Relationships Between Tropical Ascent and High Cloud Fraction Changes With Warming Revealed by Perturbation Physics Experiments in CAM5

Abstract

Abstract Tropical ascent area (Aa) and high cloud fraction (HCF) are projected to decrease with surface warming in most Coupled Model Intercomparison Project Phase 5 (CMIP5) models. Perturbing deep convective parameters in the Community Atmosphere Model (CAM5) results in a similar spread and correlation between HCF and Aa responses to interannual warming compared to the CMIP5 ensemble, with a narrower Aa corresponding to greater HCF reduction. Perturbing cloud physics parameters produces a comparatively smaller range of Aa responses to warming and a dissimilar HCF‐Aa relation to that in CMIP5; a narrower Aa corresponds to less HCF reduction, likely due to cloud radiative effects. A narrowing of Aa corresponds to a regime shift toward stronger precipitation in both experiments. We infer that model differences in deep convection parameterization likely play a greater role than differing cloud physics in determining the diverse responses of Aa and HCF to warming in CMIP5.

Keywords

• tropical circulation
• moist convection
• cloud microphysics
• deep convection parameterization
• cloud feedbacks