Validation of cloud parameter retrieval methods with objective ground based measurements

To understand and model the radiative transport in a cloudy atmosphere information on the cloud height and optical thickness is indispensable. Therefore retrieval techniques for cloud parameters are developed for the AVHRR, ATSR and the future MSG. Mainly synoptic observations are used for validation despite their subjective nature and varying quality. To validate advanced cloud parameter retrieval methods objective physical measurements are necessary. At KNMI retrieval methods of cloud fraction, cloud top temperature, optical thickness and LWP from AVHRR observations are developed. Much effort is put in building an inf;infrastructure for validation. The retrieval methods are validated with a two-year data set from the KNMI Cloud Detection System (CDS). Detailed analysis is performed with observations from the CLARA-campaigns, when a number of advanced remote sensing and in-situ instruments were added to the CDS. The collocated lidar, radar, microwave radiometer and aircraft measurements from the CLARA data set allow for the evaluation of the assumptions in cloud parameter retrieval methods. Furthermore, the study presented here shows that combining measurements from lidar, radar and AVHRR provide information on cloud properties that cannot be retrieved from any of these instruments alone. (C) 1999 Elsevier Science Ltd. All rights reserved.