Observations of Cloud-Top Entrainment in Marine Stratocumulus Clouds

Abstract

Measurements of the thermodynamic and dynamic properties of entrainment events in marine stratocumulus are used to explain why cloud-top entrainment instability may not lead to the breakup of the clouds and to define the role of cloud-top entrainment on the turbulent mixing processes when buoyancy reversal due to mixing is released. The measurements were made off the coast of California during the First ISCCP Regional Experiment (FIRE 1987) by the NCAR Electra research aircraft. The data used in this study were collected on a day when the cloud-top jump conditions indicate possible buoyancy reversal for the entrained parcels that mix with cloudy air. The entrainment events are identified using a conditional sampling method. Ozone concentration is used as a tracer of inversion air to define the entrainment mixing fraction. It is found that cloud-top entrainment ceases to be a mere interfacial phenomenon when buoyancy reversal of the entrainment parcel occurs. Strong entrainment preferentially occurs in the downdraft branch of the boundary-layer circulation, and its effect is not confined to a region near the cloud top. In the case studied here, the contribution to the negative buoyancy in the entrainment downdrafts through evaporative cooling is comparable with that from radiative cooling. The buoyancy deficit as the result of evaporation of cloud droplets is found to be insufficient to promote enhanced entrainment that leads to the breakup of the cloud deck, as suggested by the simple application of cloud-top entrainment instability (CTEI). A conceptual model for cloud-top entrainment that results in buoyancy reversal is proposed. This model emphasizes the interaction between entrainment and the boundary-layer circulation. According to this conceptual model, while buoyancy reversal tends to maintain a well-mixed boundary layer by providing deficit negative buoyancy to drive turbulent mixing, it may also accelerate the thinning and dissipation of a cloud deck once the boundary layer is decoupled by other processes such as solar absorption or drizzle. It is suggested here that a simple criterion for CTEI based solely on the cloud-top discontinuities is unlikely to exist since the dynamics of the entire boundary layer are involved in the entrainment process.