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- Author or Editor: Olivier Jourdan x
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Abstract
Observations of halos and related phenomena due to ice crystals are commonly reported from ground observations and presented in the literature. Nevertheless, ice crystal characteristics have only been poorly documented from in situ measurements performed in halo-producing cirrus with simultaneous observations of optical phenomena. Using the Polar Nephelometer, a new instrument for in situ measuring of the scattering phase function of cloud droplets and ice particles, 22° and 46° halo features have been evidenced during a cirrus uncinus cloud case study between −30°C and −38°C. Simultaneous microphysical measurements were made with a 2D-C probe manufactured by Particle Measuring Systems Inc. (PMS). The results show that ice crystal properties derived from 2D-C measurements do not present substantial differences when comparing cirrus cloud samples with and without halos. Consequently, the cloud scattering properties appear to be dominated by small ice particles (smaller than about 100 μm), which are poorly documented with conventional PMS probes. The halo occurrences are observed in only a few cloud portions (2%), which are characterized by small horizontal scales (100–400 m). Furthermore, the observed 22° and 46° peak features are smoothed out with regard to modeling results relative to geometric pristine-crystal shape. These differences are discussed by using the new Inhomogeneous Hexagonal Monocrystal theoretical model of light scattering.
Abstract
Observations of halos and related phenomena due to ice crystals are commonly reported from ground observations and presented in the literature. Nevertheless, ice crystal characteristics have only been poorly documented from in situ measurements performed in halo-producing cirrus with simultaneous observations of optical phenomena. Using the Polar Nephelometer, a new instrument for in situ measuring of the scattering phase function of cloud droplets and ice particles, 22° and 46° halo features have been evidenced during a cirrus uncinus cloud case study between −30°C and −38°C. Simultaneous microphysical measurements were made with a 2D-C probe manufactured by Particle Measuring Systems Inc. (PMS). The results show that ice crystal properties derived from 2D-C measurements do not present substantial differences when comparing cirrus cloud samples with and without halos. Consequently, the cloud scattering properties appear to be dominated by small ice particles (smaller than about 100 μm), which are poorly documented with conventional PMS probes. The halo occurrences are observed in only a few cloud portions (2%), which are characterized by small horizontal scales (100–400 m). Furthermore, the observed 22° and 46° peak features are smoothed out with regard to modeling results relative to geometric pristine-crystal shape. These differences are discussed by using the new Inhomogeneous Hexagonal Monocrystal theoretical model of light scattering.
Abstract
A methodology of employing statistical procedures, specifically the principal component analysis (PCA) technique, to assess cirrus cloud data reliability is described. The approach is demonstrated by an example of a study of optical and microphysical characteristics measured during two campaigns performed at midlatitudes in the pristine Southern (SH) and polluted Northern (NH) Hemispheres within the international INCA project (Interhemispheric Differences in Cirrus Cloud Properties from Anthropogenic Emissions). The datasets were obtained by using state-of-the-art airborne instruments including the polar nephelometer and PMS particle size spectrometers for the ice-particle characterization. The approach is applied to both the measured angular scattering intensities and the ice-particle size distributions. It is shown that the PCA technique allows for impartial elimination of nonreliable channels of instruments. Furthermore, this technique is efficient in a study if the dataset is statistically homogeneous, and provides the possibility of removing specific records corresponding to distinguishing statistical ensembles. The results, expressed in terms of significant components and corresponding eigenvalues, show that the Southern and Northern Hemisphere datasets are in good agreement and they can be considered as statistically representative of the sampled cirrus. Furthermore, the frequency distributions of the cirrus cloud microphysical and optical properties can be regarded as arbitrary positive quantities, which are lognormally distributed. The validation of the measurements is provided by intercomparison of parameters estimated from different and independent techniques. The statistical relationships between quantities derived from angular scattering intensities and from ice-particle distributions as well as the similarity of the results obtained for the Southern and Northern Hemisphere cases serve as proof of the reliability of the measured cloud properties.
Abstract
A methodology of employing statistical procedures, specifically the principal component analysis (PCA) technique, to assess cirrus cloud data reliability is described. The approach is demonstrated by an example of a study of optical and microphysical characteristics measured during two campaigns performed at midlatitudes in the pristine Southern (SH) and polluted Northern (NH) Hemispheres within the international INCA project (Interhemispheric Differences in Cirrus Cloud Properties from Anthropogenic Emissions). The datasets were obtained by using state-of-the-art airborne instruments including the polar nephelometer and PMS particle size spectrometers for the ice-particle characterization. The approach is applied to both the measured angular scattering intensities and the ice-particle size distributions. It is shown that the PCA technique allows for impartial elimination of nonreliable channels of instruments. Furthermore, this technique is efficient in a study if the dataset is statistically homogeneous, and provides the possibility of removing specific records corresponding to distinguishing statistical ensembles. The results, expressed in terms of significant components and corresponding eigenvalues, show that the Southern and Northern Hemisphere datasets are in good agreement and they can be considered as statistically representative of the sampled cirrus. Furthermore, the frequency distributions of the cirrus cloud microphysical and optical properties can be regarded as arbitrary positive quantities, which are lognormally distributed. The validation of the measurements is provided by intercomparison of parameters estimated from different and independent techniques. The statistical relationships between quantities derived from angular scattering intensities and from ice-particle distributions as well as the similarity of the results obtained for the Southern and Northern Hemisphere cases serve as proof of the reliability of the measured cloud properties.
