| Summary: | The ability to characterize the global cloud cover from space has been greatly enhanced by the availability of MODIS, CloudSat, and CALIOP data. The three sensors provide good complementary information about clouds. In this study, we investigated unexpected observations of certain types of clouds apparent in the MODIS data but not detected by CloudSat and CALIOP. Several examples are presented and generally these undetected clouds are geometrically thin, low-level clouds. In particular, they are located in the Arctic region and have optical thicknesses of less than 14, top height altitudes of below 2.5km, and layer thickness of less than 1km. CloudSat may miss such low-level clouds because of its coarse vertical resolution of about 500m and it has limited sensitivity near the surface. Unexpectedly, CALIOP with a much higher vertical resolution of 30m also misses these clouds and this is due to the cloud's geometrically thin nature and surface proximity. Copyright 2011 by the American Geophysical Union.
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