Lidar and Radioinetric Observations of Cirrus Clouds

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.