Climate sensitivity of the CSIRO GCM: Effect of cloud modeling assumptions

The climate sensitivity of the CSIRO Global Climate Model is investigated u sing uniform sea surface temperature perturbation experiments. One experime nt (denoted DIAG) uses a diagnostic treatment of clouds, with fixed cloud r adiative properties that vary with height. The other experiment (denoted CT RL) uses a recently introduced prognostic treatment of stratiform clouds, w ith interactive calculation of cloud radiative properties. The DIAG experiment has a positive shortwave (SW) cloud feedback and a nega tive longwave (LW) feedback, due to an overall reduction of midlevel and hi gh cloudiness in the warmer climate. The signs of both the SW and LW feedba cks are opposite in the CTRL experiment due to an overall increase of cloud water content in the warmer climate. Because of cancellation between the S W and LW components, there is not a large difference in the net cloud feedb ack between the two experiments, with both having a modest negative cloud f eedback. as measured by the change in cloud radiative forcing. The CTRL experiment has a larger clear-sky climate sensitivity than the DIA G experiment. Off-line radiative calculations are used to show that this is primarily because of a stronger water vapor feedback. This is caused by di fferences in upper-tropospheric cloud radiative forcing that give a stronge r upward shift of the tropopause on warming when the prognostic scheme is u sed. A sensitivity test shows that an artificial restriction on the maximum height of high clouds that exists in the diagnostic scheme is the reason f or the different behavior. The robustness of the result obtained in the CTRL experiment is investigate d via 18 perturbation experiments, in which key parameters in the prognosti c cloud scheme are varied, while retaining the overall approach used in the CTRL experiment. As far as possible, theory and observations are used to c onstrain the ranges within which these parameters are varied. It is found t hat the behavior of the scheme under climate change is generally robust, wi th no statistically significant changes in LW cloud feedback and only modes t changes in SW cloud feedback. Overall, larger differences (both in contro l climate and in climate sensitivity) result from parameter changes that af fect cloud formation than from changes that affect precipitation processes or cloud radiative properties.