Evaluation of a Lidar Simulator for Assessment of Model Cloudiness

Clouds represent a primary uncertainty in model estimates of climate sensitivity. Cloud estimates from climate models have been evaluated against observations from passive satellite sensors, but passive measurements have limited information on cloud vertical structure and also have other limitations. With the launch of the CALIPSO and CloudSat satellites in 2006, new observations are available to evaluate model cloud estimates. The CALIPSO lidar provides a number of unique capabilities, including the ability to profile optically thin clouds with a vertical resolution of 30 to 60 meters. In recent years there has been a trend toward the use of instrument simulators to provide a more consistent basis for comparing observations with models. A “CALIPSO simulator” has been developed which accounts for effects of cloud overlap, spatial resolution, and attenuation of the lidar signal. The simulator is applied to model outputs, and the distribution of cloud layers is derived from the simulator output. CALIPSO Level 1 profiles are then processed in a consistent manner. The simulator necessarily involves assumptions on atmospheric and cloud properties, however, and the simulator itself must be evaluated. In this paper we assess assumptions built into the current CALIPSO simulator and evaluate cloud distributions produced by the simulator against standard CALIPSO cloud products.