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

A mixed-layer model is used to study the response of stratocumulus equilibrium state solutions
to perturbations of cloud controlling factors which include the sea surface temperature, the specific humid-
ity and temperature in the free troposphere, as well as the large-scale divergence and horizontal wind
speed. In the first set of experiments, we assess the effect of a change in a single forcing condition while
keeping the entrainment rate fixed, while in the second set, the entrainment rate is allowed to respond. The
role of the entrainment rate is exemplified from an experiment in which the sea surface temperature is
increased. An analysis of the budget equation for heat and moisture demonstrates that for a fixed entrain-
ment rate, the stratocumulus liquid water path (LWP) will increase since the moistening from the surface
evaporation dominates the warming effect. By contrast, if the response of the entrainment rate to the
change in the surface forcing is sufficiently strong, enhanced mixing of dry and warm inversion air will
cause a thinning of the cloud layer. If the entrainment warming effect is sufficiently strong, the surface sen-
sible heat flux will decrease, as opposed to an increase which will occur for a fixed entrainment rate. It is
argued that the surface evaporation will always increase for an increase in the sea surface temperature, and
this change will be enlarged if the entrainment rate increases. These experiments aid the interpretation of
results of similar simulations with single-column model versions of climate models carried out in the frame-
work of the CFMIP-GCSS Intercomparison of Large-Eddy and Single-Column Models (CGILS) project. Because
in a large-scale models, the entrainment response to changes in the large-scale forcing conditions depends
on the details of the parameterization of turbulent and convective transport, intermodel differences in the
sign of the LWP response may be well attributable to differences in the entrainment response.