A ground-based sky imager system consisting of two commercial digital CCD cameras with wide-angle lenses has been developed. The system can be used to derive various macroscopic cloud parameters: cloud amount, cloud-base heights and cloud-base wind (for every visible cloud layer). The method to calculate a DSM of the cloud-base is presented. It includes the precise determination of the interior and exterior orientation of the cameras, which have been carried out with a close-range photogrammetric testfield, stars and special airplane flights (equipped with DGPS). The data acquisition took place in the Upper Rhine Valley, Switzerland in October 1999 and was part of the SOP (Special Observing Period) campaign of the programme MAP (Mesoscale Alpine Programme). Cloud-base heights have been derived automatically using commercial digital photogrammetric systems and own software. A comparison of the results with operational and MAP-only lidar and radiosonde measurements is shown.
Finally, a case study of coincident ground- and satellite-based retrieval of cloud-base/cloud-top height for a vertically thin cirrus cloud formation is presented. The ground measurements with our new stereo camera system showed therefore to be an interesting technique to validate satellite-based cloud-top heights of vertically thin clouds and to additionally detect smaller scale cloud features, which is important for accurate nowcasting in mountainous terrain.
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