Remote Sounding of High Clouds. Part VI: Optical Properties of Midlatitude and Tropical Cirrus

Abstract

A lidar (0.694 ?m wavelength) and a passive radiometer (10?12 ?m) 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 ?m) 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/2? showed different characteristics in the summer and winter midlatitude cirrus when plotted against temperature, but these differences disappeared to some extent when k/2? was plotted against altitude. The values of k/2? 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 ? to be too high. Values of the elective ratio of visible extinction to infrared absorption (2α?) 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 ? 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.