Joint histograms of Cloud Top Height (CTH) and Optical Depth (OD) derived by the International Satellite Cloud Climatology Project (ISCCP) are being widely used by the climate modeling community in evaluating global climate models. Similar joint histograms of CTH-OD are now being produced by the NASA Multi-angle Imaging Spectro-Radiometer (MISR) and Moderate Resolution Imaging Spectroradiometer (MODIS) instrument teams. There are notable differences in the histograms being produced by these three projects. In this poster we analyze some of the differences and discuss how the differences relate to the retrieval approaches used and what they tell us about the observed cloud fields.
The MISR CTH retrieval is based on a stereo-imaging technique that is purely geometric and is not sensitive to the sensor calibration. The technique does, however, require finding the same cloud-top features in multiple views of the same cloud scene. The MODIS CTH retrieval is based on a combination of techniques. A CO2 slicing technique is used to infer cloud-top pressure for most clouds above about 700 hPa. For low-level clouds, the cloud top pressure is determined in a similar way to ISCCP. That is, the MODIS 11-µm band brightness temperature is used to determine a cloud-top temperature, from which the cloud-top pressure is inferred (using NCEP Global Data Assimilation System (GDAS) in the case of MODIS). This latter approach assumes the cloud is optically thick and entirely covers the satellite pixels over which the retrieval is applied.