On the diurnal cycle and susceptibility to aerosol concentration in a stratocumulus-topped mixed layer

Mixed-layer theory is used to study the diurnal cycle of the stratocumulus-topped boundary layer and its susceptibility to perturbations in atmospheric aerosol concentration. Our results show that the diurnal evolution of cloud thickness is sensitive to the entrainment efficiency. For high entrainment efficiencies, the cloud base tends to descend at a faster rate than the cloud top; this difference in descent rates leads to cloud thickening during the daytime, which is inconsistent with observations. For low entrainment efficiencies, variations in cloud-top height dominate in the cloud-thickness evolution, while cloud-base height remains almost constant; this behaviour is in better agreement with available data. We explain these effects through a consideration of the equilibrium state of cloud boundaries and their adjustment time-scales. Liquid-water path and cloud albedo are both sensitive to the entrainment efficiency; still, the susceptibility of cloud albedo to droplet number density dominates the entrainment effects. This result has significant implications for climate-sensitivity studies: it suggests that estimates of aerosol indirect effects from stratocumulus clouds will not be particularly sensitive to the way entrainment is represented in large-scale models.