Dual lidar observations at 10.6 μm and 532 nm for retrieving semitransparent cirrus cloud properties

To improve the estimation of the infrared radiances in cirrus clouds, one needs to consider the vertical inhomogeneities of the cloud properties. The position of the maximum of absorption within an ice cloud is potentially important to the improvement of the split-window techniques for retrieving particle size and for understanding the radiative effect of the cloud in the infrared spectrum. Current remote sensing techniques used for inferring ice clouds hardly reach the level of accuracy required to resolve the vertical inhomogeneities of a cloud and to determine the position of absorption. This study explores the possibility of retrieving the vertical structures of ice clouds by combining data from two lidar measurements acquired at the wavelengths of 532 nm and 10.6 mu m. A method is proposed to retrieve the variability of ice crystal absorption efficiency at 10.6 mu m, the particle concentration weighted by the crystal area, and the attenuation by absorption at 10.6 mu m. The method is tested against observations collected at Site Instrumental de Recherche en Teledetection Atmospherique (SIRTA) in Palaiseau, France. Observations and simulations both show that lidar observations collected simultaneously at those two wavelengths can be used to determine the level within the ice cloud where maximum attenuation of infrared radiation occurs. The maximum attenuation may occur near the cloud base or the cloud top, depending on the case studied.