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C. M. R. Platt

Abstract

Some computations are made of submillimeter wave extinction in clouds and fogs using recent spectrometric results of the optical properties of water. At wavelengths > 1000μ an approximate formula is adequate in which extinction is proportional to cloud water content. At 2000 and 1000μ the extinction is 6.5 and 15.2 dB km−1 per gm m−3, respectively. Between 200 and 1000μ additional extinction occurs due to large droplets of diameters >20μ. Extinction for a typical fog distribution is computed and is found to be 41.1 dB km−1 per gm m−3 at 337 μ and 92.8 dB km−1 per gm m−3 at 200μ. Comparisons with experimental data at 1200 and 337μ shows qualitative agreement, but insufficient data on the composition of the clouds and fogs investigated precludes accurate comparison.

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C. M. R. Platt

Abstract

The technique of using combined lidar (0.694µm) and radiometric (10–12µm) measurements on cirrus clouds was developed in order to determine their infrared emissivity and to examine the experimental relationships between the infrared and lidar data.

Detailed measurements of cirrus infrared emissivity and lidar backscatter were made on several cirrus systems at Adelaide in November 1970. The mean zenith emissivity of cirrus was found to be 0.245. Emissivity was only weakly correlated with thickness. The cirrus extinction coefficient, which is proportional to the particle density, was totally uncorrelated with mid-cloud temperature.

Various infrared and visible optical quantities were calculated from the experimental data and the relationships between some of these quantities were examined. The lidar backscatter amplitude integrated through the cloud was found to be well correlated with the infrared optical thickness. A simple model of absorption and scattering of radiation in a cloud of large (σ50µm) ice spheres was employed to derive a theoretical expression between the integrated lidar backscatter and the infrared optical thickness. The experimental data agreed well with the above expression. The experimentally observed backscatter-to-extinction ratio at 0.694µm was 0.25±0.06. This is considerably lower than values predicted for large ice spheres, in qualitative agreement with the laboratory measurements on ice crystals reported by Huffman and Dugin et al.

A value of the cirrus optical thicknc% al 0.694μ, was calculated from the measured mean infrared optical thickness using the theoretical values of extinction for large ice spheres. Using Fritz's relations between albedo and optical thickness, it was found that at, say, 38S latitude, the cirrus albedo would vary between 0.2 in midwinter and 0.07 in midsummer.

A visible optical thickness can be obtained directly from the lidar data but its value is lower than the true value due to near-forward, multiple-scattered radiation being detected by the laser receiver. It is hoped to remedy this in future experiments by using a narrower receiver aperture.

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C. M. R. Platt

Abstract

Monte Carlo calculations of multiple-scattered contributions to the total energy received in a lidar beam have been made for a representative cirrus ice-cloud scattering phase function. The phase function is varied arbitrarily near the back direction to give three different scattering patterns. The Monte Carlo method of Plass and Kattawar (1971) has been modified and extended to give a greatly increased efficiency for a small decrease in accuracy. This modification has reduced the computation time by ∼80% allowing routine calculations to be made on many different cloud models. When applied to two water cloud models the method gives results which agree well with those obtained by previous workers, The purpose of. the calculations is to investigate the apparent reduction in the cloud optical depth due to multiple scattering when measured by a lidar. This reduction is described here by a multiple scattering factor η It is found that for cirrus clouds the factor η varies considerably with the depth of cloud penetration, with the cloud optical depth and with the cloud extinction coefficient.

The calculations show for the first time the pattern of contribution from each order of scattering separately. It is shown that for cloud optical depths ≳0.1 a model which considers double scattering only is inadequate.

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

Abstract

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

Abstract

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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C. M. R. Platt and K. Bartusek

Abstract

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

Abstract

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, S. C. Scott, and A. C. Dilley

Abstract

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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G. L. Stephens, G. W. Paltridge, and C. M. R. Platt

Abstract

Six case studies of “uniform” planetary boundary layer clouds are reported where the solar and infrared radiation fields, liquid water content, drop-size distributions and temperature and humidity profiles were measured simultaneously. The measurements are compared with theoretical prediction from a detailed radiative transfer model in an attempt to verify the performance of the model and its associated parameterization schemes (Parts 1 and 2). The measurements support the parameterization of both shortwave and longwave radiative characteristics in terms of vertical liquid water path (LWP) i.e., without the need to define cloud drop-size distributions. Within experimental error, there are no significant discrepancies between theory and measurement. However, there is some evidence in the present study, supported by measurements of others in (generally) thicker and denser clouds that solar absorption is in excess of theoretical prediction.

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C. M. R. Platt, R. T. Austin, S. A. Young, and A. J. Heymsfield

Abstract

During the Maritime Continent Thunderstorm Experiment (MCTEX), several decaying storm anvils were observed. The anvil clouds exhibited typical patterns of fallout and decay over a number of hours of observation. The anvil bases were initially very attenuating to lidar pulses, and continued that way until anvil breakup commenced. During that time, the anvil base reached some characteristic altitude (∼7 km) below which the cloud particles had evaporated fully. Some typical “tongues” of fallout below such levels also occurred. Millimeter radar showed the storm anvil cloud tops to be much higher than detected by lidar until the anvil was well dissipated.

The infrared properties of the anvils were calculated. In three of the four anvils studied, the calculated emittance never exceeded 0.8–0.85. In the remaining case, the cloud emittance approached unity only in the period before the anvil had descended appreciably. Radiative transfer calculations showed that the infrared emission originated mostly from the layer between cloud base and the height at which complete attenuation of the lidar pulse occurred. However, the correct blackbody emission at cloud base could only be obtained by assuming the existence of an additional layer, situated above the first, 1.8 km deep and with a specific backscatter coefficient. The depressed values of emittance were interpreted as a cooling (below those temperatures measured by radiosonde) for some distance above anvil cloud base due to evaporation of the cloud. Typically, this cooling amounted to about 10°C, depending on the layer thickness above cloud base at which cooling was occurring. A reexamination of data taken in 1981 at Darwin, Northern Territory, Australia, indicated a similar depression in emittance in all cases of attenuating storm anvils. A simple model of ice-mass evaporation saturating the ambient air was used to approximate the observed cooling in one anvil. Millimeter radar reflectivity measurements, which also yielded ice water content at cloud base, were also used to find equivalent cooling rates. By varying the mean volume diameter in the calculation, cooling rates similar to those found from the radiometric method could be obtained. The values of mean volume diameter agreed, within uncertainties, with those obtained by the lidar–radar method. Estimated cooling to over 1 km above cloud base confirms earlier work on anvil mammata. Values of backscatter-to-extinction ratio at the base of the anvils showed some consistent variations, indicating a change of ice-crystal habit, or size, with time.

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