Dr. Kathleen Schiro, Jet Propulsion Laboratory
Relationships between tropical ascent and high cloud fraction changes with warming revealed by perturbation physics experiments in CESM
Tropical ascent area and high cloud fraction are projected to decrease with surface warming in Coupled Model Intercomparison Project (CMIP5) climate models. Perturbing deep convective parameters in an atmosphere-only version of the Community Earth System Model (CESM) results in a strikingly similar spread and relation between the high cloud and ascent area responses to warming compared to the CMIP5 multi-model ensemble, with a narrowing of ascent area corresponding to a greater high cloud reduction. This similarity suggests that the CMIP5 intermodel spread in the response of high cloud fraction and ascent area to warming could be attributed to model differences in deep convective parameterization. This stresses the need to inform revisions to convective parameterization with process-oriented studies of deep convection. Efforts to inform revision to convective mixing parameterization and diagnose deep convective onset in GCMs will thus be presented. While differences in cloud physics parameterization could also contribute significantly to the CMIP5 intermodel spread in high cloud fraction changes, perturbing cloud physics produces a weaker circulation response and dissimilar relation to that seen in CMIP5. This can be attributed to the fact that decreases in high clouds reduce cloud radiative heating, which opposes ascent narrowing by decreasing atmospheric stability and net energy input. Implications of these results will be discussed and ideas for future exploration will be presented.