Low-Cloud Feedback In CAM5-CLUBB:Physical Mechanisms And Parameter Sensitivity Analysis

Equilibrium climate sensitivity(ECS)is an important factor for climate projection.However,the Fifth Assessment Report(AR5)of the Intergovernmental Panel on Climate Change has shown that ECS among current climate models still exhibits a wide range of values,and the uncertainty of ECS among general circulation models(GCMs)mainly stems from low-cloud feedbacks over tropical(30°N/S)oceans.To reduce this uncertainty,it is therefore important to understand the physical mechanism of low-cloud feedbacks.In this study,the physical mechanism of low-cloud feedbacks is explored by using perturbed-parameter ensemble(PPE)experiments in a unified scheme of boundary layer turbulence and shallow clouds,named Cloud Layers Unified by Binormals(CLUBB)coupled to Community Atmosphere Model version5(CAM5).A Generalized Linear Model(GLM)is further applied to study the responses of simulated low-cloud feedbacks to 14 CLUBB selected tunable parameters and 4 parameters of the Zhang-McFarlane deep convection scheme.Our results show that the largest spread of net cloud feedback due to parameter perturbation exists in the most stable tropical regime dominated by stratocumulus clouds,attributed to the large spread of shortwave cloud feedbacks.And the shortwave cloud feedbacks in CAM5-CLUBB are generally positive,with reduced low-cloud cover and shallower low clouds in a warmer climate.The shallower and reduced low cloud is related to the weaker turbulence in the planetary boundary layer(PBL),which is possibly triggered by the strengthened stability of the cloud layer.The positive feedback between cloud cover,cloud top radiative cooling and PBL turbulent mixing could further enhance the decrease in low clouds.The stronger inversion stability of PBL partly counters the decrease in low clouds.Shallow convection strengthens 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 usually means more convective drying,but also leads to less PBL turbulence and less low clouds in the present climate,which leads to less reduction in low clouds through the "beta feedback".The parameters related to dynamic turbulent structure and double Gaussian closure in CLUBB have been found to be 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.