SIMULATION OF SATELLITE LIDAR AND RADIOMETER RETRIEVALS OF A GENERAL-CIRCULATION MODEL 3-DIMENSIONAL CLOUD DATA SET

The inclusion of a backscatter lidar on a space platform for a radiati on mission, as proposed by various space agencies, aims to bring new i nformation on three-dimensional cloud distribution, with a special emp hasis on optically thin cirrus clouds, which are presently poorly dete cted by passive sensors. Key issues for such cloud observational studi es are the detection of multilayered cloud systems, thin cirrus, and f ractional cloud cover, knowledge that would improve our understanding of the global radiation budget. To assess the impact of such lidar mea surements on cloud climatology, a 1 month cloud data set has been simu lated with a general circulation model (GCM). The cloud detection capa bility of a spaceborne scanning backscatter lidar is assessed with the use of two detection schemes, one based on limitations in the detecte d cloud optical depth and the other based on lidar signal-to-noise rat io. The cloud information retrieved from passive radiometric measureme nts using a procedure like that used in the International Satellite Cl oud Climatology Project is also simulated from the same GCM cloud data set. It is shown that a spaceborne backscatter lidar can improve sign ificantly the retrieval of thin cirrus clouds as well as underlying cl oud layers. High-level cloud retrieval from a spaceborne lidar therefo re appears as a powerful complement to radiometric measurements for im proving our knowledge of actual cloud climatology.