Lidar and Radioinetric Observations of Cirrus Clouds

Abstract

The technique of using combined lidar (0.694um) and radiometric (10-12um)measurements on cirrus clouds was developed in order to determinetheir infrared emissivity and to examine the experimental relationshipsbetween the infrared and lidar data.Detailed measurements of cirrus infrared emissivity and lidar backscatterwere made on several cirrus systems at Adelaide in November 1970.The mean zenith emissivity of cirrus was found to be 0.245. Emissivitywas only weakly correlated with thickness. The cirrus extinctioncoefficient, which is proportional to the particle density, was totallyuncorrelated with mid-cloud temperature.Various infrared and visible optical quantities were calculated fromthe experimental data and the relationships between some of thesequantities were examined. The lidar backscatter amplitude integratedthrough the cloud was found to be well correlated with the infraredoptical thickness. A simple model of absorption and scattering ofradiation in a cloud of large (sigma50um) ice spheres was employedto derive a theoretical expression between the integrated lidar backscatterand the infrared optical thickness. The experimental data agreedwell with the above expression. The experimentally observed backscatter-to-extinctionratio at 0.694um was 0.25+-0.06. This is considerably lower thanvalues predicted for large ice spheres, in qualitative agreementwith the laboratory measurements on ice crystals reported by Huffmanand Dugin et al.A value of the cirrus optical thicknc al 0.694u, was calculated fromthe measured mean infrared optical thickness using the theoreticalvalues of extinction for large ice spheres. Using Fritz's relationsbetween albedo and optical thickness, it was found that at, say,38S latitude, the cirrus albedo would vary between 0.2 in midwinterand 0.07 in midsummer.A visible optical thickness can be obtained directly from the lidardata 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 narrowerreceiver aperture.