In this paper, the possibilities of satellite-based and ground-based stereoscopy of clouds are examined, with the objective to derive cloud-top and cloud-base heights and motion. These parameters are very important for a better description of clouds for nowcasting and numerical weather prediction models.

For the satellite part, coincident images of MISR (on EOS Terra), ASTER (on EOS Terra) and ATSR2 (on ERS-2) are used. As stereo image pairs from polar-orbiting satellites are never perfectly synchronous (time delay of some seconds between the image acquisition from the different viewing angles), the height error of the cloud-top heights (CTH), introduced by the along-track motion component, is corrected with the cloud-top winds (CTW) extracted from the Meteosat-6 10min Rapid Scans. For MISR, with nine viewing angles, this height correction is only needed when two camera views are taken. With at least three images from non-symmetric cameras, it is possible to directly separate the along-track parallax (due to cloud height) from the along-track wind contribution (due to cloud motion). The CTH/CTW results from MISR L1B2 (own algorithms), MISR L2TC (operational NASA/JPL cloud product), ASTER 3N/3B, ATSR2 and Meteosat-6 are compared.

Our ground-based imager system consists of at least two commercial color digital CCD cameras, with a horizontal distance of 500-1000 m, each connected to a laptop for camera control and image storage and to a radio clock for high-precision time synchronization. The stereo-photogrammetric method to calculate height and motion of the cloud-base includes the precise determination of the interior and exterior orientation of the cameras (which has been carried out with an inhouse close-range photogrammetric testfield and an on-site calibration with stars). The time interval between subsequent images is adapted to the current meteorological situation and is normally chosen between 30 seconds and 2 minutes. The cameras were installed at Zurich-Airport, Switzerland, in September 2001 and April 2002, in coincidence with overpasses of EOS-Terra and ERS-2. The ASTER image acquisition in April 2002 was only scheduled on demand.

In principle, the same stereo matching algorithms, based on least-squares matching, are applied on both the satellite-based and ground-based images. With respect to the different spatial resolution of the sensors (ground-based cameras: 1-10m (0.1% of cloud height), ASTER: 15m, MISR: 275m, ATSR2: 1km), the matching strategy has to be adjusted accordingly. Furthermore, the main differences in the processing chain between the satellite-based and ground-based data sets are the geometric calibration of the sensors, the preprocessing procedures and the quality-control algorithms.

Finally, two case studies of coincident ground- and satellite-based retrieval of cloud-base/cloud-top height and motion are presented. The ground measurements with our new stereo camera system showed to be an interesting technique to validate satellite-based cloud-top height and motion of vertically thin clouds and to additionally detect smaller scale cloud features, which is particularly important for accurate nowcasting in mountainous terrain. The results are further validated by radiosonde ascents and ceilometer data. The described unique 3D data sets will be used in the EU project Cloudmap2 for further modeling and visualization studies.