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


The results of a ruby lidar (0.694 μm wavelength) and infrared radiometer (10–12 μm) study on cirrus clouds are reported for a period covering the autumn and winter months at Aspendale (38°S, 144°E). The lidar and radiometer data have been used to study the temperature dependence of the gross structure and optical properties of cirrus clouds. Well-defined correlations are found between the mid-cloud temperature and cloud depth, infrared absorption coefficient, infrared emittance, backscatter to extinction ratio and ratio of the visible extinction coefficient at 0.693 μm to the infrared absorption coefficient at 10–12 μm. For instance, as the mid-cloud temperature varies from −70 to −30°C, the mean cloud depth increases from 1 to 3.5 km and mean infrared absorption coefficient from 0.04 to 0,25 km−1. These two factors together cause a change in emittance from 0.11 to 0.42. The increase in absorption coefficient with temperature can be attributed to the presence of larger ice particles in the deeper clouds. Over the same temperature range the effective backscatter to extinction ratio has a fairly complex behavior with values of 0.25–0.3 below −45°C, but with a rapid increase to 0.45 at −40°C. The multiple scattering factor is found to increase from 0.54 at −60°C (∼11 km altitude) to 0.76 at −20°C (∼5.5 km altitude). Some cases of very high anomalous lidar backscatter occur for clouds of mean temperature ≳ −35° and emittance ≳0.6. The depolarization ratio of the lidar backscattered radiation also shows complicated variations with temperature.

The observed changes in backscatter to extinction ratio are attributed to a change in the ice crystal habit from simple spatial crystals at temperatures <−40°C to more complex aggregates at greater temperatures. This is based on the fact that supercooled water cannot exist at temperatures below −40°C. The high anomalous backscatter is attributed to specular reflection from horizontally oriented plate crystals or from supercooled water droplets. Changes in depolarization ratio at temperatures greater than −40°C are attributed variously to the presence of mixed-phase clouds, to crystal aggregates and to horizontally oriented hexagonal crystals.

Changes in the multiple-scattering factor with temperature (i.e., altitude) are found to agree qualitatively with theoretical predictions, the main effect being a reduction in the multiple-scattering factor (leading to a more transmitting cloud) as the range or altitude of the clouds increases.

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


The scattering and reflection components of the remotely measured effective beam emittance of high clouds are calculated using a detailed model of radiative transfer through cirrus. Two atmospheric profiles of temperature and humidity are used representing tropical and midlatitude summer atmospheres respectively. The scattering and reflection components of the measured beam emittance are shown to be appreciable, particularly for tropical atmospheres where for example the reflection component at the ground for vertical viewing is 20% of the total emittance.

Computed values of the broad-band effective flux emittance are compared with equivalent values of the narrow-band effective flux emittance at 11 μm wavelength and the narrow-band beam emittance at 11μm. It is shown that the two former quantities are well correlated and approximately equal in magnitude.

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


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


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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Andrew J. Heymsfield and C. M. R. Platt


A data set obtained in cirrus clouds has been examined to deduce any dependencies of the particle size spectral form or the crystal habit on the temperature. It was found that both form of the spectra and crystal habit changed systematically with temperature, the largest change occurring between −l40 and −50°C. These findings are consistent with previously found dependencies between linear backscatter measurements and temperature.

A preliminary scheme for parameterizing the cirrus particle size spectra for crystal dimensions greater than 20 μm in terms of the temperature and the ice water content is described. The visible extinction in cirrus is estimated.

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C. M. R. Platt and K. Bartusek


Simultaneous infrared (10–12 μm) and visible (0.694 μm) optical properties of some altocumulus and altostratus clouds have been determined using infrared radiometry and lidar.

Broadly, two types of middle-level clouds were observed; rather thin (∼200 m) dense clouds forming between 0 and −5C and with high infrared absorption coefficients (∼8 km−1), and thicker, low-density clouds, forming between −10 and −30C and with average infrared absorption coefficients of less than 1 km−1. The former type were undoubtedly liquid water clouds, whereas the latter were probably ice or mixed-phase clouds. Both types were often semi-transparent to infrared radiation.

Lidar backscatter coefficients for the higher clouds were typically between 0.1 and 3 km−1.

The experimental backscatter-to-extinction ratio at 0.694 μm and the ratio of the backscatter at 0.694 μm to the absorption at 10–12 μm are compared with theoretical computations. In certain cases, changes in these ratios could be identified with changes in the cloud microstructure.

Calculated infrared cooling rates in the clouds varied from 0.1 to 1.5C hr−1 depending on cloud density.

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


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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C. M. R. Platt, S. C. Scott, and A. C. Dilley


A lidar (0.694 μm wavelength) and a passive radiometer (10–12 μm) have been used together to remotely sense the optical properties and gross structure of cirrus (the LIRAD method).

