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

Abstract

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 dryer free troposphere support a thicker cloud layer. Furthermore cooler and dryer 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.