Remote sounding of high clouds, Pt. 4, Optical properties of midlatitude and tropical cirrus

• Published in: Journal of the atmospheric sciences 1987-01, Vol.44 (4), p.729-747
• Main Authors: Platt, CMR, Scott, J C, Dilley, A C
• Format: Article
• Language: English
• ISSN: 0022-4928

A lidar (0.694- mu m wavelength) and a passive radiometer (10-12 mu m) have been used together for remote sensing of 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 IR emittance, optical depth, cloud depth, depolarization ratio, anomalous backscatter, the effective ratio of backscatter to extinction at the lidar wavelength and of visible to IR extinction, and the backscatter profile of cirrus. The results show that the IR 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 consideration the limited samples. Mean values of beam emittance (10-12 mu 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 similar for the winter and summer seasons, varying from 1 to 2 km at -65 degrees C to 2-4 km at -35 degrees C, and decreasing again to 1-2.5 km at -15 degrees C. The cloud depths at Darwin showed a similar pattern, but the maximum depths of 2-3 km occurred between -55 and -70 degrees C, dropping dramatically for both higher and lower temperatures. Integrated depolarization ratios varied from 0.4 at -60 degrees C to 0.25 at -30 degrees 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 falling to similar to 0.14 at -15 degrees C. The depolarization ratios in tropical cirrus were much less variable, with values ranging from 0.3 at -75 degrees C to 0.27 at -50 degrees 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 made possible a more accurate calculation of optical quantities for normal backscatter and also indicated that anomalous backscatter was present in >50% of returns at temperatures in the -20-30 degrees C range. The backscatter-to-extinction ra