when small cloud volumes are being examined, this method of analysis is useful in sensing the size and habit of ice crystals.

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Kenneth Sassen

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No abstract available.

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Kenneth Sassen

Abstract

On 25 October during the 1986 Project FIRE Intensive Field Observations experiment, a NCAR King Air mission conducted over a ground-based polarization lidar site at Wausau, Wisconsin, sampled a highly supercooled (−28° to −31°C) altocumulus perlucidus layer. Ground-based photography shows the concurrent formation of a contrail, which glaciated and gradually spread until producing an ice crystal optical display. According to wind-advected flight tracks, the aircraft-produced ice particles (APIP)-affected cloud volume was later sampled during zenith lidar measurements, indicating a ∼1.0 m s−1 horizontal dispersion rate, and accidentally repenetrated by the aircraft during a spiral descent. In situ data obtained during the legs generating the APIP measured average liquid-water contents of 0.02–0.05 gm−3, mean droplet diameters of 12–15 μm, and droplet concentrations of 20–25 cm−3. The resampling of the APIP region yielded ice crystal concentrations of 242 I−1 at the original leg altitude, and 36 1−1 within an apparent crystal fallstreak. The background altocumulus ice particle concentration averaged ∼41−1.

The APIP mechanism most likely to explain the observations involves the homogeneous freezing of contrail and natural cloud droplets induced by the rapid cooling behind the King Air propeller tips. In view of the wide use of aircraft of this type in basic and applied cloud physics research, caution should be exercised in interpreting ice particle data obtained when the likelihood of flight track resampling exists.

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Kenneth Sassen

After reviewing the indirect evidence for the regional climatic impact of contrail-generated cirrus clouds (contrail-cirrus), the author presents a variety of new measurements indicating the nature and scope of the problem. The assessment concentrates on polarization lidar and radiometric observations of persisting contrails from Salt Lake City, Utah, where an extended Project First ISCCP (International Satellite Cloud Climatology Program) Regional Experiment (FIRE) cirrus cloud dataset from the Facility for Atmospheric Remote Sensing has captured new information in a geographical area previously identified as being affected by relatively heavy air traffic. The following contrail properties are considered: hourly and monthly frequency of occurrence; height, temperature, and relative humidity statistics; visible and infrared radiative impacts; and microphysical content evaluated from in situ data and contrail optical phenomenon such as halos and coronas. Also presented are high-resolution lidar images of contrails from the recent SUCCESS experiment, and the results of an initial attempt to numerically simulate the radiative effects of an observed contrail. The evidence indicates that the direct radiative effects of contrails display the potential for regional climate change at many midlatitude locations, even though the sign of the climatic impact may be uncertain. However, new information suggests that the unusually small particles typical of many persisting contrails may favor the albedo cooling over the greenhouse warming effect, depending on such factors as the geographic distribution and patterns in day versus night aircraft usage.

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