By simulating the observations of multiple satellite instruments, COSP enables quantitative evaluation of clouds, humidity, and precipitation processes in diverse numerical models.G eneral circulation models (GCMs) of the atmosphere, including those used for numerical weather prediction (NWP) and climate projections, operate with resolutions from a few kilometers to hundreds of kilometers. Many atmospheric processes, such as turbulence and microphysical processes within clouds, operate at smaller scales and hence cannot be resolved by current model resolutions. These processes are included by means of parameterizations, which are semiempirical or statistical models that relate gridbox mean variables to these subgrid processes. For instance, some cloud parameterizations diagnose the amount of cloud condensate and the fraction of the grid box that a cloud occupies (cloud area fraction) as a function of the relative humidity (RH) of the grid box (Slingo 1980;Smith 1990). The formulation of these parameterizations is very important for the model evolution because they modify the three-dimensional structure of temperature and humidity directly (e.g., condensation/evaporation) or indirectly by interacting with other parameterizations (e.g., radiation) and the large-scale dynamics. Therefore, the evaluation of these parameterizations is crucial to improving our weather forecasts or increasing our confidence in climate projections.Satellites have proven to be very helpful tools for this purpose because they provide global or nearglobal coverage, thereby giving a representative sample of all meteorological conditions. However, satellites do not measure directly those geophysical quantities of interest, such as the amount or phase of cloud condensate. They measure the intensity of radiation coming from a particular area and direction in a particular wavelength range (radiances). The range of wavelengths covered by past and current systems spans several orders of magnitude, from COSP