Validation of POLDER/ADEOS data using a ground-based lidar network: Preliminary results for cirrus clouds

POLDER-1 on board the Japanese ADEOS platform has collected 8 months of data (October 1996 to June 1997). POLDER is a visible radiometer with multi-directional and polarization capabilities. These features allow to obtain information both on cloud thermodynamical phase and cloud altitude. The POLDER cloud algorithms are currently under validation. For cirrus clouds, a validation based on a comparison between cloud properties inferred from POLDER-1 data and cloud properties retrieved from a ground-based lidar network is currently under consideration. In this paper, preliminary comparisons between cloud altitude inferred from POLDER-1 and lidars are presented. These comparisons show an underestimation of cloud altitude by POLDER-1 for optically thin clouds. A first theoretical analysis based on radiative transfer simulations allows to explain the difference between POLDER and lidar altitudes. Moreover. a preliminary comparison between thermodynamical cloud phase (liquid water / ice) obtained with POLDER-1 and cloud temperature inferred from radiosoundings and cloud altitudes shows that cirrus clouds at temperatures lower than -40 degrees C are correctly identified as ice layers by POLDER-1 (C) 1999 Elsevier Science Ltd. All rights reserved.