Stereo cloud-top heights and cloud fraction retrieval from ATSR-2

Abstract

An operational processing system is described for the automated retrieval of cloud-top heights (CTHs) and amounts from the Gridded Brightness Temperature (GBT) 1 km product derived from the dual view of the Along Track Scanning Radiometer (ATSR-2) instrument onboard the ESA ERS-2 satellite. A new stereo matching algorithm, called M4 is described together with the overall processing chain for retrieving cloud-top heights and amounts. M4 is based on the successful stereo matchers, M2 and M3 which have been in operation with the NASA MISR sensor since March 2000 and which have also been applied to mapping cloud fraction in the polar region using ATSR-2 (Cawkwell and Bamber 2002, Cawkwell et al. 2001). Results in companion papers (Naud et al., Denis et al. 2007) describe the accuracy achieved using this algorithm and detailed scene-by-scene results are available from http:// cloudmap.org under results-.UCL-.ATSR-2 Validation. We assess the impact of cloud-top winds on the accuracy of the retrieved cloud-top heights and conclude that for most middle and lower tropospheric clouds, cloud-top winds will have little, if any, noticeable impact. We show how ATSR-2 cloud-top heights at the different wavelengths (particularly visible compared with thermal IR) can sometimes yield information on multi-layer clouds which is unique to ATSR-2. The processing system has recently been applied to three years of ASTR-2 GBT data and results are shown. These results are available from the British Atmospheric Data Centre (http://badc.nerc.ac.uk/data/list_all_datasets. html?source5data).