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A. C. Dilley

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

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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 A. C. Dilley

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

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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, A. C. Dilley, J. C. Scott, I. J. Barton, and G. L. Stephens

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The infrared properties and structures of some anvils emanating from local thunderstorms were studied by lidar and infrared radiometry at Darwin, tropical Northern Australia. The anvils were typically from 1 to 2 km deep, at altitudes from 7 to 16 km and at temperatures from −15 to −70°C. There was a rough dependence of infrared emittance on temperature, but there was also a dependence on the age of the anvil. The average altitude and calculated wide-band greybody flux emittance were 11 km and 0.65 respectively.

One dense cloud appeared “superblack” when observed from below, due to reflection of upwelling warm radiation from the surface. The magnitude of the effect agreed within experimental error with that predicted from computations on a model cloud of ice cylinders, but was about twice that computed for a model of ice spheres.

Calculated rates of heating in the very cold clouds were very high, reaching 4°C h−1 near cloud base. The survival of these clouds for several hours suggests that the absorbed radiant heat was converted largely into sensible heat in the atmosphere rather than causing evaporation of the crystals.

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