EAPS

Title:
Relationships between tropical ascent and high cloud changes with warming and directions toward model improvement

Abstract:
Tropical ascent area (Aa) and high cloud fraction (HCF) 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 results in a strikingly similar spread and relation between the HCF and Aa responses to warming compared to the CMIP5 multi-model ensemble, with a narrowing of Aa corresponding to a greater HCF reduction. This similarity suggests that the CMIP5 intermodel spread in the response of HCF and Aa 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 HCF changes, perturbing cloud physics produces a weaker circulation response and dissimilar HCF-Aa relation to that seen in CMIP5. This can be attributed to the fact that decreases in HCF reduce cloud radiative heating, which opposes Aa narrowing by decreasing atmospheric stability and net energy input. Implications of these results will be discussed and ideas for future exploration will be presented.