CLOUD RADIATION FORCINGS AND FEEDBACKS - GENERAL-CIRCULATION MODEL TESTS AND OBSERVATIONAL VALIDATION

Using an atmospheric general circulation model (the National Center fo r Atmospheric Research Community Climate Model: CCM2), the effects on climate sensitivity of several different cloud radiation parameterizat ions have been investigated. In addition to the original cloud radiati on scheme of CCM2, four parameterizations incorporating prognostic clo ud water were tested: one version with prescribed cloud radiative prop erties and three other versions with interactive cloud radiative prope rties. The authors' numerical experiments employ perpetual July integr ations driven by globally constant sea surface temperature forcings of two degrees, both positive and negative. A diagnostic radiation calcu lation has been applied to investigate the partial contributions of hi gh, middle, and low cloud to the total cloud radiative forcing, as wel l as the contributions of water vapor, temperature, and cloud to the n et climate feedback. The high cloud net radiative forcing is positive, and the middle and low cloud net radiative forcings are negative. The total net cloud forcing is negative in all of the model versions. The effect of interactive cloud radiative properties on global climate se nsitivity is significant. The net cloud radiative feedbacks consist of quite different shortwave and longwave components between the schemes with interactive cloud radiative properties and the schemes with spec ified properties. The increase in cloud water content in the warmer cl imate leads to optically thicker middle-and low-level clouds and in tu rn to negative shortwave feedbacks for the interactive radiative schem es, while the decrease in cloud amount simply produces a positive shor twave feedback for the schemes with a specified cloud water path. For the longwave feedbacks, the decrease in high effective cloudiness for the schemes without interactive radiative properties leads to a negati ve feedback, while for the other cases, the longwave feedback is posit ive. These cloud radiation parameterizations are empirically validated by using a single-column diagnostic model, together with measurements from the Atmospheric Radiation Measurement program and from the Tropi cal Ocean Global Atmosphere Combined Ocean-Atmosphere Response Experim ent. The inclusion of prognostic cloud water produces a notable improv ement in the realism of the parameterizations, as judged by these obse rvations. Furthermore, the observational evidence suggests that derivi ng cloud radiative properties from cloud water content and microphysic al characteristics is a promising route to further improvement.