Cirrus Classification at Midlatitude from Systematic Lidar Observations

Abstract

Systematic cirrus lidar measurements performed in the south of France during 2000 are analyzed statistically to search for cloud classes. The classes are based on cloud characteristics (cloud thickness, light backscattering efficiency, and its variance), cloud absolute geometric height, cloud height relative to the tropopause, and the temperature at the cloud level. The successive use of principal component analysis, cluster methods, and linear discriminant analysis allows the identification of four cirrus classes. Almost all the cirrus detections correspond to three classes with similar proportion of the total cirrus detected (around 30%). The absolute geometric height and the thickness are found to be the main discriminant variables. The first cirrus class corresponds to thin clouds above the local tropopause (absolute geometric height: 11.5 km), or at least around the tropopause, while another class corresponds also to thin clouds but at a lower altitude range in the troposphere (absolute geometric height: 8.6 km). The third class corresponds to thick clouds (thickness of 3.2 km) located below the tropopause, in an altitude range between the two first classes (absolute geometric height: 9.8 km). As expected, the high-altitude cirrus class is characterized with the lowest mean temperature. It is noted that the temperature is closely related to the altitude and so the role of temperature in the cirrus classes cannot be disentangled from the role of the altitude.