Global distribution of cirrus clouds from CloudSat/Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) measurements

The cirrus clouds of the upper troposphere are globally widespread and are important regulators of the radiative balance, and hence climate, of the Earth-atmosphere system. Despite their wide distribution, however, cirrus are difficult to study from satellite radiance measurements or from scattered ground observing sites because they can occur as part of multilayered cloud systems and are characteristically optically thin. The need to better characterize the global distribution of cirrus clouds was therefore a major justification for the formation flying of the CloudSat and CALIPSO satellites, which support a cloud radar and polarization lidar, respectively. Measurements by these active remote sensors, when analyzed by appropriate algorithms, have the ability to identify and accurately measure the locations and heights of this category of clouds. The combined CloudSat/Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) data cirrus cloud algorithm used in this study is aimed at identifying those clouds that would likely be classified as cirrus by a surface weather observer: it is based on previous experience with multiple remote sensor approaches and knowledge gleaned from extensive surface lidar and radar observations of visually identified cirrus clouds with a minimum of a priori assumptions. We report on the global and seasonal frequencies of cirrus clouds, and on their heights and thicknesses obtained over the initial 1 year of data collected. We find a global average frequency of cirrus cloud occurrence of 16.7%. These new results are compared with other cirrus cloud climatologies and are interpreted in terms of local cirrus cloud formation mechanisms and the responsible global weather phenomena.