A hierarchical model study of MBL cloud feedback mechanisms

Marine boundary layer (MBL) clouds strongly influence Earth's radiation budget, and their response to climate changes is one of the central uncertainties in climate projections. To study this response, we use a hierarchical modeling framework spanning an idealized GCM, large-eddy simulations (LES), a single-column model (SCM), and a conceptual bulk model. The idealized GCM has an eddy diffusivity/mass flux (EDMF) turbulence closure and a PDF-based cloud scheme. The subtropical MBL clouds are simulated in a wide range of model climates. Their coverage strongly decreases as the climate warms, suggesting a positive climate feedback. The SCM with the same parameterizations is run with large-scale forcing terms similar to the GCM. The statistical equilibrium states from the SCM are comparable to the GCM results. Controlled experiments are performed in the SCM to separate dynamic and thermodynamic effects. A further simplified bulk model of the boundary layer is used to understand the energy and moisture balances in the GCM and SCM models. Possible physical mechanisms of the cloud response are discussed in the context of this bulk model, and preliminary scaling results are derived. An LES model is also run towards statistical equilibrium with similar large-scale forcing terms as in the SCM, and the validity and robustness of the SCM results are tested by comparison to the LES results.