Abstract
This study illustrates the high potential of RALI, the French airborne radar–lidar instrument, for studying cloud processes and evaluating satellite products when satellite overpasses are available. For an Arctic nimbostratus ice cloud collected on 1 April 2008 during the Polar Study using Aircraft, Remote Sensing, Surface Measurements and Models, of Climate, Chemistry, Aerosols, and Transport (POLARCAT) campaign, the capability of this synergistic instrument to retrieve cloud properties and to characterize the cloud phase at scales smaller than a kilometer, which is crucial for cloud process analysis, is demonstrated. A variational approach, which combines radar and lidar, is used to retrieve the ice-water content (IWC), extinction, and effective radius. The combination of radar and lidar is shown to provide better retrievals than do stand-alone methods and, in general, the radar overestimates and the lidar underestimates IWC. As the sampled ice cloud was simultaneously observed by CloudSat and Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) satellites, a new way to assess satellite cloud products by combining in situ and active remote sensing measurements is identified. It was then possible to compare RALI to three satellite ice cloud products: CloudSat, CALIPSO, and the Cloud-Aerosol-Water-Radiation Interactions (ICARE) center’s radar–lidar project (DARDAR).
Abstract
This study illustrates the high potential of RALI, the French airborne radar–lidar instrument, for studying cloud processes and evaluating satellite products when satellite overpasses are available. For an Arctic nimbostratus ice cloud collected on 1 April 2008 during the Polar Study using Aircraft, Remote Sensing, Surface Measurements and Models, of Climate, Chemistry, Aerosols, and Transport (POLARCAT) campaign, the capability of this synergistic instrument to retrieve cloud properties and to characterize the cloud phase at scales smaller than a kilometer, which is crucial for cloud process analysis, is demonstrated. A variational approach, which combines radar and lidar, is used to retrieve the ice-water content (IWC), extinction, and effective radius. The combination of radar and lidar is shown to provide better retrievals than do stand-alone methods and, in general, the radar overestimates and the lidar underestimates IWC. As the sampled ice cloud was simultaneously observed by CloudSat and Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) satellites, a new way to assess satellite cloud products by combining in situ and active remote sensing measurements is identified. It was then possible to compare RALI to three satellite ice cloud products: CloudSat, CALIPSO, and the Cloud-Aerosol-Water-Radiation Interactions (ICARE) center’s radar–lidar project (DARDAR).
Abstract
Homogeneous freezing of supercooled droplets occurs in convective systems in low and midlatitudes. This droplet-freezing process leads to the formation of a large amount of small ice particles, so-called frozen droplets, that are transported to the upper parts of anvil outflows, where they can influence the cloud radiative properties. However, the detailed microphysics and, thus, the scattering properties of these small ice particles are highly uncertain. Here, the link between the microphysical and optical properties of frozen droplets is investigated in cloud chamber experiments, where the frozen droplets were formed, grown, and sublimated under controlled conditions. It was found that frozen droplets developed a high degree of small-scale complexity after their initial formation and subsequent growth. During sublimation, the small-scale complexity disappeared, releasing a smooth and near-spherical ice particle. Angular light scattering and depolarization measurements confirmed that these sublimating frozen droplets scattered light similar to spherical particles: that is, they had angular light-scattering properties similar to water droplets. The knowledge gained from this laboratory study was applied to two case studies of aircraft measurements in midlatitude and tropical convective systems. The in situ aircraft measurements confirmed that the microphysics of frozen droplets is dependent on the humidity conditions they are exposed to (growth or sublimation). The existence of optically spherical frozen droplets can be important for the radiative properties of detraining convective outflows.
Abstract
Homogeneous freezing of supercooled droplets occurs in convective systems in low and midlatitudes. This droplet-freezing process leads to the formation of a large amount of small ice particles, so-called frozen droplets, that are transported to the upper parts of anvil outflows, where they can influence the cloud radiative properties. However, the detailed microphysics and, thus, the scattering properties of these small ice particles are highly uncertain. Here, the link between the microphysical and optical properties of frozen droplets is investigated in cloud chamber experiments, where the frozen droplets were formed, grown, and sublimated under controlled conditions. It was found that frozen droplets developed a high degree of small-scale complexity after their initial formation and subsequent growth. During sublimation, the small-scale complexity disappeared, releasing a smooth and near-spherical ice particle. Angular light scattering and depolarization measurements confirmed that these sublimating frozen droplets scattered light similar to spherical particles: that is, they had angular light-scattering properties similar to water droplets. The knowledge gained from this laboratory study was applied to two case studies of aircraft measurements in midlatitude and tropical convective systems. The in situ aircraft measurements confirmed that the microphysics of frozen droplets is dependent on the humidity conditions they are exposed to (growth or sublimation). The existence of optically spherical frozen droplets can be important for the radiative properties of detraining convective outflows.
