Evaluation of GLAS Cloud and Aerosol Data Products

NASA launched the Geoscience Laser Altimeter System (GLAS) onboard the ICESat spacecraft on January 12, 2003. The atmospheric lidar subsystem of GLAS has the capability to detect, discriminate, and vertically locate multiple layers of clouds and aerosols up to 40 km above the earth's surface. The results of these operations are a component of the standard data products that make up the GLAS data product archive. The layer detection procedures are a necessary step in the effort to find atmospheric layer optical properties from GLAS data. For the first time using and active optical instrument, GLAS makes these products available to the atmospheric research community on a world-wide basis over an extended period of time. In this paper, we evaluate the effectiveness of the layer detection algorithms and associated products from intercomparison studies. The studies are designed to address the following issues regarding GLAS layer detection: 1) what is the sensitivity of the layer detection; 2) what is the accuracy of the top and bottom boundaries; 3) how much does multiple scattering influence the accuracy of layer boundary estimates: 4) how accurate is the cloud/aerosol layer discrimination technique; 5) what are the frequencies of false positives and false negatives; 6) how well do layer detection results from the 1064nm channel compare with results from the 532 nm channel; 7) how do results from nighttime observations compare with those from daytime observations; 8) how well does GLAS atmospheric layer detection compare to other satellite-based detection, such as from MODIS. We also present in this paper early results of cloud layer climatological studies that show frequency of occurrence of various cloud types and altitudes at various locations. These studies demonstrate the advantage of lidar's capability to unambiguously discern multiple cloud layers.