Low-cloud feedback in CAM5-CLUBB: Physical mechanisms and parameter sensitivity analysis

The physical mechanism of low-cloud feedbacks is examined by using perturbed-parameter ensemble experiments in a unified scheme of boundary layer turbulence and shallow convection, named Cloud Layers Unified by Binormals (CLUBB) coupled to Community Atmosphere Model version 5 (CAM5). The shortwave cloud feedbacks in CAM5-CLUBB are positive in the most stable tropical regime, which is related to the weaker turbulence in the planetary boundary layer (PBL) in a warmer climate that is possibly triggered by the strengthened stability of the cloud layer. The positive feedback between low cloud cover (LCC), cloud top radiative cooling, and PBL turbulent mixing may further enhance the decrease in LCC. The stronger inversion stability of PBL partly counters the decrease in LCC, and a recently developed index, the estimated cloud-top entrainment index, is a better predictor for LCC changes than conventional stability indices. The relative strength of shallow convection increases in the warmer climate, but its effect on low-cloud feedback is complicated by the unified treatment of shallow convection and PBL turbulence in CLUBB. Stronger shallow convection means more convective drying but also less PBL turbulence and less LCC in the present climate, which leads to less reduction in LCC. The parameters related to dynamic turbulent structure and double Gaussian closure in CLUBB are the most influential parameters on low-cloud feedbacks. Our results suggest that a unified treatment of shallow convection and turbulence may give rise to the predominate role of the PBL turbulent mixing in determining low-cloud feedback.