A layer of intensive mixing (entrainment interface layer, [EIL]) at the top of marine stratocumulus under a strong inversion has been investigated with 10 cm resolution using an ultrafast thermometer (UFT-F; temperature), a particle volume monitor PVM-100A (liquid water content), and a fast forward scattering spectrometer probe (FFSSP; droplet spectra). Measurements were collected on board the NCAR C-130 aircraft during research flight RF05 of DYCOMS-II field study. The EIL consists of mutual filaments of cloudy and clear air at different stages of stirring, mixing, and homogenization. Borders between these filaments are often very sharp, with the 10 cm resolution of the instruments being insufficient to characterize them properly in many cases. Certain classifications of these filaments and hypotheses about the mechanisms of their formation have been proposed. The common occurrence of filaments of sizes smaller than the resolution of instruments has been indirectly confirmed. This is in agreement with the observed cloud droplet spectra showing variations of droplet number concentration without significant change of the mean droplet diameter and spectrum width.

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.

This study addresses key aspects of shallow moist convection, as simulated by a multiplume eddy-diffusivity/mass-flux (EDMF) model. Two factors suggested in the literature to be essential for the development of convective plumes are investigated: surface conditions and lateral entrainment. The model consistently decomposes the subgrid vertical mixing into convective plumes and the nonconvective environment. The modeled convection shows low sensitivity to the surface plume area. The results indicate that plume development in the subcloud layer is controlled by both surface conditions and lateral entrainment. Their impact significantly changes in the cloud layer where the surface conditions are no longer important. The development of shallow convection is dominated by the interactions between the plumes and the large-scale field and is sensitive to the representation of the variability of thermodynamic properties between the plumes. A simple two-layer model of steady-state convection is proposed to help understand the role of these processes in shaping the properties of moist convection.

A fully unified parameterization of boundary layer and moist convection (shallow and deep) is presented. The new parameterization is based on the stochastic multiplume eddy-diffusivity/mass-flux (EDMF) approach, which distinguishes between convective plumes and nonconvective mixing. The convective plumes represent both surface-forced updrafts and evaporatively driven downdrafts. The type of convection (i.e., dry, shallow, or deep) represented by the updrafts is not defined a priori, but rather depends on the near-surface updraft properties and the stochastic interactions between the plumes and the environment through lateral entrainment. Consequently, some updrafts may contribute only to the nonlocal transport within the subcloud layer, while others may condense and form shallow or even deep convection. Such a formulation is void of trigger functions and additional closures typical of modular parameterizations. The updrafts are coupled to relatively simple warm-, mixed-, and ice-phase microphysics. Each precipitating updraft forms a downdraft driven by the evaporation of detrained precipitation. The downdrafts control the development of cold pools near the surface that can invigorate convection. The new parameterization is tested in a single-column model against large-eddy simulations (LESs) for cases representing weakly precipitating marine convection and the diurnal cycle of continental deep convection. The results of these EDMF experiments compare well with the LES reference simulations. In particular, the transitions between the different dominant convection regimes are realistically simulated.