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Kenneth Sassen

Abstract

Coordinated polarization lidar, Ku-band radar and dual-channel microwave radiometer observations of a deep orographic cloud system were collected from a mountain-base site in northwestern Colorado as part of the Colorado Orographic Seeding Experiment (COSE) research effort. The remote sensing observations are presented for three distinct storm stages, corresponding to a pre-frontal altostratus cloud layer, a local orographically-induced cloud development, and a peak in storm activity accompanying the passage of a weak cold front. Supercooled liquid water in the form of thin but often dense liquid layers, and expansive, more weakly mixed-phase cloud regions were usually present even to temperatures approaching −40°C. The liquid water amounts present were often below the detection threshold of the vertically-pointing radiometer measurements, but during one brief interval a liquid water content as high as 0.5 g m−3 may have occurred. The lidar depolarization data also show the presence of a persistent layer of oriented planar ice crystals at the −15°C level, which was responsible for generating aggregates and light snowfall in the downwind mountains. The monitoring of the quantities of the water substance present in the vapor, liquid, and solid phases provides a unique image of the behavior of the storm, and it is concluded that this remote sensor ensemble is well suited for the study of orographic clouds and their potential for modification.

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Kenneth Sassen

Abstract

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

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

Abstract

The ice equivalent radar reflectivity factor (Zi = 5.3Ze) is found from previous measurements to be related to the precipitation rate (R mm h−1) and ice mass content (M, g m−3) of ice crystal clouds by R = 0.067Zi 0.804 and M = 0.037Zi 0.696. Comparison with other empirical equations suggests that changes in the ice crystal size distribution accompanying the formation of precipitation particles begin to modify these relations within deep-ice clouds and lead to a different class of relations for snowfall. Accordingly, the above equations are considered valid for Zi ≲ 10 dBZ, and should be particularly appropriate for short-wavelength radar observations of cirrus clouds.

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Kenneth Sassen

Abstract

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

Abstract

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

Abstract

Polarization measurements have been obtained with lidar from ice virga and precipitation. Linear depolarization ratios are characteristically near 0.5 for the ice phase, but can display significantly higher values under some conditions. Such anomalous occurrences appear to result from the interrogation of populations of oriented ice crystals, and also as a consequence of the geometry of the monostatic lidar system when viewing targets at short ranges. In agreement with continuous wave laser scattering measurements, melting snowflakes produce a depolarization bright band analogous to that observed with microwave radar, yielding depolarization ratios up to ∼0.7 a few hundred meters below the freezing level.

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Kenneth Sassen

Abstract

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

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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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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