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  • Author or Editor: C. M. R. Platt x
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C. M. R. Platt

Abstract

Measurements by monostatic lidar of linear depolarization ratios and backscatter coefficients in an altostratus cloud revealed a horizontally layered structure. Three different types of layers were observed. The bottom and central layers had depolarization ratios varying from 0.3 to 0.4, which identified them as layers containing mainly ice. The backscatter coefficients were similar to those found in cirrus ice clouds. A central, transient layer had depolarization ratios characteristic of a high-density water cloud, although the total integrated backscatter of 2.3 ± 1.2 was high for this type of cloud. The top layer had a depolarization ratio of 0.2 at the cloud base, decreasing to 0.04 at the cloud center. Backscatter coefficients ranged up to 30 km−1 and the total integrated backscatter was about 7.6 ± 3.8. This value is considerably higher than the range of values predicted for water or cirrus ice clouds and one possible explanation is that specular reflection was occurring from horizontally aligned ice crystal plates.

The variation of backscatter coefficient within each layer was rather regular, with a maximum at the center of the layers. The cloud was situated in a stable air stream and its evolution appeared to be slow.

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C. M. R. Platt

Abstract

Using a physical model, bispectral curves of visible albedo versus infrared brightness temperature are derived for various idealized cloud layers. These layers vary between those which are unbroken with variable optical depth (cirrus) to those which are broken, but the clouds have a uniform high optical depth (strato-cumulus). It is shown that the above two cases can be distinguished by the different shapes of the bispectral curves. The effects of variable solar angle and satellite viewing angle on the shapes of the bispectral curves are investigated. The effects of infrared scattering and deep cloud layers are also investigated and found to be important for high clouds with variable optical depth, such as cirrus.

The predicted curves are compared with two-dimensional bispectral histograms obtained from the VISSR radiometer of the GMS-1 satellite. The theoretical curves compare quite well qualitatively. Two detailed quantitative comparisons for clouds which are assumed to be semitransparent high clouds give good agreement.

The limitatins of the model are discussed.

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C. M. R. Platt

Abstract

The design and performance of a narrow-beam radiometer for atmospheric studies are described. It has a beamwidth of 6 mrad and a minimum detectable radiance of 0.0056 mW cm−2 sr−1 in a 1-HZ output bandwidth. The system incorporates a novel method of comparing flux incident on the aperture against a temperature-stabilized blackbody so that effects due to variations in radiation emitted by or reflected from the chopper blades are eliminated. The radiometer is being applied initially to studies in the 10–12 μm spectral band of the atmospheric “window”. Several applications, including the study of water vapor continuum absorption and the emissivity of high layer clouds, are described briefly.

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C. M. R. Platt

Abstract

A method of minimizing the effects of direct solar scatter in aureole measurements is described. Experimental results for “clear” and polluted conditions are illustrated. The effects of large particles on the shape of the aureole are evident. The solar aureole irradiances at several angles from solar center and on different days are compared with conventionally measured spectral turbidities.

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C. M. R. Platt

Abstract

This article describes a method of determining the visible and infrared properties of high ice clouds using a ground-based lidar and infrared (IR) radiometer. A method of calibrating the lidar is described. This is followed by a method for the correction of the cloud backscatter coefficients for pulse attenuation in the clouds, using an experimentally determined backscatter to extinction ratio k. The IR emissivity is then calculated by assuming a value for the ratio between the visible extinction coefficient and the IR absorption coefficient which is invariant with altitude. This ratio is altered until the computed radiance is equal to the measured cloud radiance. Errors in the calculation of the backscatter coefficient, the visible and the IR optical depths and the IR emissivity are assessed for various errors in the backscatter to extinction ratio. It is found that errors in the cloud optical depth become extremely sensitive to errors in k when the cloud visible optical depth becomes large. However, errors in the IR optical depth and IR emissivity are considerably less because they are constrained to agree with IR radiance measurements. For ”typical“ cirrus, having an emissivity of 0.24, and for a standard error in k of 20%, the error in the visible optical depth is 20–30%, whereas the errors in the IR optical depth and emissivity are only 1 and 2%, respectively. The effects of a variable multiple-scattering factor on the above errors appear to be small. However, the variation of this factor is not known well enough yet in high clouds to assess the errors accurately.

Other sources of error, which will be discussed in later papers of this series, include experimental errors in the measured IR radiance and errors in the determination of the calibration factor S.

