A Comparison of Three Global Satellite Cloud Products and Implications for GCM Validation

The majority of existing satellite products pertain mostly to bulk cloud properties like the column-integrated cloud amount, cloud optical depth (COD), and cloud top height (CTH). These products have been used for validating general circulation models (GCMs), but they may not meet the increasing demand for evaluating cloud properties in different layers. Since satellite algorithms generally assume single-layer cloud, large uncertainties are incurred in layered cloud quantities due to frequent occurrence of overlapped clouds. These uncertainties are illustrated through a comparison of three global satellite products including 1) a new product of Chang and Li (2005, JCL), 2) a standard product of the Moderate-resolution Imaging Spectroradiometer (MODIS), and 3) a bispectral visible-infrared (VIS-IR) product similar to the ISCCP. The first product identifies cirrus-overlapping low cloud and retrieves their individual layer properties. This leads to 10%-20% (absolute values) more low clouds found beneath the cirrus in the tropics and mid-latitude regions than the latter two products. For the cirrus overlapped clouds, the MODIS standard product gives only the CTH for cirrus with no information concerning the presence and/or properties of the underlying clouds; whereas the bispectral VIS-IR simulated product misidentifies them as single-layer mid-level clouds. The latter leads to overestimation of mid-level clouds but underestimation of high and low clouds thereby shows more uniformly distributed high, mid and low clouds. This is in contrast to the MODIS standard and new Chang-and-Li products where they display a bimodal distribution with distinct high and low cloud regimes that are similar to most simulations by GCMs.