Comparison of Co-Incident MISR, Terra MODIS and ISS-CATS Cloud Top Heights

Abstract:

Cloud-top height (CTH) is an Essential Climate Variable, with our longest and most stable single-platform satellite record provided by Terra’s Multi-angle Imaging Spectroradiometer (MISR), which employs stereoscopy on visible radiation, and the Moderate Resolution Imaging Spectroradiometer (MODIS), which employs either a CO₂-slicing or a 11μm-brightness temperature technique, in the infrared. Global validation of CTH from these instruments has been hindered by the lack of coincident data from a space-based lidar, until the 2015-2017 operation of Cloud-Aerosol Transport System (CATS) lidar on-board the International Space Station. In this study, co-incident (< 1 km) and concurrent (< 5 minutes) Level 2 CTH observations have been analyzed from CATS, MISR, and MODIS.

For single-layered and optically thick cloud as detected by CATS, MODIS has an accuracy = 497 m and a precision = 1.02 km, while MISR accuracy = 315 m and precision = 413 m. For multi-layered clouds detected by CATS, MODIS has an accuracy = 1.4 km and a precision = 2.3 km. MODIS CTH at times may be indicative of a “radiative mean” CTH. MISR on the other hand, owing to visible stereoscopy, retrieves the lower-layer CTH in CATS-detected multi-layered cloud conditions, if the top-cloud layer optical depth < ~ 0.3, with almost the same accuracy as noted above for single layer clouds. Systematic and random error sources are examined and propagated to explain inter-sensor disagreements. While the accuracy and precision of MISR and MODIS CTH relative to CATS reported above are global, they are observed to systematically vary with cloud heterogeneity and altitude, which will have important consequences in interpreting long term changes in CTH from Terra’s MISR and MODIS 20-year record.