Entrainment Drying of the Stratocumulus-Topped Boundary Layer

It is known that entrainment at the boundary layer top results in the growth of the boundary layer in cloud free conditions. This concept has been adapted to the stratocumulus-topped boundary layer even though the physical processes involved are much more complex. The results in this study suggest that the presence of the cloud layer indeed may modify the effects of entrainment through evaporation of the cloud droplets and cloud-top radiative cooling, which are processes not present in the cloud free cases. From multiple soundings we found layered structure within one or two hundred meters above the stratocumulus top. Analysis of the coherent signal within each layer and the presence of turbulence and cloud droplets suggest that the layers were part of the boundary layer in its history of evolution. We formed a conceptual model that describes the formation of the layered structure as the result of entrainment mixing and the subsequent evaporation of the cloud droplets in the mixture of inversion air and the boundary layer air. The observed properties of the layers above the cloud support the hypothesis in the conceptual model.

The findings from this study suggest the importance of droplet evaporation related to cloud-top entrainment and of radiative cooling and turbulence mixing in establishing a new interface at the cloud top. The role of entrainment thus can be very different from that in the clear boundary layer. The cause of the entrainment drying of the cloud layer may have to do with the time scale of boundary layer turbulence mixing and the time scale of entrainment mixing, which are subjects requiring further investigation.