This article reports on observations of midlatitude cirrus taken during two extended experiments at Aspendale, Victoria, Australia, covering one winter season and one summer season and a six-week period of observations of tropical cirrus at Darwin, Northern Territory.

Information has been obtained on the infrared emittance, optical depth, cloud depth, depolarization ratio, “anomalous” backscatter, the effective ratios of backscatter to extinction at the lidar wavelength and the visible to infrared extinction, and the backscatter profile of cirrus.

The results show that the infrared emittance and volume absorption coefficient of midlatitude cirrus, when averaged over a year, are close functions of the midcloud temperature. Very similar relationships hold for tropical cirrus, taking into amount the limited samples. Mean values of beam emittance (10–12 μm) at Aspendale and Darwin were 0.33 and 0. 115, respectively, translating into broadband flux emittance values of 0.52 and 0.30, respectively.

Cirrus cloud depths at Aspendale were quite similar for the winter and summer seasons, varying from 1 to 2 km at −65°C to 2 to 4 km at −35°C, and decreasing again to 1 to 2.5 km at −15°C. The cloud depths at Darwin showed a similar pattern, but the maximum depths of 2 to 3 km occurred between −55° and −70°C, dropping dramatically for both higher and lower temperatures.

Integrated depolarization ratios varied from 0.4 at −60°C to 0.25 at −30°C in the midlatitude cirrus. At higher temperatures, the ratios showed a branching behavior, with some values clustered around a value of 0.38 and others around a value of 0.07. This branching was less evident in summer, with values failing to about 0. 14 at −15°C. The depolarization ratios in tropical cirrus were much less variable, with values ranging from 0.3 at −75°C to 0.27 at −50°C.

A method was developed for separating “normal” and “anomalous” backscatter, the latter being characterized by very intense backscatter coupled with a low depolarization ratio. This allowed a more accurate calculation of optical quantities for normal backscatter and also indicated that anomalous backscatter was present in over 50% of returns at temperatures in the −20° to −30°C range.

The backscatter-to-extinction ratio k/2η showed different characteristics in the summer and winter midlatitude cirrus when plotted against temperature, but these differences disappeared to some extent when k/2η was plotted against altitude. The values of k/2η in tropical cirrus appeared to be rather independent of temperature.

The effects of a variable multiple scattering factor were investigated, and it was found that a variable factor tended to cause the values deduced on the simple theory of a constant η to be too high. Values of the elective ratio of visible extinction to infrared absorption (2αη) deduced for the midlatitude cirrus showed little variation between summer and winter, with values varying from about 2.3 to 1.8 between temperatures of −50° and −20°C. In tropical cirrus, the corresponding values was 3.3.

Average cloud profiles of backscatter indicated large differences between temperatures greater and less than −30°C due to anomalous backscatter in the former case. The profiles also indicated a systematic decrease in backscatter toward cloud base, thought to be due to evaporation of crystals near the base.

Uncertainty in the behavior of η is still the largest stumbling block to the calculation of fundamental quantities such as α and k. The visible optical depth can be calculated to only about 50% accuracy using the lidar backscatter values and derived values of k. A value to about 30% accuracy can be calculated from the infrared volume absorption coefficient σA together with theoretical values of α.

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C. M. R. Platt, David W. Reynolds, and N. L. Abshire


Radiometric data from the SMS-2 and GOES-1 geostationary satellites together with ground-based lidar scans have been combined to determine the visible albedo, infrared emittance and visible optical depth of cirrus clouds. The combined observations were made on an area of cirrus of about 10 km by 10 km square at Boulder, Colorado during two days.

A method of analysis was developed to separate out the cloud albedo from surface albedo effects, to allow for possible anisotropy in the bi-directional reflectance of solar radiation from the clouds, and to compare the data with results of theoretical calculations.

Relations between the visible albedo and the infrared emittance, which were derived from satellite data, and the visible optical depth, which was derived from lidar measurements, were compared with theoretical relations derived from two models of cloud particle scattering. The first model assumes that the cloud is composed of water (or ice) spheres and the second that it is composed of long ice cylinders. It was found that the observational data agree best with the latter model, although there are still some discrepancies.

The infrared emittances varied between 0.2 and 0.95, the corresponding albedos between 0.10 and 0.32 and the visible optical depths between 0.5 and 3.5.

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C. M. R. Platt, N. L. Abshire, and G. T. McNice


Lidar observations of a winter ice cloud in the zenith gave a very high reflection but a very small depolarization. When the lidar was tilted more than 0.5° away from the zenith, the reflection amplitude fell to 3% of it's zenith value, but the depolarization increased. The above properties proved unambiguously that reflection was occurring from the specular surfaces of horizontal crystals. These properties were used to estimate some cloud microphysical properties. At a selected time, the estimates gave a mean “diameter” of 74 μm for the horizontal faces, a crystal number density of 0.78 −1, and a maximum departure of the crystal axis from the horizontal of 0.5°. The fraction of the total crystal cross section which was specularly reflecting was estimated as unity.

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