Séminaire
SPOOKIE is a recent initiative building on the experimental protocol of the Cloud Feedback Model Intercomparison Project (CFMIP). Its aims are to establish the relative contributions of different areas of model physics to inter-model spread in cloud feedback, and their roles in mechanisms of robustly simulated cloud feedback. The approach is to perform "mechanism denial" sensitivity experiments where specific processes such as parametrized convection are removed or simplified. Pilot amip/amip4K "CONVOFF" experiments with parametrized convection switched off are presented. We find that models are able to run without parameterized convection at current climate model resolutions. Four of the models (MRI-CGCM3, MIROC5, HadGEM2-A and MPI-ESM-LR) retain positive low-level cloud feedbacks in stable regimes in the subtropics in CONVOFF experiments, indicating that processes other than convection contribute to positive subtropical feedback. However these models’ cloud feedbacks exhibit a strong convergence in character in CONVOFF experiments compared to the standard model versions, with the range in the global mean net cloud feedback being reduced substantially. The net cloud feedback shows a reduced or similar spread across all tropical stability regimes, and a relatively smooth and monotonic transition from weaker net cloud feedback in unstable regimes to more positive feedback in stable regimes.Hence we conclude that inter-model differences in the details of convective parametrizations contribute substantially to inter-model spread in cloud feedback in these models. The CNRM-CM5 model behaves somewhat differently however; removing parametrized convection introduces a strong negative cloud feedback in ascending regions, largely driven by increases in cloud fraction and cloud liquid water at around the freezing level. Future plans to develop this approach, for example by simplifying convective parametrizations or by suppressing mixed phase cloud feedbacks will be discussed.
MP Lefebvre (0144272799)