Systematic comparisons of clouds as observed by MODIS and ISCCP

Since 1984 the ISCCP (International Satellite Cloud Climatology) project has been providing global estimates of those cloud parameters that affect the earth's radiation budget, including cloud amounts, cloud top pressure and temperature, and cloud optical depth. These estimates are made from observations in a few spectral channels made by a changing set of geostationary and polar-orbiting satellites, and have become a valuable data set for evaluating the predictions of climate models (among many other uses). The MODIS instrument, flying aboard the polar-orbiting Terra and Aqua platforms since 2000, also estimates these quantities. Substantially more spectral information is used in the MODIS retrievals than in ISCCP, making the retrievals more sensitive to marginal clouds, at least in principle. What can be gained by using MODIS observations, and how can models (for example) be compared with both ISCCP and MODIS?

We compare MODIS and ISCCP estimates of cloud properties for the entire time period during which both are available. Estimates of monthly-mean cloud fraction within 2.5 degree regions are almost always within a few percent, indicating that the additional spectral information used by MODIS does not detect a large number of very thin clouds. Cloud top temperture, on the other hand, differs by several K, indicating that the CO2 slicing used by MODIS is more likely to predict higher cloud tops than the thermal emission tests used by ISCCP. We compare the predictions as a function of time, season, land/ocean, and cloud phase.