Climate Sensitivity of the CSIRO GCM: Effect of Cloud Modeling Assumptions

The climate sensitivity of the CSIRO Global Climate Model is investigated using uniform sea surface temperature perturbation experiments. One experiment (denoted DIAG) uses a diagnostic treatment of clouds, with fixed cloud radiative properties that vary with height. The other experiment (denoted CTRL) uses a recently introduced prognostic treatment of stratiform clouds, with interactive calculation of cloud radiative properties. The DIAG experiment has a positive shortwave (SW) cloud feedback and a negative longwave (LW) feedback, due to an overall reduction of midlevel and high cloudiness in the warmer climate. The signs of both the SW and LW feedbacks are opposite in the CTRL experiment due to an overall increase of cloud water content in the warmer climate. Because of cancellation between the SW and LW components, there is not a large difference in the net cloud feedback between the two experiments, with both having a modest negative cloud feedback, as measured by the change in cloud radiative forcing. The CTRL experiment has a larger clear-sky climate sensitivity than the DIAG experiment. Off-line radiative calculations are used to show that this is primarily because of a stronger water vapor feedback. This is caused by differences in upper-tropospheric cloud radiative forcing that give a stronger upward shift of the tropopause on warming when the prognostic scheme is used. A sensitivity test shows that an artificial restriction on the maximum height of high clouds that exists in the diagnostic scheme is the reason for the different behavior. The robustness of the result obtained in the CTRL experiment is investigated via 18 perturbation experiments, in which key parameters in the prognostic cloud scheme are varied, while retaining the overall approach used in the CTRL experiment. As far as possible, theory and observations are used to constrain the ranges within which these parameters are varied. It is found that the behavior of the scheme under climate change is generally robust, with no statistically significant changes in LW cloud feedback and only modest changes in SW cloud feedback. Overall, larger differences (both in control climate and in climate sensitivity) result from parameter changes that affect cloud formation than from changes that affect precipitation processes or cloud radiative properties.