A numerical investigation of entrainment and transport within a stratocumulus‐topped boundary layer

Abstract

Entrainment into the stratocumulus-topped boundary layer (STBL) is investigated by means of large-eddy simulations. Set-up of the numerical experiment is based on the research flight RF-01 in the DYCOMS-II field campaign. We focus on the stability of the flow in the cloud-top region known as the Entrainment Interface Layer (EIL). We calculate the local gradient Richardson number, Ri, at the surface of maximum static stability and at the material top of the STBL defined by a threshold of the total water content. We found that regions in which updraughts impinge upon the inversion and diverge horizontally are characterized by small values of Ri. Resulting turbulence is responsible for entrainment and formation of the EIL. Volumes of the STBL air and the free-tropospheric air from above it, mixed in proportion resulting in negative buoyancy and typically void of cloud water, form 'cloud holes' -trenches of descending cloud-free air which surround updraught areas.The entrainment process is further analyzed using a passive scalar introduced after three hours of the simulation above the layer of maximum static stability. The mixing fraction of this scalar in the air within the cloud holes falls within the range corresponding to the buoyancy reversal. Some of the negatively buoyant mixtures sinking through the cloud holes are wrapped around the edge of cloudy regions and recirculated into the cloud, which causes a local increase of the cloud-base height. The rest of the entrained free-tropospheric air sinks slowly into the STBL and leads to its gradual dilution.

Observation of a Self-Limiting, Shear-Induced Turbulent Inversion Layer Above Marine Stratocumulus

“…For example, one might envisage pulsating bursts of turbulence and entrainment as the boundary layer 'squeezes' the TIL and therefore sharpens the wind-shear gradient. This sharpening would lead, in turn, to a local reduction in Ri, analogous to that observed by Kurowski et al (2009) in the weak shear scenario. In this view, while the mean vertical shear of horizontal wind speed is a property of the large-scale synoptic flow, the local thickness of the TIL itself is still coupled to boundary-layer dynamics, as is the local entrainment rate through its dependence on TIL thickness.…”

“…First, this large-scale, shear-dominated situation contrasts with the picture of local-scale shear resulting from large-scale convective eddies (e.g., Sullivan et al 1998;Kurowski et al 2009). We can speculate that the large-scale convective eddies in this case are not so important in generating the shear, but possibly in driving vertical undulations that lead to variations in the TIL thickness.…”

Droplet growth in warm turbulent clouds

“…A detailed study of the cloud-top structure of stratocumulus by Moeng et al (2005) led to the question: where is the interface and which interface should be considered as the interface through which mixing occurs? These questions have been addressed by Kurowski et al (2009), who followed passive tracers in LES of stratocumulus. Their simulations confirm the existence of an entrainment interfacial layer and show that only a small fraction of air undergoing mixing at the cloud top, in proportions which are just right to produce negative buoyancy, sinks down into the cloud layer.…”

Two-fluid formulation of the cloud-top mixing layer for direct numerical simulation

“…There are many different aspects of the problem, which can be considered. One of them, the role of buoyancy reversal due to the evaporative cooling that is promoted by the evaporation of the droplets under certain mixing conditions, has been long debated using theory, field and laboratory measurements, and numerical simulations [7,14,25,28,29,36,46,51,54,61]. Although this problem explicitly involves molecular transport processes, in particular, the transport of latent heat, direct numerical simulation (DNS) has not been employed to address questions related to buoyancy reversal as extensively as it has been used in other areas [39].…”

“…Albrecht et al [1] introduced the mixture fraction into the analysis of cloud-topped mixed layers, and the formulation has been discussed by Bretherton [8] in a very comprehensive manner, with special attention to the thermodynamics. Since then, this methodology has been often employed as a physical model to investigate the role of latent heat effects in the dynamics of cloud interfaces [24,25,28,29,36,[49][50][51]. However, a detailed derivation starting from the basic equations governing the conservation of mass, momentum, and energy has not been presented in the literature, and this is one of the two objectives of this study.…”