A Numerical Study of the Interaction between Stratocumulus and the Air Overlying It

Abstract

The structure and evolution of stratocumulus cloud decks have long been recognized to depend upon a balance of numerous processes including the downward entrainment of the overlying free tropospheric air into the cloud deck. While the cloud is understood to have an impact on the overlying air as it subsides toward the inversion, the nature of the interaction between the stratocumulus and the overlying air has not been studied in detail. We have developed a simple, one-dimensional model of the air in the lower free troposphere as it subsides to the stratocumulus cloud deck. First, we fixed the mixed layer in order to determine how the thermodynamic structure of the overlying air impacts the cloud. Our model shows that this overlying air can have a large impact on the net longwave flux across the cloud top and thus the longwave cloud-top cooling. The overlying air, which may require days to subside through a couple of kilometers, undergoes cooling at a rate that depends on its vapor content. The total cooling is thus strongly dependent on the large-scale divergence. This cooling is greatest immediately above the cloud top and can substantially change the strength of the inversion and the potential for a buoyancy reversal upon entrainment. We then coupled this model of the overlying air to a mixed-layer model. We find that the temperature at which air is entrained into the cloud, and thus the strength of the inversion, is not a direct function of the altitude of cloud top as is normally modeled. The entrainment rate and the strength of the inversion are found to be in a loose balance. If the entrainment rate is too great, the strength of the inversion increases, which reduces the entrainment rate.