A Mixed-Layer Model perspective on stratocumulus steady-states in a perturbed climate

Equilibrium states of stratocumulus are evaluated for a range of free tropospheric conditions in a mixed-layer model framework using a number of different entrainment formulations. The equilibrium states show that a reduced lower tropospheric stability (LTS) and a drier free troposphere support a thicker cloud layer. Furthermore, cooler and drier free-tropospheric conditions promote decoupling which is the first stage of stratocumulus break-up into cumulus. The qualitative results hold for all the considered entrainment formulations, although the precise quantitative details of the boundary-layer state do vary with the choice of entrainment parametrization. Perturbations of the equilibrium states by increasing the sea-surface temperature while keeping the LTS and the free-tropospheric relative humidity constant leads to cloud thinning and an increased occurrence of decoupling regime. These results are in line with recent large-eddy simulation studies and increase the confidence in them by showing their validity for a large range of free tropospheric conditions.