A mixed-layer model study of the stratocumulus response to changes in large-scale conditions

The response of stratocumulus clouds to global warming as predicted by earth system models is uncertain. Idealized simulations of low clouds in the Hadley cell with single-column model versions of climate models have shown a divergent behavior in the sign of the low cloud radiative feedback when perturbations in multiple large-scale conditions are applied. To understand these different model outcomes steady-state solutions of the stratocumulus-topped boundary layer are studied with a mixed-layer model. \color{black} In particular we diagnose the response of the liquid water path (LWP) to a perturbation of a single cloud controlling factor like the sea surface temperature, the specific humidity and temperature in the free troposphere, as well as the large-scale divergence and horizontal wind speed. The total LWP response is decomposed in a direct contribution that is due to a change in a single quantity and for a fixed entrainment rate, and an indirect contribution due to a change in the entrainment rate. Analytical expressions for the direct response of boundary layer properties to changes in the cloud controlling factors are presented. The indirect LWP response comprises a multiplier term that gives the LWP response to changes in the entrainment rate, which sign depends on the free tropospheric conditions. It is demonstrated that the indirect entrainment effect can either magnify, dampen or even change the sign of the LWP response as compared to the direct response. Since the entrainment rate in a large-scale model depends on the details of the physics parameterization, intermodel differences in the sign of the LWP response to changes in the large-scale forcing conditions may be well attributable to differences in the entrainment response.