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C. M. R. Platt

Abstract

Some unusual lidar returns from an altostratus cloud are interpreted in terms of reflections from hexagonal ice plates falling with their long axes aligned in the horizontal. Such an explanation is consistent with the observed high backscatter coefficients and low depolarization ratios and also with the temperature range (−12 to −20°C) of the cloud layers, as well as the known fall characteristics of naturally occurring ice platelets. Backscatter efficiencies are calculated for “perfect” ice platelets when illuminated at or near an axis orthogonal to the crystal long axis. It is shown that very high backscatter coefficients can potentially be measured from a cloud of ice plates, depending on the fraction of crystals which are “perfect,&rdquo,; the degree to which the plates' long axes stay horizontal, and the angle of the lidar to the vertical.

It is further shown that reflection from a single crystal gives an appreciable signal-to-noise ratio at the receiver and that only a few crystals will be correctly aligned in the horizontal in a typical laser pulse volume, and for realistic particle number densities.

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C. M. R. Platt and A. C. Dilley

Abstract

The results from a series of measurements of the beam emissivity of cirrostratus at 10–12 μm wavelengths are presented, using methods of analysis which were discussed in Part I. A ruby lidar and infrared radiometer were used to gather data remotely from the ground. The various sources and magnitudes of error are discussed. The results for eight large cirrostratus systems which were observed on different days gave a mean beam emissivity of 0.54 (or flux emissivity of 0.70). This compares with a value of 0.245 (0.38 for flux obtained during an earlier period (Platt, 1973). The measurements were obtained at 35°S (Adelaide) and 38°S (Aspendale). The cloud systems at Aspendale all formed in similar synoptic situations.

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W. L. Smith and C. M. R. Platt

Abstract

Cloud altitudes specified from the Infrared Temperature Profile Radiometer on the Nimbus 5 satellite are compared with simultaneous observations by radiosonde and ground-based ranging measurements conducted with the lidar system at CSIRO in Aspendale, Victoria, Australia, during September 1976. The results show that the cloud altitudes deduced by the CO2 channel absorption method are in general agreement with the lidar and radiosonde determinations, regardless of the cloud opacity and amount.

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Graeme L. Stephens and C. M. R. Platt

Abstract

This paper reports on a series of flights that were conducted off the east coast of Australia through and over stratocumulus and fair-weather cumulus cloud fields. The CSIRO Fokker F-27 research aircraft was used to obtain radiation and in situ cloud microphysical and thermodynamical measurements. Central to the analyses presented in this paper were the measurements obtained by a spectrally scanning visible near-infrared radiometer (SPERAD) which was designed specifically for the experiments reported herein.

Analyses of the data obtained during the flights that are reported in this paper showed that the clouds sampled were warm and mainly maritime in character, with both low droplet concentrations and liquid water contents. The stratiform clouds were shallow, with optical depths of about 10. Despite the lack of cloud vertical development significant concentrations of large droplets were recorded by the Knollenberg 2D probe. Variance analyses of the cloud optical properties indicated that the sampled cloud layers possessed highly variable volume extinction coefficients with fractional deviations exceeding 0.5 at most levels, whereas the single-scattering albedo and the asymmetry parameter were more uniform along any given level. Variance analyses of the bidirectional reflected radiation from Sc clouds indicated a variability of cloud reflectance on two distinct horizontal scales, which could in turn be related to the scale of the relevant mixing processes. It was also found that the reflected radiances from cumulus clouds were far more anisotropic in character than thou reflected from stratocumulus clouds. The spectral variation of cloud reflectance with wavelength also exhibited features that, on the basis of the comparisons reported, could not be fully explained by existing theory.

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C. M. R. Platt, M. A. Vaughan, and R. T. Austin

Abstract

Following the discovery of anomalously high values of lidar integrated attenuated backscatter near the top center layers of mesoscale convective systems (MCSs) observed by the NASA Lidar In-Space Technology Experiment (LITE), a search of Cloud Aerosol Lidar with Orthogonal Polarization (CALIOP) data on board the Cloud Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) platform revealed the same phenomena in a sample of eight MCSs investigated. The backscatter depolarization ratio also showed changes concurrent with the high integrated backscatter and either increased or decreased concurrently with the anomalous backscatter. Simultaneous CloudSat data in the A-Train formation showed a cloud-top altitude similar to that measured by CALIOP, indicating fairly large ice crystals were reaching cloud top. Based on previous work, the CALIOP and CloudSat returns were likely due to a mix of small ice droxtals or frozen drops extending in a continuous spectrum to large crystals composed of well-formed hexagonal columns, thick hexagonal plates, spheroids, and irregular particles. The CALIOP lidar would detect the whole spectrum whereas CloudSat would detect ice crystals greater than ∼30 μm in effective radius; there were apparently enough of such crystals to allow CloudSat to detect a cloud-top height similar to that found by CALIOP. Using such a model, it was estimated that the measured backscatter phase function in the most active part of the cloud could be reconciled approximately with theoretical values of the various crystal habits. However, it was harder to reconcile the changes in depolarization ratio given the absence of values of this parameter for small droxtal crystals.

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